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How Eclipses Work
Remember that old riddle about the tree falling in the forest? If no one is there, does it make a sound? I like to apply that to eclipses, especially the effects that the 53 eclipses I've seen have had on me. I've always felt that we, as observers, are vital parts of these events. So suppose they gave an eclipse, and no one came?
As a scientific event, an eclipse is a comic coincidence, a curiosity. Planets don't crash into each other, and stars do not explode. But put yourself into that event, and an eclipse can have a most powerful effect. Even a barely noticeable penumbral lunar eclipse does that to me. The Moon's supposed to be full, but as its brightness dims and the rays stretching away from the craters Tycho and Copernicus become so much more prominent, I become aware that inexorably, the Moon is passing through the outer reaches of the shadow of the Earth. At the other extreme, of course, is a total eclipse of the Sun, an event that stabs like a knife to the core of my emotions. Sure, if no one sees an eclipse, then the event is nothing more than a coincidence. But those who do see it leave subtly changed, and moved by its power.
Let's look at the mechanism behind that power. A total eclipse of the Moon happens here on Earth, and with planets and moons elsewhere in the solar system. As the Moon orbits the Earth once every 29 1/2 days, it forms some angle between it, the Earth, and the Sun. Twice each month, at New or Full Moon, that angle becomes a straight line. If it precisely straight, then an eclipse takesplace. Eclipses of the Sun or Moon can occur only when the Sun, Earth and Moon are exactly lined up. This can occur during Eclipse seasons, which happen twice each year. The simple geometry of planetary bodies orbiting one another in space is common enough, and because the solar system is essentially on one plane, like a record or disc, these lineups occur frequently. But on these other worlds, the effect is far less dramatic. Mars, for example, has lineups when either of its tiny moons, Phobos and Deimos, pass in front of the Sun. But these moons are so small that they produce virtually no effect at allif you were standing on the surface of Mars, you might barely detect such an event in progress as a tiny dot crossing the Sun's surface. Jupiter's moons would, on the other hand, block out the Sun so completely that they would cause several hours of darkness. But Earth's single Moon is now at just the right distance from us that it appears to be the same size as the Sun. The result is an exquisite blocking of the Sun's bright surface, revealing the prominences of the Sun's inner atmosphere, or chromosphere, and the pearly outer atmosphere, or corona. The fact that we get to see the prominences and the corona at all is a miracle in itself, for the Moon is slowly moving farther from Earth. As it continues to recede, its apparent size will shrink, and it will no longer be able to completely blot out the Sun.
ECLIPSES, TRANSITS, AND OCCULTATIONS
Technically, the event called an eclipse takes place only when the body being covered is the same apparent size as the body doing the covering. A transit takes place when a small body passes in front of a larger one, as when Mercury or Venus pass in front of the Sun. An occultation is the term we use to describe the passing of a large body in front of an apparently smaller one. The Moon frequently passing in front of a star is an example.
As the Moon continues slowly to move away from Earth at a rate of one and a half inches each year, it will eventually seem to take up less space in the sky. Some 620 million years in the future, there will come, a day, sadly when we see the last total eclipse. The Moon will pass in front of the Sun, covering it entirely for a fraction of a second, and when that faroff event is over, total eclipses of the Sun will be a thing of the past. Even now, almost half the times that the Sun is centrally covered by the Moon, the Moon is near the farthest point of its orbit around the Earth. At that distance it does not cover the Sun completely; the result is an annular, or ring eclipse, at the middle of which the Moon is surrounded by a ring of bright sunlight.
In May 1984, I traveled to New Orleans to see an annular eclipse. A cold front had passed through the night before, leaving the usually humid city dry and clear. At the midpoint of the eclipse, the Moon's shadow swept out of the sky and almost entirely enveloped us. Overhead, what was left of the Sun shone as the thinnest of rings. A second later, the darkness whooshed away, leaving a thin crescent of sunlight. On the way home from this particular ring eclipse I stopped by to visit Clyde Tombaugh, the discoverer of the planet Pluto, and an old and close friend. "How was the eclipse?" he asked. "Did you have a ringside seat?"
