From time to time the Sun, Earth, and Moon all happen to lie along a straight line. When this occurs, the shadow of the Earth can fall on the Moon or the shadow of the Moon can fall on the Earth. Such phenomena are called eclipses. They are perhaps the most dramatic astronomical events that can be seen with the naked eye. And that is called the Eclipse.
The Important Basic
A lunar eclipse occurs when the Moon passes through the Earth’s shadow. This occurs when the Sun, Earth, and Moon are in a straight line, with the Earth between the Sun and Moon so that the Moon is at full phase. At this point in the Moon’s orbit, the face of the Moon seen from Earth would normally be fully illuminated by the Sun. Instead, it appears quite dim because the Earth casts a shadow on the Moon.
A solar eclipse occurs when the Earth passes through the Moon’s shadow. As seen from Earth, the Moon moves in front of the Sun. Once again, this can happen only when the Sun, Moon, and Earth are in a straight line. However, for a solar eclipse to occur, the Moon must be between the Earth and the Sun. Therefore, a solar eclipse can occur only at the new moon.
Both new moon and full moon occur at intervals of 29½ days. Hence, you might expect that there would be a solar eclipse every 29½ days, followed by a lunar eclipse about two weeks (half a lunar orbit) later. But in fact, there are only a few solar eclipses and lunar eclipses per year. Solar and lunar eclipses are so infrequent because the plane of the Moon’s orbit and the plane of the Earth’s orbit are not exactly aligned, as shows. The angle between the plane of the Earth’s orbit and the plane of the Moon’s orbit is about 5°. Because of this tilt, new moon and full moon usually occur when the Moon is either above or below the plane of the Earth’s orbit. When the Moon is not in the plane of the Earth’s orbit, the Sun, Moon, and Earth cannot align perfectly, and an eclipse cannot occur.
In order for the Sun, Earth, and Moon to be lined up for an eclipse, the Moon must lie in the same plane as the Earth’s orbit around the Sun. This plane is called the ecliptic plane because it is the same as the plane of the Sun’s apparent path around the sky, or ecliptic. Thus, when an eclipse occurs, the Moon appears from Earth to be on the ecliptic— which is how the ecliptic gets its name.
The planes of the Earth’s orbit and the Moon’s orbit intersect along a line called the line of nodes. The line of nodes passes through the Earth and is pointed in a particular direction in space. Eclipses can occur only if the line of nodes is pointed toward the Sun—that is, if the Sun lies on or near the line of nodes—and if, at the same time, the Moon lies on or very near the line of nodes. Only then do the Sun, Earth, and Moon lie in a line straight enough for an eclipse to occur.
Anyone who wants to predict eclipses must know the orientation of the line of nodes. But the line of nodes is gradually shifting because of the gravitational pull of the Sun on the Moon. As a result, the line of nodes rotates slowly westward. Astronomers calculate such details to fix the dates and times of upcoming eclipses.
There are at least two—but never more than five—solar eclipses each year. The last year in which five solar eclipses occurred was 1935. The least number of eclipses possible (two solars, zero lunar) happened in 1969. Lunar eclipses occur just about as frequently as solar eclipses, but the maximum possible number of eclipses (lunar and solar combined) in a single year is seven.
Lunar Eclipses – In Depth
The character of a lunar eclipse depends on exactly how the Moon travels through the Earth’s shadow. The shadow of the Earth has two distinct parts. In the umbra, the darkest part of the shadow, no portion of the Moon’s surface can be seen. A portion of the Moon’s surface is visible in the penumbra, which therefore is not quite as dark. Most people notice a lunar eclipse only if the Moon passes into the Earth’s umbra. As this umbral phase of the eclipse begins, a bite seems to be taken out of the Moon.
If you were on the Moon during a total lunar eclipse, the Sun would be hidden behind the Earth. But some sunlight would be visible through the thin ring of the atmosphere around the Earth, just as you would see sunlight through a person’s hair whose head was between your eyes and the Sun. As a result, a small amount of light reaches the Moon during a total lunar eclipse, and so the Moon does not completely disappear from the sky as seen from Earth. Most of the sunlight that passes through the Earth’s atmosphere is red, and thus the eclipsed Moon glows faintly in reddish hues.
Lunar eclipses occur at the full moon when the Moon is directly opposite the Sun in the sky. Hence, a lunar eclipse can be seen at any place on Earth where the Sun is below the horizon (that is, where it is nighttime). A lunar eclipse has the maximum possible duration of the Moon travels directly through the center of the umbra. The Moon’s speed through the Earth’s shadow is roughly 1 kilometer per second (3600 kilometers per hour, or 2280 miles per hour), which means that totality—the period when the Moon is completely within the Earth’s umbra—can last for as long as 1 hour and 42 minutes.
