The Sciences
by Edward Holden
Book I - Astronomy
Contents
The Science of the Sun, Moon, and Stars.. . . . . . . . . . . . . . 9
The Earth as a Planet . . . . . .. . . . . . . . . . . . . . . . . . .
. . . . 9
Distance of the Moon from the Earth. . .. . . . . . . . . . . . . . .11
Distance of the Sun from The Earth . . . . . . . . . . . . . . . . . 11
The Diameter of the Earth . . . . . . . . . . . . . . . . . . . .
. .. . .12
Distance of the Sun from the Earth. . . . . . . . . . . . . . . .
. . 14
The Planets Mercury and Venus . . . . . . . . . . .. . . . . . .
. . .16
The Planets Mars, Jupiter, Saturn, Uranus, and Neptune .. . . .16
Distances of the Planets from the Sun . . . . . . . . . . . . . . . .
17
How to make a Map that shows the Sun and Planets . . . . .
.17
Scale of the Map . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .. . . 18
Sizes of the Planets compare to the Sun . . . . . . . . . . .
. . . .19
The Solar System; the Sun and Planets . . . . . . . . . . .. . . . . .25
Relative Sizes of the Planets . . . . . . . . . . . . . . . . . .
.. . . . . 28
The Moons of the Planets . . . . . . . . . . . . . . . . . . . . . . .
. . 30
The Minor Planets; the Asteroids . . . . . . . . . . . . . . .. .
. . . 32
Comets . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .
. . . . . .. . .32
The Stars . . . . . . . . . . . . . . . . . . . . . .. . . . . .
. . . . . . . . 32
Distances of the Stars . . . . . . . . . . . . . . . . . . . . . . .
. . . . 32
What is a Planet? . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . .33
Phases of the Moon (New Moon, Full Moon. . . . . . . . . . . . 34
Number of the Stars . . . . . . . . . . . . . . . . . . . . . . . . . .
.. . 38
Clusters of Stars . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 39
The Pleiades . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 39
The Milky Way . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.. . .41
Do the Stars have Planets as the Sun does? . . . . . . . . . . .
. 42
Shooting Stars; Meteors; Fire-balls . . . . . . . . . . . . . . .
. . ..44
The Zodiacal Light . . . . . . . . . . . . . . . . . . . . . . . . . .
. .. . .46
Nebulae . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . .
. . .. . .47
Rising and Setting of the Sun . . . . . . . . . . . . . . . . . . . ..
. . 48
How the Sun appears to move From Rising to Setting . . . . 49
The Celestial Sphere . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 48
The Northern Stars . . . . . . . . . . . . . . . . . . . . . . . . . .
... . .51
The Great Bear (the Dipper) . . . . . . . . . . . . . . . . . . . . ...
. .53
The Southern Stars . . . . . . . . . . . . . . . . . . . . . . . . . .
.. . . 54
Time and Timekeeping . . . . . . . . . . . . . . . . . . . . . .
. .. . . .56
Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . .. . 56
A Meridian Circle . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . .57
The Lick Telescope . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . .61
The Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 62
Mountains on the Moon . . . . . . . . . .. . . . . . . . . . . .
. . .. . 62
Life on the Planets . . . . . . . . . . . .. . . . . . . . . . . . . .
. . . . .64
The Planet Mars . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . .64
The Planet Jupiter . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . .64
Appendix (Statistics of the Solar System). . . . . . . . . . . . . 66-70
pg 8
The Moon, from a photograph taken with the great telescope of the Lick
Observatory.
pg 9
Book I -
ASTRONOMY
The Science of the Sun, Moon, and Stars
The Earth as a Planet.--The
children were looking at a map of the world
one fine afternoon and studying the way the land and water are
distributed, when Agnes said: "I never knew before how little land
there was on the earth. Why, there is very
much more water than land."
"Oh, yes," said Tom, "there's very much more water on the surface; but
it's all land at the bottom of the ocean. The sea is about three miles
deep, you know, and then you come to the ocean bottom, and that is
solid land again. The earth is nearly all rocks and soil; only a little
of it is water, after all, but that little is on the surface, of
course, and that is why it shows."
Agnes. So the earth is almost
all land; if you dig down deep enough,
you should come to rocks, even below the oceans?
Tom. Yes, and if you went up
high enough, you would come to nothing.
You would come to air first, and then by and by to no air, and then you
would come to just nothing--to empty space.
Agnes. Well, it isn't quite
empty, as you call it. There are other
globes in space. There are other planets, and the sun and the moon, and
there are simply thousands of stars. So space isn't empty; it is pretty
full!
pg 10
Fig 6 America
Fig 7 The Old World
pg 11
Distance of the Moon and of the Sun
from the Earth.--Here Tom's big
brother Jack looked up from his book and said: "Well, that depends on
what you call full. It is 240,000 miles from here to the moon, and the
moon is the very nearest of all the heavenly bodies to us. There is a
good deal of empty space between us and the moon, it seems to me."
Agnes. Two hundred and forty
thousand miles! Oh, Jack, is that right?
Jack. Why, that isn't a
beginning; how far off do you suppose the sun
is? It is 93,000,000 miles--millions this time, not thousands; and some
of the planets are much farther off yet, and every one of the stars is
farther off still.
Fig 8 This picture
shows the height of land on the earth compared to the depth of the
sea. If you could cut the earth through and through with a knife
and look at one part only, it would look something like the picture.
All the shaded part is land. The curved line drawn all across the
picture, near the top, is the curve of the surface of the oceans.
Part of one of the oceans is shown by the white space below this curved
line and above the floor of the ocean itself,--the shaded land.
The curve of the ocean surface is continued across the picture
underneath the mountains. If the surface of the earth were all
water, the bounding line would be this curve. From side to side
of the picture is about 350 miles. If the whole circle of the
earth were drawn, it would be about eight feet in diameter. That
is the scale of the drawing.
Agnes.
Jack, tell us about it,
will you? We don't know, and you do.
Jack. The very first thing you
have to think about is the size of the
earth. How far is it through and through the earth, Tom? If you pushed
a stick through the earth from New York to China, how long would the
stick be?
pg 12
The Diameter of the Earth.--Tom. The geography says that the
diameter
of the earth is 8000 miles; so the stick would have to be 8000 miles
long,--as long as from Cape Horn to Hudson Bay, my teacher says.
Fig. 9. A Balloon--Balloons carrying men have
gone up more than five miles, and small balloons carrying thermometers,
etc., have been sent nearly ten miles high. The atmosphere of the earth
extends upwards a hundred miles or so, but beyond this there is no
air--nothing but empty space.
pg 13
Jack. That's about right.
Suppose there were a railway from Hudson Bay
to Cape Horn, and express trains running on it at the rate of 40 miles
an hour. Let us see how long they would take to go the 8000 miles. They
would go 40 miles in one hour, and 80 miles in two hours, and 960 miles
in a day--say 1000 miles a day. Well, they would take eight days to go
the 8000 miles, then. Now, suppose we could build a railway to the
moon. How long would an express train take to go the distance? Take
your pencil, Tom, and cipher it out.
Fig. 10. The Full Moon
Rising in the East
pg 14
Tom. You said the distance
from the earth to the moon was 240,000
miles. If the train goes 1000 miles a day, it would take 240 days. I
don't need any pencil.
Jack. Sure enough; and 240
days is eight months (8 x 30 = 240). It
would take the train eight months to go from the earth to the moon,
then--eight whole months, traveling night and day at forty miles and
more every hour.
Agnes. I should be nearly a
year older when I got there than when I
started, then.
Jack. Yes, and recollect that
there are no stations on the railway to
the moon. The moon is the heavenly body that is nearest to us, so that
space is pretty nearly empty, after all.
