More or Less at the Center of It

Near the center of the solar system—more accurately, at one focus of the elliptical orbits of the planets (see Chapter 4, "Astronomy Reborn: 1543-1687")—is the sun. The sun is most of the solar system, containing more than 99.9 percent of the matter that makes up the solar system. All of Chapter 16, "Our Star," is devoted to the sun, but be aware now just how massive an object it is. Jupiter, the largest planet in the solar system, is over 300 times the mass of the earth. And the sun, in turn, is more than a thousand times more massive than Jupiter (and about 300,000 times more massive than the earth).

Let's make a survey of the planets. Here's what we'll be measuring and comparing in the table that follows:

V Semi-major axis of orbit. You'll recall from Chapter 4 that the planets orbit the sun not in perfectly circular paths, but elliptical ones. The semi-major axis of an ellipse is the distance from the center of the ellipse to its farthest point. This distance does not exactly correspond to the distance from the sun to the farthest point of a planet's orbit, since the sun is not at the center of the ellipse, but at one of the ellipse's two foci. We will express this number in A.U.

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