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Look Up Tonight: The Tetrad Lunar Eclipse Explained

If you're living in North or South America, look toward the sky tonight for a total lunar eclipse. Beginning around 2 AM Eastern Time tonight (technically April 15th), the moon will pass into the Earth's shadow, and the eclipse will peak between 3 and 4 AM Eastern Time.

Lunar eclipses (varying from total eclipses to barely noticeable ones) are fairly common — typically two to three occur each year. Tonight's eclipse, however, is just the beginning.

After tonight, three subsequent lunar eclipses will all be total eclipses as well, each separated by about six month intervals. This "tetrad" of lunar eclipses occurs infrequently; in fact, only about 16 percent of lunar eclipses ever belong to a tetrad such as the one beginning tonight.

A lunar eclipse from 2007 captured by Jens Hackman of Weikersheim, Germany.
Image Credit: Jens Hackman/NASA

Eclipse Dissection

For a total lunar eclipse to occur, the moon, Earth, and sun must all align perfectly in what is known as a syzygy (perhaps the best physics term to say aloud). During this alignment, the moon falls into the Earth's shadow.

A diagram of a lunar eclipse. The Umbra and Penumbra form the shadow of the Earth.
Image Credit: NASA/Sagredo via Wikimedia Commons

Nonetheless, light that passes through the Earth's atmosphere will still reach the moon. Instead of the white light you typically see shining on the moon's surface during a full moon, however, primarily red light reaches the moon during an eclipse.

This is because the shorter wavelengths in the visible spectrum are scattered during the long trip through our atmosphere while the longer red wavelengths tend to survive. Hence the moon typically has a reddish hue during a lunar eclipse, and the subtleties of its color depend on the composition of the atmosphere at the time of the eclipse.

Due to the varied composition of the atmosphere at different altitudes, different light bands may pass through, giving the moon a turquoise hue near the edges like in the image at the top of this post.

Astronomers define the evolution of a lunar eclipse by several important points of contact:

P1 (First Contact): When the moon first passes into the penumbra portion of the Earth's shadow.
U1 (Second Contact): When the moon first passes into the umbra portion of the Earth's shadow.
U2 (Third Contact): The first point where the entire moon passes into the umbra — the start of the total eclipse (the moon will shine red at this point).
Greatest Eclipse: The peak of the eclipse.
U3 (Fourth Contact): The first point where the moon starts to pass out of the umbra.
U4 (Fifth Contact): The first point where the moon starts to pass out of the penumbra.
P2 (Sixth Contact): The end of the eclipse when the moon completely leaves the penumbra.

A diagram of the upcoming April 15, 2014 eclipse with the various points of contact shown.
Image Credit: F. Espenak/NASA Goddard

Below you can see a map for viewing tonight's eclipse with all of the contact points listed. Tonight's eclipse favors the United States for optimal viewing.

A viewing map for tonight's eclipse. All of the eclipse will be viewable across the vast majority of the United States.
Image Credit: F. Espenak/NASA Goddard

So if you're in North or South America tonight (or way out in the Pacific Ocean!), look toward the sky late tonight for a ruddy red moon. The total eclipse should last for over an hour, so there's plenty of viewing time barring any cloudy weather.

For more information on lunar eclipses, check out NASA's dedicated web page.


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