Fig. 11. A School Globe
Distance of the Sun from the Earth.--Tom. How far did you say it was
from the earth to the sun--93,000,000 miles?
Jack. That's right. You will
need your pencil to figure out how long
the express train would take to go from the earth to the sun, Tom.
Tom. Yes, it is like this,
isn't it? The train goes 1000 miles in a
day; then it will take 93,000 days to get to get to the sun.
30 ) 93000 days
12 )3100 months
258 1/2 years
It would take 3100 months, that is more than 258 years, to get to the
sun. That's a long journey! You would have 258 birthdays on the road,
Agnes.
pg 15
Jack. Put it this way, Tom:
258 years ago takes you back to the year
1643 (1901 - 258 = 1643). The Pilgrims had been in New England only
twenty-three years in 1643, for they came in 1620 (1643 - 1620 = 23).
Suppose one of those Pilgrims to have stepped on to the train at
Plymouth Rock; he would have been traveling all these years, and he
would only have arrived at the sun a few years ago; that is, if he had
lived to make the journey.
Fig. 12. The Pilgrims
landing on Plymouth Rock from their ship the "Mayflower," Dec. 20, 1620
Tom. Two hundred and
fifty-eight years!
pg 16
The Planets Mercury and Venus.--Jack. Yes, and nearly all that
space is
empty too. There are only two planets between the earth and the
sun--Mercury and Venus.
Agnes. Venus, the evening star?
Jack. Yes, Venus is the
evening star sometimes. Venus and Mercury are
the only planets that the Pilgrim would pass on the road from earth to
the sun. Space is rather empty, isn't it?
Agnes. Aren't there any stars
in between the earth and the sun, Jack?
Jack. Not one; the real stars
are thousands and thousands of times
farther off. We call Venus the "evening star," but Venus is not a star
at all, but a planet. Let me tell you, so that you can make a sort of
picture of it all in your minds. The sun is there in the middle of
space and all the planets move around him, just as the earth does.
Nearest to the sun is the planet Mercury, and then comes the planet
Venus, and then the planet Earth.
Agnes. That sounds
queerly--"the planet Earth"--though of course we
know the Earth is a planet.
The Planets Mars, Jupiter, Saturn,
Uranus, (1) and Neptune.--Jack.
Yes,
exactly so. And then there are other planets farther away from the sun
than the earth; Mars for one, and then Jupiter, and then Saturn, and
then Uranus, and then Neptune. That is all we know of; there may be
more of them. Neptune is thirty times as far from the sun as the earth
is. Here is a little table that I will write down for you to keep. You
need not memorize it, only recollect that Mercury and Venus are nearer
to the sun than we are, and that all the others are farther away.
(1) Pronounced U ra-nus.
pg 17
Distances of the Planets from the Sun
The planet Mercury
is 36 million miles from the sun
The planet Venus
is 67 million miles from the sun
The planet Earth
is 93 million miles from the sun
The planet Mars
is 141 million
miles from the sun
The planet Jupiter
is 483 million miles
from the sun
The planet Saturn
is 886 million miles
from the sun
The planet Uranus
is 1782 million miles from
the sun
The planet Neptune
is 2791 million miles from the sun
Jupiter is five times, and Neptune is thirty times, as far from the sun
as the earth is.
Tom. Isn't there a map of all
these planets that we can see?
Jack. No, and there's a good
reason why. Suppose you tried to make a
map of them, and suppose you took the distance from the sun to the
Earth on the map to be an inch. Don't you see that the distance from
the Sun to Neptune would have to be thirty times one inch, and the page
of your book thirty inches wide--nearly a yard wide?
Tom. Of course, no book has a
page as big as that; but you might make
little maps.
How to Make a Map that shows the Sun
and Planets.--Jack. You
and Agnes
can make a map yourselves to-morrow morning, if you want to, when you
go out for a walk, and I'll tell you how to do it.
Suppose you take the large globe in the library, that you were looking
at just now, to stand for the Sun. It is two feet in diameter. Well,
the diameter of the real sun is 870,000 miles, and your map has to be
made all to one scale. Every step of yours is about two feet long,
isn't it, Tom? Try it.
Tom. Yes, my steps are almost
exactly two feet long.
pg 18
Jack. Well, remember to-morrow
that every step you take along the road
to the village is really two feet long, but that it stands on the map
for 870,000 miles.
Agnes. Are we going to make
the map along the road?
Fig. 13. The Road to
the Village
Jack. My dear, you have to do
it that way; your map is going to be
nearly a mile and a quarter long. You have to use the whole country
round to make it.
Agnes. Well, that is a map!
Tom. How shall we make it,
Jack?
Jack. You start, you know,
with this globe in the house to stand for
the Sun. The globe is two feet in diameter, and the real Sun is 870,000
miles in diameter.
Scale of the Map.--"So,
recollect, every two feet on your map is
870,000 miles. Every one of your steps, Tom, stands for 870,000 miles.
pg 19
"You must take with you
a very small grain of canary-bird seed to stand for the planet Mercury;
a very small green pea to stand for the planet Venus;
a common green pea to stand for the planet Earth;
a rather large pin out of Agnes' work box, and let its round head stand
for the planet Mars;
and orange to stand for the planet Jupiter;
a golf ball to stand for the planet Saturn;
a common marble to stand for the planet Uranus;
a rather large marble to stand for the planet Neptune.
Sizes of the Planets compared to the
Sun.--"If this globe, two feet in
diameter, stands for the Sun (which is really 870,000 miles in
diameter), then a common green pea is just the right
Fig. 14. The sizes of
the planets of the Solar System (the Sun's Family) compared with each
other.
pg 20
size to stand for the Earth (which is really 8000 miles in diameter)
and an orange is just the right size to stand for Jupiter, and so on.
You are going to carry all the planets off in your pocket, and when you
have put them down in the right places you have made your map."
Tom. How shall we know where
to put them down?
Jack. I will give you the
right number of steps to take between the Sun
and every one of the planets. If one of Tom's steps is 870,000 miles,
then
Mercury (the canary seed) is
41 steps from the Sun (the globe at the
house);
Venus (the small pea) is 77
steps from the globe that stands for the
Sun;
Earth (the pea) is 107 steps
from the globe that stands for the Sun
Mars (the pin's head) is 162
steps from the globe that stands for the
Sun
Jupiter (the orange) is 555
steps from the globe that stands for the Sun
Saturn (the golf ball) is 1019
steps from the globe that stands for the
Sun
Uranus (the small marble) is
2048 steps from the globe that stands for
the Sun
Neptune (the large marble) is
3208 steps from the globe that stands for
the Sun
Those are the right distances, and you can make your map tomorrow
morning when you go for a walk. Recollect that the globe in the house
stands for the Sun. You are to walk away from it along the road to the
village until you've take 41 steps. Stop there and put down the canary
seed to stand for the planet Mercury. Then go on 36 steps more and you
will be 77 steps from the model of the Sun. This will be the place to
put the small green pea that stands for the planet Venus; then go on 30
steps more and you will be 107 steps away from the Sun. This will be
the place to put down the green pea that stands for the Earth, and so
on. The last planet--Neptune--will be 3208 steps away from the
house,--about one and a fifth miles away.
pg 21
Agnes. I don't believe we can
count such large numbers, Jack; we shall
be sure to forget them and lose the count.
