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The Moon Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 28, 2014

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).

The Moon and the Sun The Moon Period, Perigee, And Apogee

Figure 4-2. The synodic lunar period is longer than the sidereal lunar period. (This drawing is not to scale.)

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.

Libration

As the Moon makes its way around Earth, it keeps the same face toward us, more or less. This is so because the Moon’s mass is not uniformly distributed within the globe, and Earth’s gravity has managed, over millions of centuries, to tug the Moon’s rotation rate into near-perfect lockstep with its revolution. But the Moon still wobbles back and forth a little; it has not completely “settled down.” We can see 59 percent of the Moon’s surface from Earth if we make enough observations. The wobbling of the Moon’s face relative to Earth is called libration (not to be confused with libation).

Libration can give rise to interesting phenomena. For example, when amateur radio operators bounce their signals off the Moon to communicate with their fellows on the opposite side of the world, libration produces multiple signal paths whose lengths vary constantly, making the radio waves add and cancel in a manner so complicated that precise analysis would challenge any computer. The resulting received signals sound like someone babbling or hooting underwater. The wavelengths of light are too short for this effect to be observed visually. If we could see at radio wavelengths, the Moon would seem to sparkle and scintillate as if fireworks were constantly being set off all over its surface.

Phase

The appearance of the Moon is drastically affected by its orientation relative to the Sun. When the Earth, the Moon, and the Sun are in line or nearly in line, the Moon is said to be new , and its existence is not visually apparent unless there happens to be a solar eclipse. As the Moon orbits Earth, a journey that takes place in a counterclockwise direction as viewed from high above Earth’s north pole (Fig. 4-3), it presents more and more and then less and less of its lit-up face to us. Three or four days after the new Moon, it is a waxing crescent . About a week after the new Moon, we see half its globe illuminated by the Sun; this is first quarter . Three or four days after that, most of the Moon is illuminated as we see it; this is waxing gibbous . Two weeks and 18 hours after the new Moon, it is entirely illuminated for us unless a lunar eclipse happens to be taking place. This is the full Moon . Phases proceed in timely fashion after the full Moon through waning gibbous, last quarter, waning crescent , and finally, back to new again.

The Moon and the Sun The Moon Phase

Figure 4-3. The Moon’s phases result from the relative orientation of the Moon and the Sun, as seen from Earth.

Almost nobody lives in this world without getting to know the lunar phases before they get into kindergarten. The waxing crescent is visible just after sunset; the first-quarter Moon can be seen until midnight. The waxing gibbous Moon stays in the sky into the wee hours of the morning, and the full Moon is above the horizon all night, setting as the Sun rises. After the full phase, the waning gibbous Moon rises a couple of hours after sunset; the last-quarter Moon rises around midnight; the waning crescent Moon waits until the predawn hours to rise. Moonset in the waning phases takes place in the daytime, and some people say that there is “no moonset” during this half of the lunar cycle.

Of North And South

Most people envision the Moon’s phase-to-phase progress and appearance as seen from the northern hemisphere. This is natural because there are more people living north of the equator than south of it. As far as Earth itself is concerned, however, this is only half the story.

Figure 4-4 shows the way the Moon looks at various stages in its orbit around Earth as seen from a midlatitude northern location such as Kansas City, Missouri, or Rome, Italy. (There is some variance in the tilt, depending on the season of the year; moonrise and moonset occur somewhat north or south of due east or west.) The waxing crescent appears in the southwestern or western sky just after sunset and sets 2 to 4 hours after the Sun. The Moon at first quarter is in the southern sky at sunset, moves generally westward, and sets around midnight. The waxing gibbous Moon is in the southeast at sunset, moves generally westward, and sets in the predawn hours. The full Moon is opposite the Sun, rising at sunset and setting at or near sunrise. The waning gibbous Moon rises some time after sunset and sets after sunrise the next day. The last-quarter Moon rises around midnight and sets around noon. The waning crescent waits until the predawn hours to rise and sets in the afternoon.

The Moon and the Sun The Moon Of North And South

Figure 4-4. Lunar phases as seen from middle latitudes in the northern hemisphere. The Moon’s tilt varies somewhat, depending on the season and the time of night.

Figure 4-5 illustrates the appearance of lunar phases as seen from a midlatitude southern location such as Perth, Australia, or Napier, New Zealand. (As with the northern-hemispheric situation, there is some variance in the Moon’s tilt. Depending on the season of the year, moonrise and moonset occur somewhat north or south of due east or west.) The waxing crescent is in the northwestern or western sky just after sunset and sets 2 to 4 hours after the Sun. The Moon at first quarter is in the northern sky at sunset, moves generally westward, and sets around midnight. The waxing gibbous Moon is in the northeast at sunset, moves to the west, and sets a couple of hours before dawn. The full Moon rises around sunset and sets around sunrise, tracking across the northern half of the sky during the night. The waning gibbous Moon rises shortly after sunset and sets after sunrise the next day. The last-quarter Moon rises at about midnight and sets around noon. The waning crescent rises a couple of hours before dawn and sets in the afternoon.

The Moon and the Sun The Moon Of North And South

Figure 4-5. Lunar phases as seen from middle latitudes in the southern hemisphere. The Moon’s tilt varies somewhat, depending on the season and the time of night.

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