"As we have already," began the Professor, "had a talk about the stars
in general, let us this morning give a little attention to our own
particular star."
"Is there a star that we can call our own?" asked May, with unusual
animation. "How nice! I wonder if it can be the one I saw from our front
window last evening, that looked so bright and beautiful?"
"I am sure it was not," said the Professor, "if you saw it in the
evening."
"Is it hard to see our star, then?" she said.
"By no means," replied the Professor; "rather it is hard not to see it.
But you must be careful about looking directly at it, or your eyes will
be badly dazzled, it is so very bright. Our star is no other than the
sun. And we are right in calling it a star, because all the stars are
suns, and very likely give light and heat to worlds as large as our
earth, though they are all so far off that we can not see them. Our star
seems so much brighter and hotter than the others, only because it is so
much nearer to us than they are, though still it is some ninety-two
millions of miles away."
"How big is the sun?" asked Joe.
"You can get the clearest idea of its size by a comparison. The earth is
7920 miles in diameter, that is, as measured right through the centre.
Now suppose it to be only one inch, or about as large as a plum or a
half-grown peach; then we would have to regard the sun as three yards in
diameter, so that if it were in this room it would reach from the floor
to the ceiling."
"How do they find out the distance of the sun?" asked Joe.
"Until lately," replied the Professor, "the same method was pursued as
in surveying, that is, by measuring lines and angles. An angle, you
know, is the corner made by two lines coming together, as in the letter
V. But that method did not answer very well, as it did not make the
distance certain within several millions of miles. Quite recently
Professor Newcomb has found out a way of measuring the sun's distance by
the velocity of its light. He has invented a means of learning exactly
how fast light moves; and then, by comparing this with the time light
takes to come from the sun to us, he is able to tell how far off the sun
is. Thus, if a man knows how many miles he walks in an hour, and how
many hours it takes him to walk to a certain place, he can very easily
figure up the number of miles it is away."
"Why," said Gus, "that sounds just like what Bob Stebbins said the other
day in school. He has a big silver watch that he is mighty fond of
hauling out of his pocket before everybody. A caterpillar came crawling
through the door, and went right toward the teacher's desk at the other
end of the room. 'Now,' said Bob, 'if that fellow will only keep
straight ahead, I can tell how long the room is.' So out came the watch,
and Bob wrote down the time and how many inches the caterpillar
travelled in a minute. But just then Sally Smith came across his track
with her long dress, and swept him to Jericho. We boys all laughed out;
Sally blushed and got angry; and the teacher kept us in after school."
"Astronomers have the same kind of troubles," said the Professor. "They
incur great labor and expense to take some particular observation that
is possible only once in a number of years, and then for only a few
minutes. And after their instruments are all carefully set up, and their
calculations made, the clouds spread over the sky, and hide everything
they wish to see. People, too, are very apt to laugh at their
disappointment.
"There would, however, be no science of astronomy if those who pursued
it were discouraged by common difficulties. To explain the heavenly
bodies they sometimes try to make little systems or images of the sun
and the planets; but they are never able to show the sizes and distances
correctly. If they were to begin by making the sun one inch in diameter,
then the earth would have to be three yards off, and as small as a grain
of dust; some of the planets would have to be across the street, and
others away beyond the opposite houses. So when you look at these little
solar systems, as they are called, you must remember that the sizes and
distances are all wrong.
"Still, you can get from them some idea how the sun stands in the
middle, and the earth and other planets go round, and how the earth,
while going round the sun, keeps also turning itself around. You have
seen how a top, while spinning, sometimes runs round in a circle. That
is just the way our earth does. And if you imagine a candle in the
centre of the circle that the top makes, you will see why it is
sometimes day and sometimes night. When the side of the earth we are on
is turned toward the sun, we have day; and when we have spun past the
sun, night comes.
"The sun seems to go past us, and people used to think it really did.
But we know now that it is as if we were in a rail-car, and the trees
and houses seemed to be rushing along, when we ourselves are the ones
that are moving. The sun and all the stars seem to move through the sky
from east to west; but it is only our earth that is turning itself the
other way, and carrying us with it."
"What makes summer and winter?" asked Joe.
"I think that the top will help you to understand that too. You have
noticed that when it spins it does not always stand straight up, but
often leans over to one side. So sometimes the upper part of it would be
over toward the candle, and sometimes over away from it. The earth leans
over too in this same manner; and that is the reason why we have summer
and winter. When by this leaning our part of the earth is toward the
sun, we get more heat, and have a warm season; when we are leaning away
from the sun, and are more in the shadow, the cold weather comes, and
continues until we get into a good position to be warmed up again.
"A kind Providence brings this all around very regularly, and there is
no danger of our being kept so long in the cold that we would freeze to
death. Everything works like a clock that is never allowed to run down
or get out of order. In spinning, the earth carries us round twelve or
fifteen times as fast as the fastest railway train has ever yet been
made to run; and in making its circle round the sun, it moves as fast as
a shot from a gun."
"Oh! oh!" exclaimed the children; and Joe asked, "Why are we not all
dashed to pieces?"
"Because," said the Professor, "we do not run against anything large
enough to do any harm; and we do not realize how fast we are moving, or
that we are moving at all, because we do not pass near anything that is
standing still. You know that in riding we look at the trees and fences
by the road-side to see how rapidly we are going. The hills in the
distance do not show our speed, but seem to be following us. Unless we
look outside we can not know anything about it, excepting, perhaps, we
may guess from the noise and jostling of the vehicle. But as the earth
moves smoothly and without the least noise, we would think it stood
entirely still did not astronomers assure us of its wonderfully rapid
motion. It took them a great while to find it out. When they began to
suspect it there was a great dispute over it. Some said it moved; others
said it did not. The two parties were for a time very bitter against
each other; but now all agree in the belief of its rapid motion."
"A queer thing to quarrel about, I must say," remarked Gus. "I wouldn't
have cared a straw whether it moved or not, if I could only have been
allowed to move about on it as I pleased."
"I hope you are not getting uneasy, Gus," said Joe.
"There is evident reason," observed Jack, "to suspect that his
appreciation of the marvels of science is insufficient to preserve--"
"Oh, bother! Jack, don't give us your college stuff now, after the
Professor has told us so much. We like to hear him, of course. I do, for
one, a great deal better than I thought I should. But then a fellow
can't help getting tired."