Chapter 3 Continued
To summarize, let us follow the moon through one cycle of phases. At new moon, the moon is nearly in line with the sun and sets in the west with the sun. Thus we see no moon at new moon. A few days after new moon, we see the waxing crescent above the western horizon soon after sunset. On such evenings we might be able to see the dark part of the moon in addition to the brighter crescent. This has been called "the old moon in the new moon's arms" and is caused by earthshine, sunlight reflected from Earth and illuminating the night side of the moon.
Each evening the crescent moon is fatter and higher above the horizon, until, about 1 week after new moon, it reaches first quarter and stands high in the southern sky at sunset. The first-quarter moon does not set until about midnight. In the days following first quarter, the moon waxes fatter, becoming gibbous waxing. Each evening we find it farther east among the stars, and it sets later and later. About 2 weeks after new moon, the moon reaches full, rising in the east as the sun sets in the west. The full moon is visible all night, setting in the west at sunrise.
The waning phases of the moon may be less familiar because the moon is not visible in the early evening sky. As it wanes through gibbous, it rises later and later; by the time it reaches third quarter, it does not rise until midnight. The waning crescent does not rise until even later, and if we wish to see the thin waning crescent just before new moon, we must get up before sunrise and look for the moon above the eastern horizon.
Almost everyone is familiar with the changing phases of the moon, but those who live near the seashore are probably familiar with another phenomenon related to the lunar cycle--the periodic advance and retreat of the ocean tides.
-Tides-
The moon's gravity has dramatic effects on Earth. The side of Earth facing the moon is about 4,000 miles closer to the moon than the center of Earth is, and the moon's gravity pulls on the near side of Earth more strongly than on Earth's center. Though we think of our planet as solid, it is not perfectly rigid, so the moon's gravity draws the rocky surface of the near side up into a bulge a few centimeters high.
The oceans respond to the force of the moon's gravity by flowing into a bulge of water on the side of Earth facing the moon. There is also a bulge on the side away from the moon, which develops because the moon pulls more strongly on Earth's center than on the far side. Thus the moon pulls Earth away from the oceans, which flow into a bulge on the far side.
We can see dramatic evidence of this effect if we watch the ocean shore for a few hours. Though Earth rotates on its axis, the tidal bulges remain fixed with respect to the moon. As the turning Earth carries us into a tidal bulge, the ocean water deepens and the tide crawls up the beach. Later, when Earth carries us out of the bulge, the water becomes shallower and the tide falls. Because there are two bulges on opposite sides of Earth, the tides should rise and fall twice a day on an ideal coast.
In reality, the tide cycle at any given location can be quite complex because of the latitude of the site, shape of the shore, winds, and so on. Tides in the Bay of Fundy (New Brunswick, Canada), for example, occur twice a day and can exceed 40 feet, while the northern coast of the Gulf of Mexico has only one tidal cycle, of roughly 1 foot, each day.
The sun, too, produces tidal bulges on Earth. At new moon and at full moon, the moon and sun produce tidal bulges that add together and produce extreme tidal changes; high tide is very high, and low tide is very low. Such tides are called spring tides, even though they occur at every new and full moon, and not just in the spring. Neap tides occur at first- and third-quarter moon, when the moon and sun pull at right angles to each other. Then the tides caused by the sun reduce the tides caused by the moon, and the rise and fall of the ocean is less extreme than usual.
Tidal forces can have surprising effects. The friction of the ocean waters with the seabeds slows the rotation of the earth and makes the day grow by 0.0023 second per century. Fossils of marine animals and tidal sediments confirm that only 900 million years ago Earth's day was 18 hours long. In addition, Earth's gravitational field exerts tidal forces on the moon, and, although there are no bodies of water on the moon, friction within the flexing rock has slowed the moon's rotation to the point that it now keeps the same face toward Earth.
Tidal forces can also affect orbital motion. Friction with the ocean beds drags the tidal bulges eastward out of a direct Earth-moon line. These tidal bulges contain a large amount of mass, and their gravitational field pulls the moon forward in its orbit. As a result, the moon's orbit is growing larger, and it is receding from Earth at about 4 cm per year, an effect that astronomers can measure by bouncing laser beams off reflectors left on the lunar surface by the Apollo astronauts.
These and other tidal effects are important in many areas of astronomy. In later chapters, we will see how tidal forces can pull gas away from stars, rip galaxies apart, and melt the interiors of satellites orbiting near massive planets. For now, however, we must consider other aspects of the lunar cycle. The stately progression of the lunar phases and the ebb and flow of the ocean tides are commonplace, but occasionally something peculiar happens: The moon darkens and turns copper-red in a lunar eclipse.
