We begin our study of the Moon and Mercury by examining their orbits. Knowledge of these will, in turn, aid us in determining and explaining many of the other properties of these worlds. Detailed orbital and physical data are presented in the Moon Data box and the Mercury Data box.
Parallax methods, described in Chapter 1, can provide us with quite accurate measurements of the distance to the Moon, using Earth's diameter as a baseline. 
(Sec. 1.5) Radar yields a more accurate distance. The Moon is much closer than any of the planets and the radar echo bounced off the Moon's surface is strong. A radio telescope receives the echo after about a 2.56-second wait. Dividing this time by 2 (to account for the round trip taken by the signal) and multiplying it by the speed of light (300,000 km/s) gives us a mean distance of 384,000 km. (The actual distance at any specific time depends on the Moon's location in its slightly elliptical orbit around Earth.)
Current laser-ranging technology, using reflectors placed on the lunar surface by Apollo astronauts to reflect laser beams fired from Earth, allows astronomers to measure the round-trip time to submicrosecond accuracy. Repeated measurements have allowed astronomers to determine the Moon's orbit to within a few centimeters. This precision is needed for programming unmanned spacecraft to land successfully on the lunar surface.
Viewed from Earth, Mercury never strays far from the Sun: the angular distance between the Sun and Mercury is never greater than 28°. Consequently, the planet is visible to the naked eye only when the Sun's light is blotted out—just before dawn or just after sunset (or, much less frequently, during a total solar eclipse), and it is not possible to follow Mercury through a full cycle of phases. In fact, although Mercury was well known to ancient astronomers, they originally believed that this companion to the Sun was two different objects, and the connection between the planet's morning and evening appearances took some time to establish. However, later Greek astronomers were certainly aware that the "two planets" were really different alignments of a single body. Figure 8.1, a photograph taken just after sunset, shows Mercury above the western horizon, along with three other planets and the Moon.
Figure 8.1 Four planets, together with the Moon, are visible in this photograph taken shortly after sunset. To the right of the Moon (top left) is the brightest planet, Venus. A little farther to the right, at top center, is Mars, with the star Regulus just below and to its left. At the lower right, at the edge of the Sun's glare, is Jupiter, with Mercury just below it. (The moon appears round rather than crescent-shaped because the "dark" portion of its disk is indirectly illuminated by sunlight reflected from Earth. This "earthshine," relatively faint to the naked eye, is exaggerated in the overexposed photographic image.)
Because Earth rotates at a rate of 15° per hour, Mercury is visible for at most 2 hours on any given night, even under the most favorable circumstances. For most observers at most times of the year, Mercury is considerably less than 28° above the horizon, so it is generally visible for a much shorter period (see Figure 8.2). Nowadays, large telescopes can filter out the Sun's glare and observe Mercury even during the daytime, when the planet is higher in the sky and the atmosphere's effects are reduced. (The amount of air that the light from the planet has to traverse before reaching our telescope decreases as the height above the horizon increases.) In fact, some of the best views of Mercury have been obtained in this way. The naked-eye or amateur astronomer is generally limited to nighttime observations, however.
Figure 8.2 Favorable and unfavorable orientations of Mercury's orbit result from different Earth orientations and observer locations. At the most unfavorable orientations, Mercury is close to both the Sun and the horizon.
In all cases, it becomes progressively more difficult to view Mercury the closer (in the sky) its orbit takes it to the Sun. The best images of the planet therefore show a "half Mercury," close to its maximum angular separation from the Sun, or maximum elongation, as illustrated in Figure 8.3.
Figure 8.3 Some views of Mercury at different points along its orbit. The best images of the planet are taken when it is at its maximum elongation (greatest apparent distance from the Sun) and show a "half Mercury." (Compare with Figure 2.14a.)