Jack. True enough, Agnes. Let
me see if I can't make it simpler for
you. I will write down on a card all that you have to remember, and we
can make the numbers that you have to count smaller. We can do it this
way; instead of counting the distances from the Sun to each planet, we
will count the number of steps between each planet and the next one:
this way. Here is the card that Jack wrote:
If one of
Tom's steps is 870,000 miles, then:
The
distance from the model of the Sun to
the canary seed that stands for the planet Mercury
is 41 steps; the distance from Mercury to
Venus is 36 steps farther; the distance from
Venus to the Earth is 30 steps farther; the distance
from the Earth to Mars is 55 steps farther;
the distance from Mars to Jupiter is 393
steps farther; the distance from Jupiter to Saturn
is 464 steps farther; the distance from Saturn to
Uranus is 1029 steps farther; the distance from
Uranus to Neptune is 1160 steps farther.
pg 22
NOTE--The numbers that are needed to make the map are
obtained in
this way: If one step is 870,000 miles, then
the distance from the Sun to Mercury = 36,000,000 miles = 41 steps
the distance from Venus = 67,000,000 miles = 77 steps (36 steps
difference)
the distance from Earth = 92,000,000 miles = 107 steps (30 steps
difference)
the distance from Mars = 141,000,000 miles = 162 steps (55 steps
difference)
the distance from Jupiter = 483,000,000 miles = 555 steps (393 steps
difference)
the distance from Saturn = 886,000,000 miles = 1019 steps (464 steps
difference)
the distance from Uranus = 1,782,000,000 miles = 2048 steps (1029 steps
difference)
the distance from Neptune = 2,791,000,000 miles = 3208 steps (1160
steps difference)
In the last column are the differences between the numbers just
preceding; 77 less 41 is 36, 107 less 77 is 30, 162 less 107 is 55,
and so on. If the model of the planet Mercury must be 41 steps
from the model of the Sun, and if the model of the planet Venus
must be 77 steps from the Sun, then the model of Venus must be 30
steps away from the model of Mercury, and so on for the others.
When the next day came, Tom and Agnes set out to make the map of
the Sun and all the planets. The school globe in the house
stood for the Sun, and they carried the models of the planets
with them, as well as the card that showed how far apart the
planets were to be on the scale of their map. Agnes kept the card
in her hand and told Tom how many steps he was to take. At the
house she said: "Tom, you must take 41 steps, and then
stop." So Tom walked off, counting his steps till he had made 41,
then he put down the little canary seed that stood for the planet
Mercury. The globe in the library stood for the Sun; this tiny
seed stood for the planet Mercury; the distance from the globe to
the seed stood for the real distance of the real planet Mercury
from the real Sun. Thirty-six steps farther they put down the
small green pea that stood for the planet Venus; and 30 steps
farther still they put down the green pea that was to stand
for the Earth.
Here they stopped for a minute to think about it all. This little
bit of a green pea was the huge Earth, very, very much
pg 23
smaller than the globe that stood for the Sun. They could not even
see the small green pea that stood for Venus, nor the little seed
that stood for Mercury, though they knew about where they
were, of course. There were no other planets in the real space
between the real Earth and the real Sun except
Fig. 15 A plan of the orbits of Mercury, Venus, the Earth,
and Mars.
just those two, Mercury and Venus, and space was almost empty,
after all, as Jack had said, except for few, very few, planets
that were exceedingly far apart. "Why, we can't even see the
models of Mercury and Venus from here," said Agnes. "No,"
said Tom, "but if they were shining things, as the
pg 24
planets are, we could see them. They ought to be painted white so
that the sunlight would make them glisten."
So the children went on putting the models down in the road at
the right distances apart. Agnes read the right numbers
from the card, and Tom walked away counting his steps
FIG. 16 A plan of the
orbits of Mars, Jupiter, Saturn,
Uranus,
and Neptune.
(The scale of this
drawing is much smaller than that
of the preceding one.)
up to the thousands. He got rather tired of it, but they kept on
until finally all the models were put down at the right distances
apart, and their map was made. By this time they were nearly a
mile and a quarter away from home, and they had spent the whole
morning in the work. But the work was not
pg 25
wasted. They really understood what they had been doing, and
realized, as very few people--even grown people--do, how
immensely large space is, and how few--very few--planets there
are to fill it. (1)
When the children came home that day there was a great deal of
talk about the map--the model--that they had made. All the older
people and some of the neighbors were interested in it. They
found their work had not been wasted and that they had really
learned something.
The Solar System; the Sun and Planets.--Jack
told them some
interesting things about the sun and the planets. They knew
already, of course, that all the planets moved round the
sun in paths that were called orbits. The earth, for instance,
goes once round the sun every year,--every 365 1/4 days. Every
one of the planets goes round the sun, too, in its own particular
orbit, in its own year. For instance,
Mercury goes round the Sun in 88 days = about 3 months
Venus goes round the Sun in 225 days = about 7 months
Earth goes round the Sun in 365 days = about 12 months
Mars goes round the Sun in 687 days = about 23 months
Jupiter goes round the Sun in 12 years
Saturn goes round the Sun in 29 years
Uranus goes round the Sun in 84 years
Neptune goes round the Sun in 165 years
(1) It is strongly recommended that the teacher should
make such a
model of the solar system as had just been described, with the
aid of his pupils. If actually made, it will lead to a true and
living realization of the dimensions of the solar system. No
amount of mere class-room instruction can do this for young
children.
Tom's father told them about one of the kings of Spain who, long
ago, used to play chess on a huge chessboard with real living
persons for chessmen. These men moved from square
pg 26
to square on the chessboard as the game went on; and Tom's father
said that the map of the solar system with its eight planets
ought to have had eight little boys who would walk in
Fig. 17 In this
picture the large circle stands for the
sun.
Each of
the small dots stands for the earth.
The size of the dots and of
the circle are in the right proportion.
It would take 109
earths in a row stretched across the disk of the sun
to
reach from edge to edge. Count them.
circles round the model of the sun, carrying the models of the
planets in their hands. One boy would carry the canary seed that
stood for Mercury, and he would have to walk once round his
circle in three months; another boy would carry the small
pg 27
green pea that stood for Venus, and he would have to walk around
a larger circle once in seven months; still another would carry
the green pea that stood for the Earth, and he would have to walk
around the circle of the Earth's orbit once in each year; and so
on for all the other planets. The boy that carried the
Fig. 18 Three drawings
of Jupiter as seen in a
telescope.
The lower
drawing shows Jupiter with his four bright satellites.
It is on a
smaller scale than the others.
marble that stood for Neptune would not get all the way around
his circle for 165 years. "He would be quite grown up by the time
he got round, wouldn't he?" said Agnes. "Well," said Jack, "Papa
is right; that is the real way to make the model. The sun is in
the middle. All the planets move round him in circles; each one
of the planets takes a different time
pg 28
to go once around its orbit. All of these planets together make up
the solar system,--the family of the sun."
Tom. Why do they call it the
solar system, Jack?
Jack. Just because it is the
sun's system; sol, in Latin, means "the sun," and solar means
"belonging to the sun." All the
planets go round the sun, and round nothing else. That's why. The
sun is so much larger than any of the planets, or than all of
them put together for that matter, that it is the sun's system.
Relative Sizes of the Planets.--"You
see," said Jack, "that the sun
is very large indeed. He is as much larger than the earth as the
library globe is larger than a green pea. If all the solar system
were to shrink and shrink until the earth--the huge
earth--had shrunk to the size of one green pea, the sun would
still be as big as the globe in the library--it would be two feet
in diameter."
The real diameters of the sun and planets are:
The Sun is 866,400 miles in diameter
The smaller
planets
Mercury is 3,030 miles in diameter
Venus is 7,700 miles in diameter
The Earth is 7,918 miles in diameter
The Moon is 2,162 miles in diameter
Mars is 4,230 miles in diameter
The giant
planets
Jupiter is 86,500 miles in diameter
Saturn is 73,000 miles in diameter
Uranus is 31,900 miles in diameter
Neptune is 34,800 miles in diameter
"Oh!" said Agnes, "we left the Moon out of our model."
"So we did," said Tom; "let us go this afternoon and stick a pin in
the ground to stand for the Moon, alongside of the green pea that
stands for the Earth."
pg 29
Fig. 19 Drawings of
the planet Saturn as seen in a
telescope at
different times.
