Astronomy

What is an Eclipse



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Today, of course, we know that an eclipse is simply an astronomical event when the Moon casts a shadow on the Earth (Solar eclipse) or the Earth casts a shadow on the Moon (Lunar eclipse). They are somewhat a rare phenomena at any specific place on Earth, although they occur regularly. If you have ever had the chance to observe one, particularly a full eclipse, you might agree that it is an awe-inspiring spectacle.

The word eclipse is derived from the ancient Greek [ekleipsis] which means forsaking. But long before the Greeks were around, ancient peoples observed the occasional eclipse and associated it with acts of celestial gods foretelling or giving warnings of impending doom and dread.

By the 7th century BCE however, astronomers and astrologers began to better understand the orbital trajectories of the celestial orbs and from this understanding gained the ability to predict when eclipses would occur.

The Greek Philosopher Thales of Miletus foretold the event of an eclipse to occur on May 28, 585, BCE. The eclipse happened just as he had predicted, and in that time, predicting something and having the prediction come true elevated a person to almost godly status.

In suggesting that eclipses and other phenomena were natural occurrences, as opposed to the handiwork of deities, Thales initiated the practice of scientific investigation and understanding.

Since the Moon travels around the Earth approximately once every 27.3 days, one could assume there would be a solar eclipse with the same frequency. In actuality, a total solar eclipse occurs on average every 19 months, but at least a partial solar eclipse occurs twice per year.

The reason for the anomaly, is that the Moon's obit is inclined by 6.7 degrees to the earths equatorial plane. When the Moon reaches the New Moon phase, while at a point which puts it in direct alignment with the Earth and Sun, a solar eclipse occurs somewhere on the Earth.

In a full Solar eclipse, the Moon's shadow on the Earth can be as wide as 150 miles across, and at it center, only the Sun's corona is visible (Astronomy The Definitive Guide, Burnham Dyer and Kanipe).

In 1919, British astronomer Sir Arthur Eddington used just such a vantage point to prove Einstein's theory of general relativity and prediction in support thereof, that light coming from stars behind the Sun would be bent by the Sun's immense gravity causing the stars position to appear changed from its normal locale.

Just as with Solar eclipses, Lunar eclipses occur twice per year, with a total lunar eclipse occurring on average every 17 months. In a total lunar eclipse, the Moon is completely engulfed in the earths shadow.

The only light coming from the Moon at that point is from the red end of the spectrum because this lower frequency light is actually refracted by Earth's atmosphere and projected on the Moon. If not for this refracted light, the Moon would become invisible during the eclipse.

There are still probably humans on Earth that believe eclipses hold some mystical influence over them or forewarn of doom. Most people understand that eclipses are simply an astrophysical phenomenon cause by the inclination and orbital trajectory of the Moon as it orbits the Earth.

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