“The next circle we have to draw is a very interesting one to us.  The compasses must be opened two and one-half inches.  The path made represents the journey we take in three hundred sixty-five days.

“One more circle must be drawn to complete our little plan.  This circle must be eight inches in diameter.  You see Mars is much farther from the sun than our earth is.  It takes him six hundred eighty-seven days to make the trip around the sun.  The other planets are too far away to be put in this plan.”

“O, Frank, you have missed the biggest of all—­the moon!” said Philip.

“O, no, no!” exclaimed Frank.  “The moon is quite a little ball.  It is less than seven thousand miles around her, while our earth is twenty-five thousand miles around.”

“Is that a little ball, Frank?”

“Yes, compared with the sun and the planets.  The moon is what is called a satellite—­that is, a servant or an attendant.  She is a satellite of our earth.  She keeps circling round and round our earth, while we go 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.