In the upper figure we are looking at
Saturn's rings edgewise,
and they appear as a thin line.
In the
next drawing we are looking down on the rings.
In the third
drawing we are also looking down on the rings.
pg 30
The Moons of the Planets.--"Well,"
said Jack, "that's all right.
Only you must choose a pin with a very small head. And,
while you are about it, you had better put in some more pins, for
several of the other planets have moons--satellites, they are
called--and they go around their planets just as the Moon goes
around the Earth. Mercury has no satellite that we know of;
Venus has no satellite that we know of; the Earth has the Moon
for satellite; Mars has two very small satellites;
Fig. 20 The Starlit Sky
pg 31
Fig. 21 The Great
Comet of 1858
pg 32
Jupiter has four large satellites about the size of our Moon, and
one extremely small one; Saturn has eight satellites, one larger
than our Moon; Uranus has four satellites; Neptune has one
satellite almost the same size as our Moon."
The Minor Planets; the Asteroids.--"Yes,
and at the same time
you might as well sprinkle about 500 grains of sand in the
space between Mars and Jupiter to stand for the 500 minor planets
that they call asteroids.
There are about 500 of them known now,
and, I've no doubt, hundreds more not yet discovered. When
you read in the newspaper that a new planet was discovered
last night by some astronomer, that means that another one
of these minor planets has been found. They find them by
photography with a large telescope."
Comets.--"And, by the way, put
in two or three thin wisps of cotton
wool somewhere to stand for comets. Comets are mostly made out of
shining gas--they aren't solid. But they look a little like wisps
of cotton wool, anyway."
Tom. Is that all? Shall we put
in anything else?
Jack. That is all for the
solar system, except clouds of very
little stones, almost like dust, that make the shooting stars or
meteors.
The Stars.--"What about the
stars?" said Agnes.
Jack. Oh, the stars are not
part of the solar system, Agnes; they
are millions and millions of miles outside of it; the very
nearest star is thousands and thousands of times farther from us
than even the planet Neptune.
Tom. How far off are they,
Jack, anyway? Could we get the nearest
of the stars on our model? Where would it be? In the next
country?
Distances of the Stars.--"Let
me see," said Jack, "the nearest star
of all is 20,000,000,000,000 miles from the sun--
pg 33
twenty millions of millions of miles! If you were to put it on
your map, it would have to be about 9000 miles from where
we are now--it would have to be somewhere in China."
Agnes. Is that the nearest star, Jack?
Jack. Yes, the very nearest.
If you should put another school globe
in the Chinese emperor's palace at Peking, that would stand for
the nearest star to our sun, which our school globe in the
library stands for. The sun is a star, and stars are about of the
same size. So a school globe may stand for any one of them.
Tom. Well, space is empty if planets and stars
aren't any closer
than that. What is the difference between a planet and a star,
anyway?
What is a Planet?--"The
greatest difference," said Jack, "is this:
the stars shine by their own light, just as an electric street
lamp shines; and the planets shine by light reflected from the
sun, just as a football would shine if you help it up in the
sunlight."
Tom. Do you mean that Venus
and Jupiter do not shine by their own
light?
Jack. I mean just that. Venus
and Jupiter are two great globes
something like the earth, made out of rocks and soil, with clouds
all around them--clouds something like our clouds. The sun shines
on them, and they shine, and we see them. If the sun were to stop
shining on them, they'd go out like a candle.
Agnes. But, Jack, Venus shines
at night, in the dark sky, when the
sun has stopped shining.
Jack. The sun has stopped
shining on you and me at night because
the earth has turned round and we are in the earth's shadow; you
know that. But all the while the sun is shining
pg 34
just the same. It is shining on the other side of the earth, where
it is daytime, and it is sending out sunbeams above the
earth and below it, everywhere all the time. Some of these
sunbeams fall on Jupiter and Venus and make them bright, and we
see them. What we really see is the sun's brightness reflected
back to us, just as you might see an electric light at
night shining on a mirror. You might be in the dark yourself; the
electric light might be round the corner of the street, but the
mirror would be bright.
Tom. So planets are bright
because the sun shines on them. Why are
the stars bright then?
Jack. Stars are bright just as
the sun is bright. The sun makes its
own light as an electric lamp makes its
own light. The stars are
like the sun. They shine by their own light. Planets shine by
borrowed light. They borrow their light from the sun. If you were
to go off and sit on the nearest star and look at the solar
system, you might see the sun in the middle of it shining away
all the time--all day and all night, too. And if you could see
our little group of eight planets wheeling around it, they would
be bright on the side nearest the sun--on the side shined upon;
and it would be dark on the side away from the sun. The sunlight cannot
go through them. The sun can shine only on that part of a
planet that is turned towards it.
Phases of the Moon (New Moon, Full
Moon, etc.).--"Don't you know
the moon is often only half bright, and so on? Venus looks
that way in a telescope sometimes; in a telescope you can see
Venus like a crescent moon--like a sickle. You do not see it like
that with your eye, because Venus is so bright that your eyes are
dazzled. You see the glare, and it looks like any other
dazzling glare; you do not see its true shape."
pg 35
Tom. You can't see the true
shape of a sheet of tin that the sun
shines on; it looks just like a dazzle of light.
Fig. 22 The New Moon
Setting in the West
Fig. 23 The Moon in
the First Quarter
Fig. 24 Fred Watching
the Full Moon Rise in the East
Jack. That is the way with the
planets when you do not use a
telescope. Now the moon looks so large, and the light from any
part of it is so faint, that you can
see its shape. It
pg 36
does not dazzle your eyes. They call those different shapes of
the bright part of the moon its phases.
Venus has phases, too.
The moon is a globe, you know, about 2000 miles in
diameter. One half of it is always turned towards the sun, and
that half of it is always bright, day and night. If we were on
the sun, we should always see the whole circle of the moon
bright, But we are on the earth, and the bright part of the moon
is not
Fig. 25 A schoolroom experiment to show how the sun
lights up
half of every one of the planets, and only half. The room should
be darkened; the lamp should have a ground-glass shade; the
orange that stands for the earth or planet should be fastened by
a knitting needle to a pincushion. The pupils should see that
half, and only half, of a globe (a planet, the earth, the moon)
is illuminated. They should also see that by going to different
parts of the room different portions (phases) of the
illuminated part are visible. The phases of the moon can be
explained by this experiment. Half of the moon is lighted by the
sun; all of the illuminated half that is turned towards the earth
is seen bright; the moon moves round the earth and turns
different parts to it at different times.
always turned towards us. We see only so much of the bright part as
is turned towards us--so much and no more.
Agnes. Sometimes we see the
whole circle of the moon bright--at
full moon.
Jack. Yes, we see it so when
the sun is setting in the west and
the moon rising in the east. The sun is shining full on the
moon, and the bright half of the moon is turned full towards us.
pg 37
Tom. When the moon is a sickle
it is often in the west, not far
from the sun about sunset.
Jack. That is the phase we
call new moon.
Tom. The moon goes round the
earth, doesn't it?
Fig. 26 This picture shows why the moon's disk has
different shapes
at different times. The sun is supposed to be far away in the
direction of the top of the page. It shines on the earth and
lights half of it. It is night on the unlighted half of the
earth. The moon goes around the earth in its orbit in the
direction of the arrow. Wherever the moon is, one half of it is
lighted--the half turned towards him. The little circles outside
the orbit in the picture show the shape that the bright part of
the moon will have at new moon, full moon, etc.
pg 38
Jack. It goes round the earth
once in every month. The moon's
month begins when the moon is a new
moon. Every night the bright
part gets larger, and in about a week, a quarter of a month, we
see a quarter of the moon bright; that is the first quarter. Two
weeks after the new moon the full moon comes; and a week
later comes a moon that is only partly bright again; that is the
third quarter. By and by, in
four weeks, comes another new
moon, and so on forever.
