"Do you know, the appearance and disappearance of the little moons of
Jupiter once gave a great deal of trouble to astronomers. They had a
way of appearing a little too soon or a little too late. They were very
seldom on time. This was very provoking, as astronomers were rather
proud of being able to tell exactly when these little moons could be
seen. At last they found out what was the matter, and that they were to
blame and not the moons. We see the little moons on account of their
light, and light takes time to travel. Don't you remember, I told you
sound travels a mile in five seconds. Light travels even more quickly,
for it only takes a little over a second in coming to us from the moon.
It takes about eight minutes in coming to us from the sun; but Jupiter
is about five times as far away from us as the sun, so that light takes
about half an hour in coming to us from Jupiter. We do not see it as
it is, but as it was more than half an hour ago, when its rays of light
started out to Planet Earth.
"Now, Jupiter, in going around the sun, is sometimes on the same side
of the sun as we are. Then the light from the moons reaches us in about
thirty-two minutes. But when Jupiter is on the opposite side of the
sun, and as far away from us as it can be, then light takes as much as
forty-eight minutes in coming here--over a quarter of an hour longer. So
a clever astronomer decided that when Jupiter and his moons are nearest
to us, it does not take as long for their light to reach us as when they
are farther away, and this is because light, like sound, must have time
to travel.
"Even though light can go round the earth seven times in a second,
traveling at the rate of about 186,000 miles a second, yet, as Jupiter
is millions of miles away, it takes light about half an hour, and some
times forty-eight minutes, for it to cross that great distance. It is
just the same as if Uncle Robert were in India. It would take him a much
longer time to come and see you than if he were at his home just a few
hundred yards away. It takes time for him to travel here, just as it
takes time for light to travel from the little moons of Jupiter."
"I wish we had five moons shining on our earth," said Harry; "how pretty
it would be! Does it take the moons as long as our moon to get around
Jupiter?"
"They are much livelier than our moon," replied Mary; "and the second
moon flies right around Jupiter in a little more than a day and a half,
and even the outside moon only takes about two weeks; so there must
always be a moon shining in the sky for Jupiter. These moons, except
the moon discovered by Professor Barnard, are all larger than our moon,
and the fourth one is nearly as large as Mars. But I hear the bell for
lunch, Harry, and I must run away now. I will tell you about the other
planets later."
"How many are there?" said Harry, as his sister kissed him good-by.
"Only three," replied Mary; "and I shall tell you about them to-morrow,
if you are not too tired."
"Too tired!" said Harry. "I am never too tired to listen to you."