Clyde got the chance to live down that pun. Ten years later a second ring eclipse took place directly over his house. Although poor weather was forecast, the day dawned bright and clear and we saw a magnificent ring. The Moon was close to apogee, and took up considerably less space in the sky than the Sun. Thus, the ring was bigger, and it lasted 11 minutes!
THE ORBIT OF THE MOON
The moon circles the Earth once every 29 1/2 days, a period loosely coinciding with a month; the word "month," in fact, is derived from old German for "moon." The Moon's orbit is the only determinant of the Moslem calendar, and is the prime base for the Jewish calendar, whose months, though timed to the Moon's orbit, are kept in step with the solar year by the occasional addition of a leap month.
Does the Moon's phase affect human behavior? There has been much conjecture and debate on that question. Anecdotal evidence does support the idea that people tend to get rowdier around the times of full Moon. But as to whether the Moon has an effect on the Earth, the answer is absolutely clear. When the Moon is near perigee (its closet approach to Earth), its gravity causes an increase in the strength of the tides. And when Moon and Sun are approximately lined up with the Sun, tides increase.
Eclipses happen because of the relation between the orbit of the Earth around the Sun, and the Moon's orbit around the Earth. The Earth orbits the Sun in a near-circle once every year, and the Moon orbits the Earth, reaching the Sun's position, every 29 1/2 days. The two orbits are tilted relative to each other; if they were not, eclipses would happen every two weeks, a solar eclipse every new Moon and a lunar eclipse at every full Moon. Instead, the Moon spends part of its month-long orbit below the plane of the Earth's orbit around the Sun, and part of it above that plane. Twice each month the Moon crosses the plane of Earth's orbit. The two points of crossing, or intersection, of the two orbits are called nodes.
The Moon crosses a node twice each month. If the Moon is moving northward in its orbit, it's called the ascending node; if it is going south, it crosses the descending node. The node crossings take place at different phases of the Moon each month. Now, in addition to the Moon's orbit of the Earth, the Earth travels around the Sun, so the Sun appears to cross one of the nodes twice each year. When that happens, an eclipse can occur.
ON THE MOON
When eclipses occur on Earth, does anything happen on the Moon? During a lunar eclipse, the entire Moon is bathed in the shadow of the Earth, which means that a person or a camera on the Moon should witness an eclipse of the Sun (by the disk of the Earth). On April 24, 1967, humanity saw a total eclipse of the Sun by the Earth through the eyes of the U.S. space probe called Surveyor 3. The spacecraft's camera took two sets of pictures of the event. The exposures revealed that an eclipse of the Sun by the Earth, seen from the Moon, is less spectacular than one of the Sun by the Moon. From the Moon, the Earth takes up almost four times as much sky as the Moon does as seen from Earth. Thus, at the middle of the eclipse the Earth would cover the Sun and most of its corona. What part of the outermost corona was left might have been recorded by the Surveyor camera, but the exposures were not long enough to reveal any of it. At the same time that Surveyor was working to take those pictures of a solar eclipse, a lunar eclipse was taking place on Earth that Passover night. (Since they always occur at full Moon, Passover Seders often coincide with eclipses of the Moon. I remember leaving our Seder to observe that particular eclipse.)
AN ECLIPSE OF THE EARTH
What would an eclipse of the Sun by the Earth look like from the Moon? Because the Moon's shadow is so much smaller than the Earth's, the effect for lunar inhabitants would be barely noticeable. The Earth, visible in the lunar night in full phase, would dominate the sky. If you looked closely, you could watch a tiny patch of darkness, surrounded by a fainter outer shadow, wend its way across the Earth's surface, crossing land and sea, from one side of the planet to the other, in about two hours. Truly, the cosmic pinball game of Earth, Moon, and Sun is something to see from any vantage point.