|
|
|
The Moon's the North Wind's cookie He bites it, day by day Until there's but a rim of scraps That crumble all away. | The South Wind is the baker He kneads clouds in his den, And bakes a crisp new moon that ... greedy.... North.... Wind ....eats....again! |
| "What the Little Girl Said" Vachel (Nicholas) Lindsay, 1879-1931. |
The Month
The monthly cycle of the moon (we won't capitalize the word here) must have mystified early humans--"waxing" from thin crescent ("new moon") to half-moon, then to a "gibbous" moon and a full one, and afterwards "waning" to a crescent again. That cycle, lasting about 29.5 days, gave us the word "month"--related to "moon," as is "Monday."The civil year, January to December, no longer ties its months to the moon, but some traditions still do and their terms for "month" reflect the connection--in Arabic, "shahr", in biblical Hebrew "yerach" and also "chodesh" from "new," since it was reckoned from one new moon to the next. Jericho (pronounced Yericho), one of the oldest cities on Earth, took its name from "yerach," and of course, legends tell of many moon-gods and goddesses, e.g. Artemis and Diana.
|
|
Early astronomers understood the different shapes of the moon, noting that each was linked to a certain relative position between moon and Sun: for instance, full moon always occured when moon and Sun were at opposite ends of the sky. All this suggested that the moon was a sphere, illuminated by the Sun. The moon's path across the sky was found to be close to the ecliptic, inclined to it by about 5 degrees. Eclipses of the Sun always occured when moon and Sun were due to occupy the same spot in the sky, suggesting that the moon was nearer to us and obscured the Sun. Eclipses of the moon, similarly, always occured at full moon, with the two on opposite sides of the Earth, and could be explained by the shadow of the Earth falling on the moon. Lunar eclipses allowed the Greek astronomer Aristarchus, around 220 BC, to estimate the distance to the moon (see section #8c). If the moon and the Sun followed exactly the same path across the sky, eclipses of both kinds would happen each month. Actually they are relatively rare, because the 5-degree angle between the paths only allows eclipses when Sun and moon are near one of the points where the paths intersect. . The cycle from each new moon to next one takes 29.5 days, but the actual orbital period of the moon is only 27.3217 days. That is the time it takes the moon to return to (approximately) the same position among the stars.
|
Why the difference? Suppose we start counting from the moment when the moon in its motion across the sky is just overtaking the Sun; we will call this the "new moon," even though the thin crescent of the moon will only be visible some time later, and only shortly after sunset. Wait 27.3217 days: the moon has returned to approximately the same place in the sky, but the Sun has meanwhile moved away, on its annual journey around the heavens. It takes the moon about 2 more days to catch up with the Sun, to the position of the next "new moon," which is why times of the new moon are separated by 29.5 days.
The Face of the MoonThe visible face of the moon has light and dark patches, which people interpreted in different ways, depending on their culture. Europeans see a face and talk of "the man in the moon" while children in China and Thailand recognize "the rabbit in the moon." All agree, however, that the moon does not change, that it always presents the same face to Earth.Does that mean the moon doesn't rotate? No, it does rotate--one rotation for each revolution around Earth! The drawings on the left, covering half an orbit, should make this clear. In them we look at the moon's orbit from high above the north pole, and imagine a clock dial around the moon, and a feature on it, marked by an arrow, which initially (bottom position in each drawing) points at 12 oclock. |
|
In the top drawing the marked feature continues to point at Earth, and as the moon goes around the Earth, it points to the hours 10, 8 and 6 on the clock dial. As the moon goes through half a revolution, it also undergoes half a rotation
If the moon did not rotate, the situation would be as in the bottom drawing. The arrow would continue to point in the 12-oclock direction, and after half an orbit, people on Earth would be able to see the other side of the moon. This does not happen. We need to go aboard a spaceship and fly halfway around the Moon before we get a view of its other side--as did the Apollo astronauts who took the picture below.
The Gravity GradientThis strange rotation of the moon is maintained because the moon is slightly elongated along the axis which points towards earth. To understand the effect we look at the motion of a body with a much more pronounced elongation--an artificial satellite with the shape of a symmetric dumbbell (drawing).It can be shown that if the forces on the dumbbell (or indeed on a satellite of any shape) are unbalanced, it rotates around its center of gravity. That point will be defined later, but in a symmetric dumbbell with two equal masses marked A and B, the center of gravity is right in the middle between them. |
Of course, if it started from an off-equilibrium position, it can overshoot the mark and then swing back and forth around equilibrium, like a pendulum. The elongated moon acts like a dumbbell too, and does in fact swing a little from side to side, like a pendulum of like balanced scales; this has allowed astronomers even before the space age to see a little more than half the moon. Such motion is known as "libration," from " libra", balance-beam scales--also the name of a constellation of the zodiac and of an ancient unit of weight, from which our abbreviation lb is derived. The rotating force which lines up the moon or an orbiting dumbbell therefore depends not on how strongly gravity pulls it, but on how rapidly the pull of gravity changes with distance--on the "gravity gradient." Near Earth that is a gentle force, though it is strong enough to line up elongated satellites. Among those was Triad, shaped like a long dumbbell with an additional payload in the middle, the first satellite to map the electrical currents associated with the polar aurora. It did swing back and forth with a 6-minute period, complicating the analysis of its data. Near a black hole or pulsar, on the other hand, the gravity-gradient force is fierce enough to rip a spacecraft apart.
|
Next Stop: #4b The Moon: A Closer Look
|Astro Net ® Home Page | Kanishkas Home Page|
Copyright © 01/2000 Kanishka H. Lankatillake