-Lunar Eclipses-
A lunar eclipse occurs at full moon when the moon moves through the shadow of Earth. Because the moon shines only by reflected sunlight, we see the moon gradually darken as it enters the shadow.
Earth's shadow consists of two parts. The umbra is the region of total shadow. If we were in the umbra of Earth's shadow, we would see no portion of the sun. If we moved into the penumbra, however, we would be in partial shadow and would see part of the sun peeking around the edge of Earth. Thus in the penumbra the sunlight is dimmed but not extinguished.
If the orbit of the moon carries it through the umbra, we see a total lunar eclipse. As we watch the moon in the sky, it first moves into the penumbra and dims slightly; the deeper it moves into the penumbra, the more it dims. In about an hour, the moon reaches the umbra, and we see the umbral shadow darken part of the moon. It takes about an hour for the moon to enter the umbra completely and become totally eclipsed. Totality, the period of total eclipse, may last as long as 1 hour 45 minutes, though the timing of the eclipse depends on where the moon crosses the shadow.
When the moon is totally eclipsed, it does not disappear completely. Although it receives no direct sunlight, it does receive some sunlight refracted (bent) through Earth's atmosphere. If we were on the moon during totality, we would not see any part of the sun because it would be entirely hidden behind Earth. However, we would be able to see Earth's atmosphere illuminated from behind by the sun. The red glow from this "sunset" illuminates the moon during totality and makes it glow coppery red.
If the moon does not move completely into the umbra, we see a partial lunar eclipse. The part of the moon that remains outside the umbra receives some direct sunlight, and the glare usually prevents our seeing the faint coppery glow of the part of the moon in the umbra.
A penumbral lunar eclipse occurs when the moon passes through the penumbra but misses the umbra entirely. Since the penumbra is a region of partial shadow, the moon is only partially dimmed. A penumbral eclipse is not very impressive.
While there are usually no more than one or two lunar eclipses each year, it is not difficult to see one. We need only be on the dark side of Earth when the moon passes through Earth's shadow. That is, the eclipse must occur between sunset and sunrise at our location. (!!Table3.1pg31!!)
-Solar Eclipses-
We who live on planet Earth can see a phenomenon that is not visible from most planets. It happens that the sun is 400 times larger than the moon and, on the average, 390 times farther away, so the sun and moon have nearly equal angular diameters--0.5°. Thus, the moon is just the right size to cover the bright disk of the sun and cause a solar eclipse. If the moon covers the entire disk of the sun, we see a total eclipse. If it covers only part of the sun, we see a partial eclipse.
Whether we see a total or partial eclipse depends on whether we are in the umbra or the penumbra of the moon's shadow. The umbra of the moon's shadow barely reaches Earth and casts a small circular shadow never larger than 270 km (168 miles) in diameter. If we are standing in that umbral spot, we are in total shadow, unable to see any part of the sun's surface, and the eclipse is total. But if we are located outside the umbra, in the penumbra, we see part of the sun peeking around the edge of the moon and the eclipse is partial. Of course, if we are outside the penumbra, we see no eclipse at all.
Because of the orbital motion of the moon and the rotation of Earth, the moon's shadow sweeps rapidly across Earth in a long, narrow path of totality. If we want to see a total solar eclipse, we must be in the path of totality. When the umbra of the moon's shadow sweeps over us, we see one of the most dramatic sights in the sky, the totally eclipsed sun.
The eclipse begins as the moon slowly crosses in front of the sun. It takes about an hour for the moon to cover the solar disk, but as the last sliver of sun disappears, dark falls in a few seconds. Automatic street lights come on, drivers of cars turn on their headlights, and birds go to roost. The sky becomes so dark we can even see the brighter stars.
The darkness lasts only a few minutes because the umbra is never more than 270 km (168 miles) in diameter and sweeps across Earth's surface at over 1,600 km/hr (1,000 mph). The sun cannot remain totally eclipsed for more than 7.5 minutes, and the average period of totality lasts only 2 or 3 minutes.
When the moon covers the bright surface of the sun, called the photosphere, we can see the bright gases of the chromosphere just above the photosphere, and the corona, the sun's faint outer atmosphere. The corona is a low-density, hot gas that glows with a pale white color. Streamers caused by the solar magnetic field streak the corona. The chromosphere is often marked by eruptions on the solar surface called prominences and is bright pink. The corona, chromosphere, and prominences are visible only when the brilliant photosphere is covered. As soon as part of the photosphere reappears, the fainter corona, chromosphere, and prominences vanish in the glare, and totality is over. The moon moves on in its orbit, and in an hour the sun is completely visible again.
