September 25 – Look! Up In The Sky!

Today’s factismal: The technical term for a supermoon eclipse is a perigee-syzygy.

Mankind has been watching the skies for millenia. And we’ve seen our share of eclipses in that time, starting with the first eclipse in recorded history was seen over Sumeria (near modern-day Iran) in 1375 BCE and going through to the “supermoon” eclipse coming up on Sunday. During that time we’ve developed some terms to describe the event (such as syzygy, or “yoked together”) and we’ve learned some things.

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.

We’ve learned that eclipses are caused when the Moon blocks the Sun’s light from part of the Earth (a solar eclipse) or when the Earth blocks the Sun’s light from the Moon (a lunar eclipse like Sunday’s). We’ve learned that happens because the Moon orbits the Earth in a near-perfect circle. If the Moon’s orbit were a perfect circle around Earth’s equator, then 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 it isn’t and we don’t. Because the Moon orbits in a tilted ellipse where it comes as close as 225,291 mi and heads out as far as 251,910 mi, eclipses are rare; we only get between two and five solar eclipses each year, and the same number of lunar eclipses. In addition, the ellipse slowly precesses around the Earth so that the close part (what astronomy wonks call perigee) doesn’t always happen at the full moon; when it does, we get a “supermoon” where the Moon appears about 14% larger than normal.

The Sun, Earth, and Moon, drawn to scale (almost - the Moon is three times as large as it should be)

The Sun, Earth, and Moon, drawn to scale (almost – the Moon is three times as large as it should be)

This month we are exceptionally lucky. The Moon will be in the right place to have an eclipse (rare) and it will happen when the moon is at its closest approach (rarer)! The last time this happened was 33 years ago, and it won’t happen again for another18 years. All told, there have been five supermoon eclipses in the past century (counting this one). So you should definitely go out and see it!

All you will need to see the supermoon eclipse is clear skies and an alarm clock. For all of North and South America, the Moon will either be in eclipse at moonrise or will goo into eclipse shortly thereafter.  And the Moon will stay in total eclipse for a good hour or so. From the start of the eclipse (when the Moon first enters Earth’s shadow) to the end will take just over five hours, so you’ll have plenty of time to see the spectacle.For folks in the Central time zone, the eclipse starts at 8:07 PM and hits totality at 9:47 PM. So skip the Simpsons and go see something truly amazing!

The perigee-syzygy will be visible over all of the Americas (Image courtesy NASA)

The perigee-syzygy will be visible over all of the Americas
(Image courtesy NASA)

If you’d like to learn more about eclipses, including if you’ll be able to see any of the four eclipses visible nextyear, then head on over to the NASA Eclipse Web Site:
http://eclipse.gsfc.nasa.gov/eclipse.html

One thought on “September 25 – Look! Up In The Sky!

  1. Pingback: Special factismal – Sale on books! | Little facts about science

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