circling round and round the sun.

"How fast the moon must travel! If I were to go rushing round a field,
and a bird should keep flying around my head, you see that the movements
of the bird would be much quicker than mine."

"I can't understand it, Frank," said Philip. "The moon always looks so
quiet in the sky. If she is darting about like lightning, why is it that
she scarcely seems to move more than an inch in ten minutes?"

"I suppose," said Frank, after a thoughtful silence, "that what to us
seems an inch in the sky is really many miles. You know how very fast
the steam cars seem to go when one is quite near them, yet I have seen a
train of cars far off which seemed to go so slowly that I could fancy it
was painted on the sky."

"Yes, that must be the reason; but how do people find out these curious
things about the sun and the stars--to know how large they are and how
fast they go?" asked Philip.

"That is something we shall understand when we are older," said Frank.
"We must gain a little knowledge every day."

"Is the earth the only planet that has a moon?" asked Philip.

"Mercury and Venus have no moons. Mars has two, and Jupiter has four,
but we can see them only when we look through a telescope." replied
Frank.

"Are all the twinkling stars which one sees on a fine clear night,
planets?" inquired Philip.

"Those that twinkle are not planets; they are fixed stars," said Frank.
"A planet does not twinkle. It has no light of its own. It shines just
as the moon shines, because the sun gives it light."

"But our earth does not shine!" said Philip.

"Indeed it does," explained Frank. "Our earth appears to Venus and Mars
as a shining planet."

"There must be many more fixed stars than planets, then, for almost
every star that I can see twinkles and sparkles like a diamond. Do these
fixed stars all go around the sun?" asked Philip.

"O, Philip! haven't you noticed that they are called fixed stars to show
that they do not move like planets? The word _planet_ means to _wander._
These fixed stars are suns themselves, which may have planets of their
own. They are so very far away that we cannot know much about them,
except that they shine of themselves just as our sun does.

"We know that our sun gives light and heat to the planets and satellites
with which he is surrounded. We know that without his warm rays there
would not be any flowers or birds or any living thing on the earth. So
we can easily im