Planetary Report Card

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

• 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.
• Sidereal period. The time it takes a planet to complete one orbit around the sun, usually expressed in Earth years.
• Mass. The quantity of matter a planet contains. The mass of the earth is 5.977 1024 kg. We will assign the earth’s mass the value of 1.0 and compare the masses of the other planets to it.
• Radius. At the equator, the radius of the earth is slightly less than 6,400 km (3,963 miles). We will assign the radius of the earth a value of 1.0 and compare the radii of other planets to it.
• Number of known moons. Self-explanatory—an ever-changing number for the outer planets.
• Average density. This value is expressed in kilograms of mass per cubic meter. The substance of the inner planets is dense and tightly packed; in the outer planets, the densities are typically lower.