"How could that happen?" asked Harry.
"Because Uranus behaved so strangely," replied his sister. "The planets
attract each other; for instance, the earth is swayed to and fro by
Jupiter and Venus, and a great struggle is always going on among the
planets in the family of Giant Sun. It could be plainly seen that Saturn
was taking part in the struggle and dragging Uranus toward it, but
something beyond the newly discovered planet was pulling it the other
way. 'There must be another planet,' said the astronomers, and they were
right. After puzzling over the problem two astronomers found the truant,
and announced exactly when and where it was to be seen. And there it was,
nearly exactly where these learned men said it would be. The new planet
was christened Neptune, and it takes about one hundred and sixty-four
years to go around the sun. It is so far away from the sun that it only
receives one nine-hundredth of the amount of light and heat we receive
on planet earth."
"Then it must be very cold on planet Neptune?" said Harry.
"And very dark also," said Mary, "since from this planet the sun only
looks as large as an electric light seen at a distance of a few feet."
[Illustration: SIZE OF PLANETS, COMPARED WITH THE SUN.]
"IS IT TRUE?"