Agnes. One of my storybooks
says the old moons are cut up to make
stars out of. They wouldn't be bright enough, would they?
Jack. Not exactly. Stars are
the brightest things there are except
the sun, which is the very brightest thing we know.
Agnes. There are faint stars,
though--some that you can scarcely see.
Tom. They are faint only
because they are far off. If you were
near them, they would be bright like the sun.
Jack. That's right. The stars
are suns, and our sun is a star. All
of them are really very much alike, though the stars do not look
at all as the sun does. The sun looks large, and it is hot,
because it is close to us. The stars look small because
they are so far off, and we get no heat at all from them,
though we get light. You know you can see the light of a
lamp much farther than you can feel its heat.
Number of the Stars.--Agnes.
There are thousands and thousands of
stars, Jack; do you know how many there are?
Jack. There are about 6000
stars that you can see with the naked
eye, not more; and you cannot see all those at once. Probably you
never see more than a couple of thousands at any one time.
Agnes. Why, there seem to be
many more than 2000.
pg 39
Jack. Well, my dear, the only
way to know is to count them. And
the astronomers have counted
them, and made maps that show every
one of them by a little dot. That is the way they know how
many there are. But if you take an opera glass, you can see very
many more; and if you take a telescope, you can see thousands and
thousands. The largest telescopes that we have will show perhaps
a hundred million stars. The brightest stars are nearest to us,
and the faint ones are very far away indeed--inconceivably far,
in fact.
Tom. You said the nearest star
was as far away from the sun on our
map as New York is from Peking. Are all the stars as far apart as
that? Aren't some of them close together?
Clusters of Stars.--Jack. Well,
there are some groups of stars
fairly close together; but generally one star is about as far
from the star nearest to it as our sun is from the nearest star.
If you were making a map of the whole universe, you would begin
by making a model of the solar system just as you did yesterday.
The library globe would stand for the sun, which is one of the
stars, you know. The nearest star to it
Fig. 27. The Group of Stars Called the Pleiades (The
six brightest
stars can be seen with the naked eye. To see the others a small
telescope must be used. The Pleiades may be seen high up in the
sky and to the south of the point overhead about 10 P.M.
December 21, about 9 P.M. January 5, about 8 P.M. January 20,
every year. Or you may see them rising to the north of the east
point of your horizon about 10 P.M. August 23, about 9 P.M.
September 8, about 8 P.M. September 23.)
pg 40
would be shown on the map by a globe set down at Peking, 8000
miles away from us, and 8000 miles from Peking there would
be another globe, and 8000 miles farther another one, and so on.
Every 8000 miles on your map there would be a globe to stand for
a star, and there would be at least a hundred million globes on
your map of the universe, because, you know, the telescopes
show us at least a hundred million stars. Of course these
stars are scattered all around us; they aren't
Fig. 28 The stars in space are arranged somewhat as in
the picture. On
the whole, no one of them is nearer to any other one than the sun
is to the nearest star,--20,000,000,000,000 miles. The Sun is
just one out of a countless number of stars--one out of millions.
No one of the planets of the solar system can be seen from the
nearest of the stars.
in a straight line one after another, but they are scattered all
over the surface of the night sky.
Agnes. The planets move around
the sun; do the stars move around
the sun, too?
Jack. No, they are so far off
from us that the sun has nothing to
do with them, nor they with the sun. The sun has its own family
of planets, and it is possible that the stars--which are
suns--have their own planets, too; but we do not know
whether they have or not.
pg 41
Agnes. Why don't you know,
Jack?
Jack. Because the stars are so
far away. We can see the stars
like bright shining points in the sky. They shine by their own
light and are bright. Now suppose any one of the stars really had
a family of planets around it. Those planets
Fig. 29 A photograph of a part of the Milky Way. Each
little dot
in the picture is a star, and there are thousands of them even on
one photographic plate. You can see the Milky Way like a bright
belt in the sky--a belt made of stars--overhead early in the
evenings of August and September or of November, December, and
January, or parallel to the northern horizon early in the
evenings of April and May.
would shine by the light from that star, and they would be faint,
much too faint for us to see, even if the planets were really
there; and the only way to know about stars and planets is to see
them; you cannot touch them or hear them. If you cannot see a
planet it does not exist, so far as you know.
pg 42
Tom. Couldn't a man on the
nearest star, looking at our sun, see
the planets of our system,--Venus and Jupiter?
Jack. No, indeed; he would see
our sun, but the light of our
planets would be too faint. He could not possibly see them.
Do the Stars have Planets as the Sun
does?--Tom. You say you
don't
know whether the stars have
planets round them. What do you think
about it? Haven't you any idea?
Jack. There is a great deal of
difference between knowing and
thinking. I certainly do not know that the stars have planets,
for I have never seen them. But I do think that it is very likely
that they have families of planets, just as the sun has. I
think it is likely--very likely; but I don't know.
Fig. 30 The Starlit Sky
pg 43
Tom. And do you think those
planets, if there are any, have people
on them? Are they inhabited as the earth is?
Jack. That is a hard question.
In the first place, it is not
certain that there are any planets around the stars, and then it
is a mere guess whether there could be inhabitants on them.
That is one of the questions we shall have to give up. It
is too difficult.
Agnes. I am going to believe
that every star has planets round it,
just as the sun has.
Jack. Well, that is reasonable
enough. Very likely you are right.
Who knows?
Agnes. And I am going to
believe that some of these planets round
the stars have men on them.
Fig. 31 The Shower of Shooting Stars seen on Nov. 13,
1866 The round
dots stand for stars; the arrows for the tracks of meteors that
were seen. Notice that nearly all the meteors radiated from a
spot near the center of the picture.
pg 44
Jack. I can't say you're
wrong; I can't prove that you are wrong.
Who knows? You can believe what you like about it. Wait till we
know more.
Shooting Stars; Meteors; Fireballs.--On
the night of August 10
the children stayed up late to watch the shooting
Fig. 32. The Great
meteor that fell in California in
1894
stars that are regularly seen every year on that particular night.
On almost any night that is clear any one who will
Fig 35 A meteoric stone that fell in Iowa in 1875
pg 45
watch for an hour will see a dozen or more; and the easiest way
to understand what they are like is to watch for them. In the
country, where the sky is dark and where there are no electric
lights, it is not hard to see them. In the city it is not so
simple; the sky is too bright and the street lamps
interfere too much. Any one can see the stars. If one of the
stars should suddenly get brighter and move quickly away
from its place and then suddenly disappear, as if it had been
blown out like a candle, it would look just as the shooting stars
do. The real stars stay in the same place night after night, year
after year, century after century. They are called fixed
stars because they are fixed in their places. The shooting stars
are small pieces of stone or iron that are moving about in space,
as the planets move. One of these pieces comes near to the earth
and falls to the ground just as a stone falls. It moves rapidly
through the air and gets hot, as your hand will get hot if
you move it very rapidly to and fro on your desk. The shooting
star moves very fast and gets very hot indeed--hot enough to
burn. Usually the meteors (shooting stars) get so
pg 46
hot in their flight through the air that they are quite burned up
before they reach the ground. Sometimes a piece of iron falls and
is picked up. The picture shows a piece of the sort. Fig.
32 shows how such a meteor (a very large one--much large than a
shooting star) looks as it is falling.
Fig. 34 The Zodiacal
Light--The best time to see it in
the United States
is in February, March, and April in the early evening, above the
western horizon.
