May 3 – In the Dark

Today’s Factismal: The first eclipse in recorded history was seen over Sumeria (near modern-day Iran) in 1375 BCE.

Eclipses are fascinating. If the universe were perfect, then moons would orbit in circles and we’d have an eclipse twice every month with a solar eclipse at new moon and a lunar eclipse two weeks later at full moon. But the universe isn’t perfect; instead of orbiting in a circle around the Earth, the Moon orbits in an ellipse where it comes as close as 225,291 mi and heads out as far as 251,910 mi. In addition, the ellipse slowly precesses around the Earth so that the close part doesn’t always happen at the full moon. And, just to make things even more complicated, the Moon does orbit around the Earth’s equator like a ring; instead, it shimmies up and down a bit like a hula hoop. As a result, we only get between two and five solar eclipses each year, and the same number of lunar eclipses.

The Earth-Moon system, draw to scale. Notice how the Moon's orbit is slightly elliptic.

The Earth-Moon system, draw to scale. Notice how the Moon’s orbit is slightly elliptic.

Because of their rarity, each eclipse was viewed as a special phenomenon by ancient cultures. The Sumerians made a close study of the motions of the Moon and Sun and were able to predict eclipses. They weren’t alone in that; the Maya also incorporated eclipses in their calenders, as did many other ancient cultures.

A partial solar eclipse as seen in Texas (My camera)

A partial solar eclipse as seen in Texas
(My camera)

Today, we still view eclipses as something special. And we have developed a rich vocabulary to describe them, from the penumbral eclipse where some sunlight refracts through the Earth’s atmosphere to light the Moon to the total solar eclipse where the Moon completely blocks the view of the Sun during syzygy (the moment of eclipse). If you’d like to learn more about eclipses, including if you’ll be able to see any of the five eclipses visible this year, then head on over to the NASA Eclipse Web Site:
http://eclipse.gsfc.nasa.gov/eclipse.html

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