In the comic movie "A Funny Thing Happened on the Way to the Forum," the character Senex, lamenting the state of his marriage, says "Never fall in love during a total eclipse." This joke is rooted in reality, because for centuries the ancient peoples of Earth regarded solar eclipses with awe and fear. The Sun gave warmth and life to the Earth, and for it to be blotted out at midday was surely an indication that the gods were displeased with humans, or at least was an omen of some impending disaster. Although the term eclipse applies to any passage of one large celestial object in front of another (two stars orbiting one another, for example, may eclipse each other if their orbital plane is tilted at the proper angle as seen from Earth), the best-known manifestations of eclipses are those involving the Earth and the Moon.
Solar eclipses occur when the Moon passes directly between the Earth and the Sun, and therefore always occur at new Moon. The Moon is about 400 times smaller than the Sun, but the Sun is 400 times farther away, so both the Moon and Sun appear to be the same size in the sky (try holding a quarter two feet from your eye and a dime one foot from your eye, and compare their apparent sizes, to see how this works). For this reason, the Moon's shadow barely reaches earth, and the path of a total solar eclipse is a narrow band. Along this path the Sun will either be blotted out entirely (in the case of a total eclipse), or reduced to a bright ring (in the case of an annular eclipse; this happens when the Moon is in the outermost part of its elliptical orbit, and is too far from Earth to obscure the solar disk completely). People outside this path of totality see only a partial eclipse, or no eclipse at all.
Lunar eclipses occur at full Moon, when the Earth passes between the Sun and the Moon. Lunar eclipses are less common than solar eclipses, but because they are visible from any place on the nighttime side of Earth, many more people get to witness one than do a solar eclipse.
Ancient astronomers, from the Greeks to the Mayans, expended much effort attempting to predict eclipses. Although they did not understand what caused eclipses, they achieved remarkable success in deducing the general patterns of solar eclipses that we now call Saros cycles. The location of an eclipse track on Earth's surface depends on a complex combination of motions involving the Earth-Moon system, but these motions are regular and lead to "families" of eclipse paths. The Saros cycle governing a given eclipse family takes about 18 years and 11.3 days to complete, so eclipses of a given family recur with this frequency. The ".3" in the figure above is important, because it translates to about 1/3 of a rotation of the Earth. Each eclipse of a Saros cycle occurs 1/3 of the way around the Earth from the previous one, so every third eclipse of a cycle traces a path roughly--though not exactly--in the same place on Earth. Early astronomers noticed this, and although the real situation was too complex for them to predict eclipses with the accuracy we can today, they nevertheless could predict when an eclipse was likely to occur within a general area. Superstitions die hard, however, and it would be thousands of years before the awesome sight of an eclipse was appreciated not as a furious deity, but as the spectacular alignment of our only natural satellite with our star.
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