The Zodiacal (1) Light.--Space
is full of such meteors, most of
them small, like dust. The sun shines on them, and you can often
see a triangle of faint light or glow, which is called
(1) Pronounced zo-DYE-a-kal.
pg 47
the zodiacal light. If you
live in the country, where the sky is
dark, be on the lookout for it. The street lamps of the city make
the sky entirely too bright for you to see it in towns.
Nebulae.--Nebula, in Latin,
means cloud; and nebulae is the
plural. There are several spots in the sky that, even with
the
Fig. 35: the Great
Nebula in Andromeda, from a
Photograph made with
a Telescope (see Fig. 53)
naked eye, on a clear night look as if the stars in those spots
were covered with a thin veil of cloud. When these spots are
looked at with a telescope you see bright forms like those
in the pictures Figures 35 and 53, and they are, in fact,
bright clouds of gas and small particles of dust. They shine by
their own light.
pg 48
Rising and Setting of the Sun.--Tom.
We know that the sun rises in
the east every day--
Agnes. And goes across the sky
and sets in the west.
Jack. Why does it? Does the
sun really move?
Agnes. No; the earth turns
round and the sun stands still; but the
sun seems to move.
Jack. The sun seems to move
across the sky from rising to setting
every day; the moon does the same thing; each one the thousands
of stars rises and then sets every night. There are just
two ways to explain these things. Either the earth stands still
and all these different heavenly bodies really move around
it--every one of them--in twenty-four hours, or the heavenly
bodies stand still and the earth turns round on its axis every
day. The last explanation is the true one, as you know very well,
and so we have to say the sun appears
to move from
Fig. 36. The Setting
Sun
pg 49
rising to setting (for the sun really does not move at all); and
we have to say the stars appear
to move from rising to
setting (for the stars do not really move at all). It is
the earth that
Fig. 37. The Way the Sun seems to move from Rising to
Setting--The man
in the picture is looking towards the south, and his arms are
stretched out to the east and to the west. If he stood there all
day, he would see the sun rise above the horizon in the
east, gradually rise higher and higher and be highest at noon,
just to the south, and then decline towards the west and set in
the west at the end of the day. The dotted line shows the
apparent motion of the sun. The picture was drawn at about three
o'’clock in the afternoon. Why? Because the sun in the picture is
where the real sun will be every day about three o'clock.
turns, and as it turns everything in the sky appears to move from
east to west.
The Celestial Sphere.--"Think
of it this way. You are on a
globe--the earth--that turns around every twenty-four hours.
Above you is the sky. It looks exactly as if it were a hollow
globe, and as if you were inside of it. In the night-time
the stars look like little shining marks fastened to the hollow
globe all around you. In the daytime the sun (and sometimes
the moon) seems to be fastened to the inside of the hollow globe
of the sky. We call the hollow globe of the sky
pg 50
the celestial sphere. You are
in the middle of it, and you see all
the stars at night slowly moving from rising towards setting. The
celestial sphere is the surface of the sky to which the sun,
moon, and stars appear to be fastened. They look as if they were
fastened there, anyway. They all see to be at the same
distance."
Tom. They can't all be
fastened to any one sphere, because
they are at very different distances from us. The sun is
very much further away from us than the moon, and the stars
are much further off than the sun.
Fig. 38 The Celestial Sphere (The Hollow Globe on Whose
Inner
Surface all Stars seem to Lie) The earth is supposed to be at O,
and some stars at p, q, r, s, t, t, t, u, v. You see the
stars as if they were all projected on the celestial sphere at P,
Q, R, S, T, U, V. You think of them as if they were all at the
same distance from you.
pg 51
Jack. True enough. If you will
look at this picture I am drawing,
you will see how it is. You are supposed to be in the middle of
the celestial sphere at O.
The earth is at O (Fig. 38),
Fig. 39 This picture shows the northern sky as it
appears in the
early hours of the evening every August to people who live in the
United States. If you face north, you see the horizon (1):the
surface of the ground. Above that comes the sky with many stars
in it. Towards the west and pretty high up is the dipper:
The Great Bear (Ursa Major). Two of its stars: the
pointers: point at the north star: Polaris, (2) it is
called. High in the east is Cassiopeia, (3) a group that is
sometimes called The Lady in the Chair. Every child that
owns this book should try to find these stars. They are always
there, in the north. If he looks in August they will be just as
in the picture. If he looks in other months the books must be
turned a little. By taking a little pains the book can be held so
that the picture will look as the stars do.
(1) Pronounced ho-RYE-zon. (2) Pronounced po-LAY-ris. (3)
Pronounced kas-ee-oh-PEE-ya.
pg 52
and you are on it. All around you are stars, p, q, r, s, etc. You
see the star q along the line Oq--along
the line that joins your
eye and the star. The line seems to pierce the celestial sphere
at Q, and you think the star
q is really at Q. In the same
way
you think the star r is at R,
the star s at S, and so
forth. If there were really three stars, t, t, t, all in one
line, Ot, you would see only
one star at T. All the stars
seem to
be lying on the surface of some sphere, and all of them seem
equally far away.
Tom. That is true, I know.
When I look at the stars at night they
certainly do seem to be all at one distance--just like shining
tacks driven into a darkish globe above my head and all around me.
Agnes. And in the daytime the
sun and, sometimes, the moon seem to
be the same way--shining circles fastened on to a shining globe.
Jack. Of course there isn't
any real globe there. It is only an
appearance. But it looks real, and we have a name for the
appearance because it is convenient to have names for things we
always see, or even for things that we always think that we
see.
Tom. You would have a model of
the celestial sphere by making a
huge hollow globe as big as a barn and getting inside of it.
Agnes. Yes, and by lining it
with black velvet and driving
bright-headed tacks into the lining for stars; only you would
have to drive them in the right places.
Jack. A model like that would
be worth making, but it would be
expensive. We shall have to do with pictures and flat maps. They
will explain what we really see in the sky.
pg 53
The next night Jack took the children out of doors. He made them
face towards the north; the east was on their right hand, the
west on their left. First of all he showed them the Dipper--the
Great Bear (Ursa Major in
Latin)--and the pointers.
The Dipper is made up of seven bright stars and is always easy to
find. Three of its stars make the handle, four make the bowl, and
two stars of the bowl are the pointers. After you have
found the pointers it is easy to find the polestar. Now if you
imagine a line drawn from the polestar to the center of the earth
(under your feet), that line will be the axis of the earth.
The earth turns round that line every day. Every part of the axis
itself stands still, and every point not in the axis moves.
The center of the earth stands still while the earth turns; and
Polaris stands still. All the parts of the earth not on the axis
appear to move, and all the stars except Polaris appear to
move--they move from rising to setting and back to rising
again. The stars in the east move upwards, then over the pole
towards the west, and then downwards (in the direction of the
little arrows in Figs. 39 and 40).
Fig. 40. The Dipper--the Great Bear--as it appears at
Different Times--Sometimes it is above the pole, sometimes below it;
but if you
lay ruler on the picture, you will see that the pointers
always point to the north star--Polaris.
pg 54
Jack kept the children out of doors till long after their bedtime
to let them see the stars rise higher and higher, but
finally they had to go to bed. They could not watch any
longer.
On the next night Jack showed the children how the southern stars
appeared to move from rising to setting. He took them
Fig. 41 A photograph of a part of the northern sky near
the pole.
A camera was pointed at the pole early in the evening and the
plate was exposed all night and only shut off at daybreak. Each
star moved about half of its course round the pole, and as it
moved it left a trail on the plate. All the trails in the picture
are half circles. The star Polaris is not exactly at the north
pole of the heavens (though it happens to be pretty near it). Its
trail is the brightest one on the plate. The other stars left
their trails, too.
pg 55
out into a large open field and made them face towards the south.
