The Moon Help
Introduction to the Moon
Earth has countless natural satellites—meteors captured by gravity and orbiting in all manner of elliptical paths. The only natural satellite of significance and the only one that can be detected without powerful observing aids, however, is the Moon. It’s interesting that we have never come up with a better name for Earth’s Moon; we speak about the moons of Jupiter and the moons of Saturn, and then we call our own Queen of the Night “the Moon.” It is as if someone had a daughter and named her “Daughter.” Sometimes the Moon is called “Luna,” but that name conjures up visions of madness and worship and is not used by astronomers.
The Moon orbits Earth at a distance of about 381,000 kilometers (237,000 miles). Sometimes it’s a little closer, and sometimes it’s a little farther away. The Moon’s diameter is 27.2 percent that of Earth, roughly 3480 kilometers (2160 miles). That’s large for a moon relative to its parent planet. The Earth-Moon system is sometimes considered a double planet, and some astronomers think the pair formed that way. But Earth is 81 times more massive than the Moon, and the Moon has essentially no atmosphere. Thus, in planetary terms, the Moon is a dull place.
Perhaps you have seen drawings of the Earth-Moon system and have come to envision the Moon as much closer to Earth than is actually the case. (The drawings in this chapter, except for Fig. 4-1, are examples of such misleading data.) There is a reason for this distortion. If the Earth-Moon system were always drawn true to scale, the illustration would be of little use for most instructive purposes. Earth is a bit less than 12,800 kilometers (7,930 miles) in diameter, and the Moon is about 381,000 kilometers (237,000 miles) away on average. That’s 30 Earth diameters. If drawn to scale, the Earth-Moon system would look like Fig. 4-1. Think of the Earth and the Moon as pieces of fruit. Suppose that Earth is a 10-centimeter-diameter grapefruit and the Moon is a 27-millimeter-diameter plum (4 inches and 1 inch across, respectively). To make a scale model, you must set the two fruits 3 meters (10 feet) apart.
Period, Perigee, And Apogee
With respect to the Sun, the Moon takes 29½ days to make one orbit around Earth. The exact synodic (sun-based) lunar orbital period varies slightly from one orbit to the next because the orbit of the Moon around Earth is not a perfect circle and the orbit of Earth around the Sun is not a perfect circle either. However, for most amateur astronomers (including us), 29½ days is a good enough figure. Relative to the stars, the Moon’s orbit is faster; the sidereal lunar orbital period is about 27 days and 7 hours.
The synodic and sidereal lunar orbital periods differ for the same reason the synodic day is longer than the sidereal day. Every time the Moon makes one trip around Earth, our planet has moved approximately one-twelfth of the way around the Sun. The Moon has to travel further to come into line with the Sun from one orbit to the next than it must travel to come into line again with some distant star (Fig. 4-2).
Have you ever looked at the Moon, especially the full Moon, and imagined it to be closer than you remember previous full Moons to have been? Maybe it’s not your imagination. The Moon orbits Earth in an elliptical path, with Earth at one focus. The Moon can get as close as 356,000 kilometers (221,000 miles) and as distant as 407,000 kilometers (253,000 miles) from Earth. This is a difference of 13.5 percent of the Moon’s mean distance. Sometimes the Moon’s disk appears 13.5 percent larger than at other times. This is enough to make a difference, especially when the Moon passes precisely between an observer and the Sun. The Moon’s closest approach is the lunar perigee ; this term also applies to the minimum-distance figure. The Moon’s furthest retreat is the lunar apogee , a term that also is used in reference to the maximum-distance figure.
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