Just as totality begins or ends, a small part of the photosphere can peek out from behind the moon through a valley at the edge of the lunar disk. Although it is intensely bright, such a small part of the photosphere does not completely drown out the fainter corona, which forms a silvery ring of light with the brilliant spot of photosphere gleaming like a diamond. This diamond ring effect is one of the most spectacular of astronomical sights, but it is not visible during every solar eclipse. Its occurrence depends on the exact orientation and motion of the moon.
Sometimes when the moon crosses in front of the sun, it is too small to fully cover the sun, and we see an annular eclipse, a solar eclipse in which a ring (or annulus) of the photosphere is visible around the disk of the moon. With a portion of the photosphere visible, the eclipse never becomes total; it never quite gets dark; and we can't see the prominences, chromosphere, and corona. Annular eclipses occur because the moon follows a slightly elliptical orbit around the earth, and thus its angular diameter can vary. When it is at perigee, its point of closest approach, it looks significantly larger than when it is at apogee, the most distant point in its orbit. Furthermore, the earth's orbit is slightly elliptical, so the Earth-sun distance varies slightly and thus the diameter of the solar disk varies slightly. If the moon is in the farther part of its orbit during totality, its angular diameter will be less than the angular diameter of the sun, and thus we see an annular eclipse.
A list of future total and annular eclipses of the sun is given in table 3.2 (!!table3.2pg35!!). If you plan to observe a solar eclipse, remember that the sun is bright enough to burn your eyes and cause permanent damage if you look at it directly. This is true whether there is an eclipse or not. Solar eclipses can tempt us to look at the sun in spite of its brilliance and thus can tempt us to risk our eyesight. During totality, the brilliant photosphere is hidden and it is safe to look at the eclipse, but the partial phases can be dangerous.
-Predicting Eclipses-
Predicting lunar or solar eclipses seems quite complex, and precise predictions require sophisticated calculations. But we can make general eclipse predictions by thinking about the geometry of an eclipse and the cyclic motions of the sun and moon.
We see a solar eclipse when the moon passes between Earth and the sun, that is, when the lunar phase is new moon. We see a lunar eclipse at full moon. However, we don't see eclipses at every new moon and every full moon. Why not?
To be eclipsed, the moon must enter Earth's shadow. However, because its orbit is tipped, the moon often misses the shadow, passing north or south of it, and no lunar eclipse occurs. Also, in order to produce a solar eclipse, the moon's shadow must sweep over Earth. The inclination of the moon's orbit, however, means that it often reaches new moon with its shadow passing north or south of Earth, and there is no solar eclipse.
For an eclipse to occur, the moon must reach full or new moon at the same time it passes through the plane of Earth's orbit; otherwise, the shadows miss. The points where it passes through the plane of Earth's orbit are called the nodes of the moon's orbit, and the line connecting these is called the line of nodes. Twice a year this line of nodes points toward the sun, and for a few weeks eclipses are possible at new moon and full moon. These intervals when eclipses are possible are called eclipse seasons, and they occur about six months apart.
If the moon's orbit were fixed in space, the eclipse seasons would always occur at the same time each year. The moon's orbit precesses slowly, however, because of the gravitational pull of the sun on the moon, and the precession slowly changes the direction of the line of nodes. The line turns westward, making one complete rotation in 18.61 years. As a result, the eclipse seasons occur about 3 weeks earlier each year. The motion of the line of nodes, combined with the periodicity of the lunar phases, means that every 6585.3 days the eclipse seasons start over and the same pattern of eclipses repeats (6585.3 days equals 18 years 11.3 days or 18 years 10.3 days depending on the number of leap days during the interval). Because this cycle, termed the Saros cycle, contains about one-third of a day more than an integer number of days, an eclipse visible in North America will recur after 18 years 11.3 days, about one-third of the way around the world, in this case in the eastern Pacific. Many ancient peoples recognized the Saros cycle from their records of previous eclipses and were able to predict when eclipses would occur, even though they did not understand what the sun and moon were or what alignments produced eclipses.
The elegant cycles of the sun and moon produce beautiful phenomena to decorate earth's sky. Many cultures have romantic myths about the dance of the sun and moon. The Greek myth of the handsome Hippomenes racing for the hand of the beautiful Atalante, is really about the sun racing the moon around the ecliptic. The nursery rhyme Jack and Jill is the story of the waxing and waning of the moon.
The cycles in the sky are a rich part of our culture, but those same cycles may be affecting our planet in much more dramatic ways. They may be affecting the ice ages.