The east was on their left hand, the west on their right hand,
and the stars appeared to move from east to west,--from
rising towards setting--just as the sun does. The apparent
motion of all the stars--of the south stars as well as of
the north stars--is caused by one thing and one thing only. The
earth turns round on its axis underneath the starry sky.
Fig. 42 A photograph of a part of the southern sky,
showing the
trails of southern stars as they moved across the plate
from rising towards setting. This photograph, and the one like it
for the northern stars, prove that the stars really move with
respect to the photographic plate. But it is not the stars
that move. The plate moves with the earth as the earth
turns round its axis. The stars stand still.
pg 56
Time and Timekeeping.--We use
the apparent motion of the sun from
rising to setting to give us the time. Watches and clocks all
over the world are now regulated by the sun. Long ago the
ancients used to tell their time by the stars. They would say: "You
must begin your journey when Pleiades are rising"; just as
we might say: "I must take the train at 9 P.M." Groups of
stars, like the Pleiades, were the moving clock hands; the dial
was the celestial sphere. The stars moved steadily across the
dial, and their motion told the hour. The sun moves regularly and
steadily from rising to setting. When it is highest up in the
heavens and exactly south of any place (a city, a town, any
place), then it is noon at
that particular place. Twelve hours
later it is midnight; and
twelve hours later than midnight it is
noon again--noon of the next day of the week. A watch is a little
machine arranged to drive a steel hand round a dial in twelve
hours. The hand is set so as to mark XII o'clock at noon, and the
machine is regulated so that when the next noon comes the hand
shall be at XII again. To set our watches exactly, we must
have a north and south line. Astronomers have a particular kind
of telescope set exactly in the north and south line (the
meridian), so that they can
observe the exact instant of
noon. Their watches are corrected so as to mark XII o'clock just
at that moment; and made to run so that when the next noon comes
they will mark XII o'clock again. They have other kinds of
telescopes also, especially made to examine distant planets and
to discover what is to be seen on their surfaces.
Telescopes.--The children were
playing with a reading glass that
belonged to their father. Tom used it to light a match with, and
then to look at the wings of a fly, and noticed how it magnified
everything--how it made it look much larger.
pg 57
Then he said: "Jack, what is the difference between this
magnifying glass and a telescope? Both of them magnify."
Jack. Well, the telescope
magnifies very much more, for one
Fig. 43. A Meridian Circle--The eye end of the
telescope
is at M. The
telescope is fastened to a horizontal axis which lies in an east
and west line, and the telescope always remains, therefore, in
the meridian. LL is a level by which the axis is made horizontal.
The axis has two circles (H and K) fastened to it. These circles
are divided into 360 degrees, and by them we can measure the
altitude (height), of any star.
pg 58
thing; and a telescope is made up of more than one lens. The burning
glass has only one.
Jack took the burning glass and showed the children how to use it to
make an image (a picture) of the window on the wall, as in Fig. 45.
Jack. You see that this glass
makes an image of the window on the wall.
Suppose that we should cut a hole in the wall just where the image is
now. The image would be there just the same, for if you put a piece of
white paper over the hole the image would show on the paper as it now
does on the wall. Now suppose that you were in the other room beyond
the wall and held another burning glass in just the right place to
magnify the image in the hole. The second burning glass magnifies
everything it looks at; well, you could use it to magnify the image
formed by the first burning glass. If you did this, you would have a
telescope. Two lenses comined so as to form a magnified image of any
object make a telescope. One lens alone is not a telescope; it is a
magnifying glass.
Agnes. Then a telescope must
have two glasses?
Jack. Yes, two at least; the
first glass forms an image of the thing
you are looking at--a picture of the window, for instance. The second
glass magnifies the image so that you can see it better and see it
larger. All opera glasses and spyglasses have at least two lenses,
usually more than two.
Fig. 44 A Reading
Glass, A Magnifying Lens, or a
Burning Glass
pg 59
Tom. Here is a drawing of the
great telescope of the Lick Observatory (Fig. 46). Where are the two
glasses there?
Jack. One of them is the upper
end of the long steel tube; they call it
the object glass, because it
is nearest the object you are lookng at.
The other glass is at the other end of the tube; they call it the
eyepiece, because it is next
your eye. In the drawing you see a man
looking through the eyepiece.
Agnes. But the telescope is
inside a house, Jack. How can the
astronomer see anything?
Fig. 45 If you hold a burning glass in a room, you can
make it form an
image (a picture) of the window on the opposite wall. The image will be
clear and distinct, but it will be upside down, as you can prove by
trying. Most lenses will need to be held nearer the wall than that in
the figure.
pg 60
Tom. Why, you know, Agnes,
that there is a long window in the dome
that is opened when they want to look out to see anything. The
telescope looks out through the open window.
Agnes. What is the long tube
for?
Jack. It is principally to
keep the object glass and the eyepiece
at exactly the right distance apart and to hold them steadily
where you want them.
Tom. The tube is on an iron
stand, and you can go to the top of
the stand by a winding stairway. What are those big circles at
the top, Jack?
Jack. The circles are fastened
to the telescope, Tom, not to the
iron stand, you see; and they are arranged to show the latitude
and the longitude of the particular star that the telescope is
pointed at.
Agnes. Do they know the
latitudes and the longitudes of stars?
Jack.
Yes, that is the way they point at them. If I tell you to find on
the map a town that has a latitude of 41 degrees and a longitude of 80
degrees you can find it, can't you?
Agnes. Here is the map, and
the town is Pittsburg.
Jack. Well, the astronomers
have maps of the stars, and they find
the star they want by knowing its latitude and longitude, and by
pointing the telescope there.
Tom. But the star would be
moving from rising to setting. How do
they manage to follow it?
Jack. If you will look at the
drawing of the telescope (Fig. 46),
you'll see a piece of machinery in the top part of the stand. It
is really a powerful clock. That clock is arranged so as to move
the telescope towards the west exactly as fast as the star moves
toward the west. When you once have the star in the
telescope the clock keeps it there.
Agnes. How large is the object
glass of the Lick telescope?
pg 61
Fig. 46. The Great
Telescope of the Lick Observatory
Its object glass
is three feet in diameter, and it is nearly sixty feet long.
pg 62
Jack. The object glass is
three feet in diameter, and the tube is
nearly sixty feet long, and the eyepiece is quite small--just the size
to be convenient for your eye to see through, Agnes.
Tom. How much can
you magnify with a telescope like that?
The Moon.--Jack. Well, you can
arrange so as to magnify more or less as
you please. For instance, you can magnify the moon about a thousand
times--you can see the moon as if it were a thousand times nearer than
it really is. How far off did I tell you the moon is?
Tom. Two hundred and forty
thousand miles.
Jack. Then if the telescope
will make it seem a thousand times nearer,
how far off will it seem to be?
Agnes. Two hundred and forty
miles.
Jack. That's right, my dear.
The Lick telescope will show you the moon
just as you would see it if you got within two hundred and forty miles
of it--just as if the moon were at Pittsburg and you at Philadelphia.
Agnes. That does not seem very
near.
Jack. Well, it isn't near; but
it is wonderful to do even so well as
that.
Fig 47 Mountains on
the moon as seen in a telescope
pg 63
Tom. Then the planets, that
are so much farther away than the moon,
cannot be seen anything like so well?
Jack. No; Mars, for instance,
is 50,000,000 miles away from us when we
see it best, and so we never can make it seem nearer to us than 50,000
miles. That is better than nothing; but it isn't very close, after all.
It is really wonderful that men have found out so much as they have
about the planets when you consider what the difficulties are. The
smallest spot that can be seen with distinctness on the moon would
contain several acres; and when you come to looking at a distant planet
like Mars a spot would have to be fifty or sixty miles square to be
visible at all.
Tom. Then you see a city on
the moon? A city covers many acres.
