"Until quite recent times there were supposed to be only the five we have mentioned. Since the beginning of the present century about two hundred little planets, called asteroids, have been discovered between the orbits, or paths, of Mars and Jupiter. Then there are Uranus and Neptune, very far off from the sun and from us, so much so that the latter was mistaken for a fixed star."
"Professor," said May, "you mentioned the moon as being near to us. Can you explain to us how its distance is measured, so that we can understand it?"
"And then, Professor," said Jack, "I would like to know what parallax means."
"There," said Gus, "is another big word of Jack's—pallylacks, knickknacks, gimcracks, slapjacks!"
"Hush, you goose."
"I think," said the Professor, "I can answer May's and Jack's questions both at once, as they are very closely connected. Suppose that at night, when you look down the street, you see two gas lamps, one much farther off than the other. Then if you go across the street, the nearer lamp will seem to move in the opposite way from what you did. Thus, in the diagram, when you are at A, the nearer lamp is on the right of the other, and when you go over to B and look at it, it is on the left. This change in direction is called parallax. Now we can imagine the nearer one of the lights to be the moon, and that an observatory, or tower with a telescope in it, is located at A, from which the direction of the moon is carefully noted at six o'clock in the morning. Then by six in the evening the earth, spinning round on its axis, will have carried the observatory about 8000 miles away from A, and placed it at, say, B. If the moon's direction be again noted, it is very easy to calculate her distance by a branch of mathematics called trigonometry, which Jack, I have no doubt, has already studied."
