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The Moon Help (page 3)

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

The Face Of The Moon

When you look at the Moon without the binoculars or a telescope, it’s impossible to know much about the true nature of the surface. Before Galileo Galilei and other astronomers began looking at the heavens through “spy glasses” a few hundred years ago, no one could be certain that the terrain was dry, scarred, and lifeless. In fact, the true austerity of the Moon would surprise even the most pessimistic dreamers of old.

The naked-eye Moon, especially the full Moon, has light and dark features. In absolute terms, the whole Moon is a rather dark object; it reflects only a few percent of the solar light that strikes it. If the Moon were as white as snow or powdered sugar, it would shine several times more brightly. Even without the help of telescopes, people long ago surmised that the Moon’s light areas represent irregular terrain and the dark regions are flat by comparison. Some people thought the light regions were clouds and the dark zones were areas of clear weather, but after observing the Moon for many nights and seeing that the “clouds” never moved, most people rejected that theory. However, these general ideas were as far as pretelescopic people got. Many people considered the dark areas to be liquid oceans made up of water. They were called maria (pronounced “MAH-ree-uh”), a word that means “seas.” To this day, flat plains or plateaus on the Moon have names such as the Sea of Tranquillity and the Sea of Crises . The light areas were assumed to be land masses, but few people supposed they were strewn with mountain ranges and crater fields.

When Galileo and others began looking at the sky with telescopes in the seventeenth century, humanity’s ideas did not change overnight, as they might have if our race had been driven more by hunger for knowledge and less by ego, fear, and superstition. People’s imaginations were more active in Galileo’s day than they had been a few centuries earlier, but they were not quite as daring as we are now. When Galileo announced that the Moon had craters and mountains, his fellow scientists became interested right away, but those who held power over people’s lives had other notions. To them, Galileo was a troublemaker, and he was treated as one. He ended up spending his last years under house arrest. It was not a tyrannical government dictator that subjected him to this, but the Pope. Imagine the reaction the Pope would get today if he demanded that some scientist spend the rest of his life under confinement!

The Tides

The Earth-Moon system stays together because of gravitation. Earth pulls on the Moon, keeping it from flying off into interplanetary space. The Moon also pulls on Earth, although we are not aware of it unless we make certain observations. The Moon’s gravitational effects vary depending on where in the sky (or beneath the horizon) it happens to be at any given time.

The lunar day is about an hour longer than the synodic day—roughly 25 hours—but the Moon, like the Sun, appears to revolve around any stationary earthbound observer. The effects of gravity propagate through space at the speed of light, some 299,792 kilometers (186,282 miles) per second. This covers the Earth-Moon distance in only a little more than 1 second, so there is essentially no lag between the Moon’s position and the direction in which its “tugging” takes place. The Moon’s pull is extremely weak compared with the gravitation of Earth at the surface; it is nowhere near enough to affect the reading you get when you stand on a scale and weigh yourself, for example. But this does not mean that the gravitational effect of the Moon can be disregarded, as any ocean beach dweller will attest.

The Moon’s gravitation, and to a lesser extent the Sun’s gravitation too, cause Earth’s oceans to be slightly distorted relative to the solid globe of the planet. When considered to scale, the oceans form a thin, viscous coating on Earth. Even the deepest undersea trenches reach less than 0.2 percent from Earth’s surface down to the center, and most of the oceans are far shallower than this. Even so, the depth of the oceans is affected by the combined external gravitational effects of the Sun and the Moon. The effect is greatest when the Sun, the Moon, and Earth are all in line, that is, at the times of new and full Moon. When the Moon is at first or last quarter, its gravitational field acts at right angles with respect to that of the Sun, and the two almost cancel each other out, although the Moon’s effect is a little greater than the Sun’s.

As Earth rotates under its slightly distorted “coating” of ocean, the level of the sea rises and falls at specific geographic locations. In certain places, the rise and fall is dramatic, and people who live on the shore must take its effects seriously. In other places, these tides are much less extreme. There are two high tides and two low tides during the course of every lunar day; the reason for this can be envisioned by looking at Fig. 4-6. However, these drawings represent an oversimplification. The actual tides are delayed by the fact that on a planetary scale water behaves more like molasses than the freely running liquid with which we are familiar. Also, the contours of the ocean floor and the continental shelves have an effect. This is not all: Land masses break the planetary ocean up, so wave effects cannot propagate unimpeded around the world. The tides are waves, although they are very long, having two crests and two troughs with the passage of every lunar day. Actually, the tides consist of two waves of different frequencies. Superimposed on the lunar tidal waves , which have a period of about 25 hours, are solar tidal waves (truly tidal in nature, unlike tsunamis , which are caused by undersea earthquakes, not by tides) with a period of 24 hours, but whose crests and troughs have smaller magnitude.

The Moon and the Sun The Moon The Tides

Figure 4-6. Simplified diagrams showing why lunar tides occur. These views are from high above Earth’s north geographic pole.

The Moon and Sun are not the only natural entities that affect sea level. Weather systems, especially ocean-going storms, have an effect, and in some places a storm surge can cause the sea to rise 10 times as much as the astronomical tide. A tsunami comes in and pounds away at the shoreline in a manner similar to that of a storm surge, except that the tsunami is caused by a jarring of the sea floor (or, occasionally, by a volcanic eruption) rather than by high winds piling the water up onto shore. Neither of these phenomena are true tides.

Tides don’t occur to a significant extent in land-locked seas and lakes. This is so because in order for water to rise in one location, it must fall somewhere else where the lunar-solar gravitational composite is different. This can’t happen in a small body of water, on which, relative to every point, the Moon and Sun are in the same positions at any given time.

There has been some debate about the effects of the solar and lunar gravitational fields on the behavior of living cells. No one has yet come out with a respected scientific study that quantifies and defines exactly how such effects, if any, are manifest. For example, so-called Moon madness (lunacy) has not been explained on the basis of increased intracellular tidal effects during the full Moon. Because a similar loss of reasoning power does not seem to grip its perennial victims during the new Moon, it is almost certain that Moon madness (if it really exists) is not caused by gravitation.

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