Jack. You could see a city on
the moon if it were there; or even a very
large building like the Capitol at Washington; but there are no such
cities or buildings on the moon. Astronomers have looked for them
thousands of times without ever finding the slightest sign of any
living thing.
Life on the Planets.--Agnes. Is
there any sign of life on the planets?
Fig 48 Mountains on
the moon as seen in a telescope
pg 64
Jack. Not one; life of some
sort may be there--plants, trees, animals,
or possibly men--but the telescope shows no sign of life at all.
Tom. Not even on Mars?
Jack. Not even on
Mars--nowhere. Some people have talked about land and
water on Mars, calling parts of Mars that are reddish, land, and parts
that are bluish, water; but no one has any proof at all that the red
parts are really land, or the blue parts water.
Tom. I have read about canals
in Mars.
Jack. Well, whatever they are,
they are not canals. The telescope shows
narrow, straight, dark lines on the planet's surface (see Fig. 49), and
they were called canals
because they crossed the red parts of Mars that
were called continents. But
the Lick telescope shows that the canals go
across the oceans, just as they go across the continents; so that it is
pretty clear that the canals are not canals at all, and that we do not
know whether Mars has any water on its surface at all.
Tom. How is it about Jupiter?
Jack. Jupiter looks as if it
were a very hot planet; like a huge
red-hot ball covered with clouds of steam. All of Saturn that we can
see
seems to be clouds; and the same is true for Uranus and Neptune, and
for
Venus, too, for that matter. Mercury and Mars have no clouds and
probably little or no atmosphere at all. All the others have
atmospheres, but no one knows whether their air is the right kind of
air to breathe. It is very doubtful whether any planet beside the earth
is fit for men to live on.
Tom. Is there air on the moon?
Jack. There is no air on the
moon at all, nor any water either; and it
is so cold on the moon, and on Mars too, that no man could possibly
live there for an instant.
pg 65
Tom. Then there isn't any
place in the whole universe where we are
really sure that men can live except just the earth?
Jack. No. Men cannot live in
the sun; the sun is too hot. Jupiter is
too hot, also. Mercury and Mars have little or no air. Venus, Saturn,
and Uranus, and Neptune are covered with clouds, and we do not know
what is underneath the clouds. Men couldn't live in the stars; they are
like the sun: too hot. And we do not know whether the stars have
planets
round them or not; very likely they have. If they have, some of their
planets may be fit for men to live on. Agnes says she is going to
believe it.
Agnes. Yes, I am. It makes the
universe more interesting to believe
that there are people like ourselves everywhere, or at least in many
places.
Jack. Well, believe it, my
dear. I half believe it myself; but there is
no way to prove it, or to disprove it, for that matter.
Fig. 49 Drawings showing two hemispheres of the planet
Mars. The narrow
lines are what have been called canals. The dark parts of the drawing
should be colored blue and most of the white parts reddish in order to
make it look as Mars does.
pg 66
APPENDIX
THE EARTH
The earth is a globe flattened at the poles. Its shortest diameter
(from pole to pole) is 7900 miles. Its longest diameter is 7927 miles.
It turns on its axis once daily. It moves in its orbit round the sun
once in a year of 365 days 5 hours 48 minutes 45 1/2 seconds. Its month
(from new moon to new moon) is 29 1/2 days. The earth is 5 1/2 times a
heavy as a globe of water of the same size. The sun weighs 333,000
times more than the earth. The distance from the earth to the sun is
93,000,000 miles.
THE MOON
The moon is 2163 miles in diameter. The moon weighs about 3 1/2 times
as much as a globe of water of the same size. The earth weighs 81 times
as much as the moon. The distance from the moon to the earth is 240,000
miles.
ECLIPSES OF THE SUN AND MOON
They are explained in Book II (Physics).
THE PLANET MERCURY
Mercury is 3030 miles in diameter. It weighs about 3 1/8 times as much
as a globe of water of the same size. It goes once round the sun every
88 days. It is 36,000,000 miles distant from the sun--less than 1/12 of
the earth's distance, therefore.
THE PLANET VENUS
Venus is 7700 miles in diameter (about the size of the earth,
therefore). It weighs 4 8/10 times as much as a globe of water of the
same size. It goes
pg 67
round the sun once every 225 days. It is 67,200,000 miles distant from
the sun:about 7/10 of the earth's distance, therefore.
THE PLANET MARS
Mars is 4230 miles in diameter. It weighs 4 times as much as a globe of
water of the same size. It turns once on its axis in 24 hours 37
minutes 22 67/100 seconds. It goes round the sun once every 687 days.
It is 141,500,000 miles from the sun--about 1 1/2 times the earth's
distance, therefore. It has two very small moons.
Fig 50 A rough drawing
of the full moon.
JUPITER
Jupiter is 86,500 miles in diameter. It weighs only 1 3/10 times as
much as a globe of water of the same size. It turns once on its axis in
9 hours
pg 68
55 minutes. It goes round the sun once every 11 9/10 years. It is
483,300,000 miles from the sun--about 5 times the earth's
distance, therefore. It has five moons. One of them is very
small; the others much larger--about the size of our own
moon, or of the planet Mars.
THE PLANET SATURN
Saturn is made up of a globe with rings around it. The diameter of
its globe is 73,000 miles. It weighs only 7/10 as much as a globe
of water of the same size. The globe turns on its axis every 10
hours 14 minutes 24 seconds. It goes round the sun once every 29 1/2
years. It is 886,000,000 miles distant from the
sun--about 9 1/2 times the earth's distance, therefore. The
rings of Saturn are made up of a swarm of countless little moons.
The rings are about 28,000 miles wide and 168,000 miles in
diameter, and only about 100 miles thick. Saturn has eight
moons--one as large as Mars, one about the size of our
moon, and the rest smaller.
THE PLANET URANUS
Uranus is 31,900 miles in diameter. It weighs only 1 2/10 as much as
a globe of water of the same size. It goes round the sun once in
84 years. It is 1,781,900,000 miles distant from the
sun--about 19 times as far as the earth, therefore. It has four
rather small moons.
THE PLANET NEPTUNE
Neptune is 34,800 miles in diameter. It weighs only 1 1/10 times
as much as a globe of water of the same size. It goes round
the sun once in 165 years. It is 2,791,600,000 miles distant form
the sun--about 30 times the earth's distance, therefore. It
has one moon about the size of our own moon.
COMETS
A few comets belong to the family of the sun and move around him as
do the planets.
pg 69
THE FIXED STARS
Stars are suns, immensely distant from our sun and from each other
except when they are grouped in clusters.
Light, which travels
nearly 200,000 miles in a second,
takes 4 years to come to us
from the nearest star. The light from Polaris
(the polestar)
takes 47 years to reach the earth.
Fig. 51. The Total Solar Eclipse of 1871 in India--The
black circle is
the disk of the moon; behind it the sun's disk is hidden. The
pale white streamers are the sun's corona, or crown. The corona
always surrounds the sun, but is not visible every day because
the streamers are so faint. Notice close to the edge of the moon's
disk a few brighter spots. These are flames of hydrogen--a gas
that is glowing as if it were white hot--in the sun's
atmosphere.
pg 70
NEBULAE
Nebulae are masses of gas at about the same distance form the sun
as the stars are. They are of all shapes and sizes. Many of
them are spiral in shape--corkscrew shaped. If, as sometimes
happens, a star burns up it may turn into a nebula; or, as
sometimes happens, a nebula may solidify and become a star.
Perhaps our sun and all the planets were once a huge nebula that
cooled and solidified into separate globes.
pg 71
Fig. 52.A Cluster of
Stars in the constellation of the Centaur--Each
white dot represents a star.
Fig. 53. Drawing of a
large nebula (in Andromeda) as seen in a
telescope.
The white dots are stars; the shining white cloud is
the nebula. (See also Fig. 35.)
