Table 6.1 lists some basic orbital and physical properties of the nine planets, with a few other solar-system objects (the Sun, the Moon, an asteroid, and a comet) included for comparison. Most of these quantities can be determined using methods described in Chapters 1 and 2.

The distance of each planet from the Sun is known from Kepler's laws once the scale of the solar system is set by radar-ranging on Venus. (Sec. 2.6) A planet's (sidereal) orbital period is easily measurable from repeated observations of its location on the sky, so long as Earth's own motion around the Sun is properly taken into account. The masses of planets with moons may be calculated by application of Newton's laws of motion and gravity, just by observing the moons' orbits around the planets. (More Precisely 2-3) The sizes of those orbits, like the sizes of the planets themselves, are found by measuring their angular sizes and then applying elementary geometry. (Sec. 1.5)

The masses of Mercury and Venus (as well as those of our own Moon and the asteroid Ceres) are a little harder to determine accurately because these bodies have no natural satellites of their own. Nevertheless, it is possible to measure their masses by careful observations of their influence on other planets or nearby bodies. Mercury and Venus produce small but measurable effects on each other's orbits, as well as that of Earth. The Moon causes small "wobbles" in Earth's motion as the two bodies orbit their common center of mass. Only in the case of Ceres is the mass still poorly known, mainly because that asteroid's gravity is so weak. All the techniques necessary for determining mass were available to astronomers well over a century ago. Today, the masses of most of the objects in Table 6.1 have been very accurately measured through their gravitational interaction with artificial satellites and space probes launched from Earth.

As indicated in Table 6.1, the Sun, having more than 1000 times the mass of the next most massive object (the planet Jupiter), is clearly the dominant member of the solar system. In fact, the Sun contains about 99.9 percent of all solar system material. The planets—including our own—are insignificant in comparison.

A planet's rotation period is determined simply by watching surface features appear and disappear again as the planet rotates. For some planets this is difficult to do, as their surfaces are hard to see or may even be nonexistent! The surface of Venus is completely obscured by clouds, whereas Jupiter, Saturn, Uranus, and Neptune have no solid surfaces at all—their atmospheres simply thicken, and eventually become liquid, as we descend deeper and deeper below the visible clouds. We will describe the methods used to measure the rotation periods of these planets in later chapters.

The final columns in Table 6.1 list a property called density. This is a measure of the "compactness" of the matter within an object. It is computed by dividing the object's mass (in kilograms, say) by its volume (in cubic meters, for example). Dividing Earth's mass (determined from observations of the Moon's orbit) by its volume (which we know because we know Earth's radius—see More Precisely 1-3), we obtain an average density of approximately 5500 kg/m³. On average, then, there are about 5500 kilograms of Earth matter in every cubic meter of Earth volume. For comparison, the density of ordinary water is 1000 kg/m³, rocks on Earth's surface have densities in the range 2000—3000 kg/m³, and iron has a density of some 8000 kg/m³. Earth's atmosphere has a density of only a few kilograms per cubic meter. Because most working astronomers are more familiar with the cgs (centimeter, gram, second) units of density (grams per cubic centimeter—g/cm³—where 1 kg/m³ = 1000 g/cm³), Table 6.1 lists density in both SI and cgs units.