Mars is the fourth planet from the Sun and the outermost of the four terrestrial worlds in the solar system. It lies outside Earth's orbit, as illustrated in Figure 10.1(a), which shows the orbits of both planets drawn to scale. Because of its exterior orbit, Mars ranges in our sky from a position close to the Sun (for example, when Earth and Mars are at the points marked A in the figure) to one far from the Sun (with the planets at points B). Contrast this orbit with the nighttime appearances of Mercury and Venus, whose interior orbits ensure that we never see them far from the Sun. From our earthly viewpoint, Mars appears to traverse a great circle in the sky, keeping close to the ecliptic and occasionally executing retrograde loops. (Sec. 2.2) The Mars Data box lists some detailed orbital and physical data on the planet.

Figure 10.1 (a) The orbit of Mars compared with that of Earth. Notice that Mars's orbit is noticeably elliptical, unlike Earth's, whose eccentricity is barely perceptible. When the planets are on opposite sides of the Sun, as at the points marked A, Mars is said to be at conjunction. The planets are at their closest at opposition, when Earth and Mars are aligned and on the same side of the Sun, as at the points marked B. (Note that to get from point A to point B, Earth must travel for nearly 13 months—all the way around its orbit and then some.) (b) Several oppositions of Mars, including the particularly favorable configuration of September 1988 and the unfavorable opposition of March 1997.

Mars's orbital eccentricity is 0.093, much larger than that of most other planets—only the innermost and the outermost planets, Mercury and Pluto, have more elongated orbits. Because of this, Mars's perihelion distance from the Sun—1.38 A.U. (207 million km)—is substantially smaller than its aphelion distance—1.67 A.U. (249 million km), resulting in a large variation in the amount of sunlight striking the planet over the course of its year. In fact, the intensity of sunlight on the Martian surface is almost 45 percent greater when the planet is at perihelion than when it is at aphelion. As we will see, this has a substantial effect on the Martian climate.

Mars is at its largest and brightest when it is at opposition—that is, when Earth lies between Mars and the Sun (location B on Figure 10.1). If this happens to occur near a Martian perihelion, the two planets can come within 0.38 A.U. (56 million km) of each other. The angular size of Mars under those circumstances is about 25". Ground-based observations of the planet at those times can distinguish surface features as small as 100 km across—about the same resolution as the unaided human eye can achieve when viewing the Moon. Such a coincidence of opposition with Martian perihelion is relatively rare. It last happened in September 1988 and will next occur in August 2003, although Mars will not be quite so close to perihelion then as it was in 1988. The dates and configurations of some oppositions of Mars are illustrated in Figure 10.1(b), which shows the locations of Earth and Mars at six oppositions (at 780-day intervals—Mars's synodic period) between September 1988 and April 1999.

Although Mars is quite bright and easily seen at opposition, the planet is still considerably fainter than Venus. This faintness results from a combination of three factors. First, Mars is more than twice as far from the Sun as is Venus, and each square meter on the Martian surface receives less than one-quarter the amount of sunlight that strikes each square meter on Venus. Second, the surface area of Mars is only about 30 percent that of Venus, so there are fewer square meters to intercept the sunlight. Finally, Mars is much less reflective than Venus—only about 15 percent of the sunlight striking the planet is reflected back into space, compared with nearly 70 percent in the case of Venus. Still, at its brightest, Mars is brighter than any star. Its characteristic red color, visible even to the naked eye, makes it easily identifiable in the night sky.