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The Moon Help (page 3)

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By — McGraw-Hill Professional
Updated on Sep 16, 2011

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.

How The Moon Was Formed

Earth-Moon pair is unique in the Solar System; no other major planet has a satellite so large in proportion to itself. (Pluto and its moon Charon are sometimes called a double-planet system, but both of them are hardly larger than asteroids and might better be classified as such.) If the Moon were only a few hundred miles across, say the size of the asteroid Ceres, there would be little doubt that it was captured from solar orbit by Earth’s gravitation. However, because the Moon practically qualifies as a planet, being almost as large as Mercury, there are competing theories about its origin.

One theory holds that the Moon was once a planet in its own right, orbiting the Sun rather than Earth, but something, such as a collision with a large asteroid, deflected it from its solar orbit and caused it to pass so close to Earth that Earth captured it permanently. Few astronomers believe this. If it were true, then the Moon’s orbit likely would be an elongated ellipse sharply inclined to the plane of the ecliptic.

Another theory suggests that Earth originally had no Moon but that a Mars-sized object struck Earth a glancing blow and sent vast quantities of material into orbit. This would have produced rings around Earth, something like those around Saturn. These rings eventually would have congealed into the Moon. One problem with this theory is that an impact severe enough to blast that much matter off Earth might have shattered our planet. However, an increasing number of astronomers accept this theory, and it is lent support by the fact that the Moon lacks a metallic core; most or all of the blown-off material would have come from Earth’s mantle and crust. It also might explain why Earth’s equator is tilted significantly with respect to the plane of the ecliptic. A major impact could have jarred Earth and caused its axis to shift by 23.5 angular degrees.

Still another theory contends that Earth and the Moon formed as a double planetary system over the same period of time as the other planets, condensed from the disk of gas and dust orbiting the newborn Sun. This is a popular theory and is fairly consistent with what we observe about the Moon. It explains why the Moon orbits Earth in a nearly circular path and why the Moon’s orbit is tilted only a little (about 5 angular degrees) with respect to Earth’s orbit around the Sun.

Practice problems of this concept can be found at: The Moon and the Sun Practice Problems

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