Page:Astronomy for Everybody.djvu/274

ball would never reach the earth, but would revolve round it like a little satellite in an orbit of its own. Could this be done the astronomer would be able, knowing the velocity of the ball, to calculate the attraction of the earth. The moon is a satellite, moving like such a ball, and an observer on Mars would be able, by measuring the orbit of the moon, to determine the attraction of the earth as well as we determine it by actually observing the motion of falling bodies around us.

Thus it is that when a planet like Mars or Jupiter has satellites revolving around it, astronomers on the earth can observe the attraction of the planet on its satellites and thus determine its mass. The rule for doing this is very simple. The cube of the distance between the planet and satellite is divided by the square of the time of revolution. The quotient is a number which is proportional to the mass of the planet. The rule applies to the motion of the moon round the earth and of the planets round the sun. If we divide the cube of the earth's distance from the sun, say ninety-three millions of miles, by the square of three hundred and sixty-five and a quarter, the days in a year, we shall get a certain quotient. Let us call this number the sun-quotient. Then, if we divide the cube of the moon's distance from the earth by the square of its time of revolution, we shall get another quotient, which we may call the earth-quotient. The sun-quotient will come out about three hundred and thirty thousand times as large as the earth-quotient. Hence it is concluded that the mass of the sun is three hundred and thirty thousand