Solar Eclipses – In Depth
As seen from Earth, the angular diameter of the Moon is almost exactly the same as the angular diameter of the far larger but more distant Sun—about 0.5°. Thanks to this coincidence of nature, the Moon just “fits” over the Sun during a total solar eclipse.
A total solar eclipse is a dramatic event. The sky begins to darken, the air temperature falls, and winds increase as the Moon gradually covers more and more of the Sun’s disk. All nature responds: Birds go to roost, flowers close their petals, and crickets begin to chirp as if evening had arrived. As the last few rays of sunlight peek out from behind the edge of the Moon and the eclipse becomes total, the landscape around you is bathed in an eerie gray or, less frequently, in shimmering bands of light and dark. Finally, for a few minutes, the Moon completely blocks out the dazzling solar disk and not much else. The Sun’s thin, hot outer atmosphere, which is normally too dim to be seen—blazes forth in the darkened daytime sky. It is an awe-inspiring sight.
To see the remarkable spectacle of a total solar eclipse, you must be inside the darkest part of the Moon’s shadow, also called the umbra, where the Moon completely blocks the Sun. Because the Sun and the Moon have nearly the same angular diameter as seen from Earth, only the tip of the Moon’s umbra reaches the Earth’s surface. As the Earth rotates, the tip of the umbra traces an eclipse path across the Earth’s surface. Only those locations within the eclipse path are treated to the spectacle of a total solar eclipse. The inset shows the dark spot on the Earth’s surface produced by the Moon’s umbra.
Immediately surrounding the Moon’s umbra is the region of partial shadow called the penumbra. As seen from this area, the Sun’s surface appears only partially covered by the Moon. During a solar eclipse, the Moon’s penumbra covers a large portion of the Earth’s surface, and anyone standing inside the penumbra sees a partial solar eclipse. Such eclipses are much less interesting events than total solar eclipses, which is why astronomy enthusiasts strive to be inside the eclipse path. If you are within the eclipse path, you will see a partial eclipse before and after the brief period of totality
The width of the eclipse path depends primarily on the Earth-Moon distance during totality. The eclipse path is widest if the Moon happens to be at perigee, the point in its orbit nearest the Earth. In this case, the width of the eclipse path can be as great as 270 kilometers (170 miles). In most eclipses, however, the path is much narrower.
Annular Solar Eclipses
In some eclipses, the Moon’s umbra does not reach all the way to the Earth’s surface. This can happen if the Moon is at or near apogee, its farthest position from Earth. In this case, the Moon appears too small to cover the Sun completely. The result is a third type of solar eclipse, called an annular eclipse. During an annular eclipse, a thin ring of the Sun is seen around the edge of the Moon. The length of the Moon’s umbra is nearly 5000 kilometers (3100 miles) less than the average distance between the Moon and the Earth’s surface. Thus, the Moon’s shadow often fails to reach the Earth even when the Sun, Moon, and Earth are properly aligned for an eclipse. Hence, annular eclipses are slightly more common—as well as far less dramatic—than total eclipses.
Even during a total eclipse, most people along the eclipse path observe totality for only a few moments. The Earth’s rotation, coupled with the orbital motion of the Moon, causes the umbra to race eastward along the eclipse path at speeds in excess of 1700 kilometers per hour (1060 miles per hour). Because of the umbra’s high speed, totality never lasts for more than 7½ minutes. In a typical total solar eclipse, the Sun-Moon-Earth alignment and the Earth-Moon distance are such that totality lasts much less than this maximum.
Ancient astronomers achieved a limited ability to predict eclipses. In those times, religious and political leaders who were able to predict such awe-inspiring events as eclipses must have made a tremendous impression on their followers. One of three priceless manuscripts to survive the devastating Spanish Conquest shows that the Mayan astronomers of Mexico and Guatemala had a fairly reliable method for predicting eclipses. The great Greek astronomer Thales of Miletus is said to have predicted the famous eclipse of 585 b.c., which occurred during the middle of a war. The sight was so unnerving that the soldiers put down their arms and declared peace.
In retrospect, it seems that what ancient astronomers actually produced were eclipse “warnings” of various degrees of reliability rather than true predictions. Working with historical records, these astronomers generally sought to discover cycles and regularities from which future eclipses could be anticipated.