Written by Andrea Lloyd

Image from EclipseWise.

On the evening of July 27th there will be the
longest total lunar eclipse in 100 years, lasting 1 hour and 43 minutes. If you’re not in North America, you’ll be able to see it. If you are from America, you’ll remember the solar eclipse from last August. But what is an eclipse?

Typically, planets, stars, and other celestial bodies pass in elliptical orbits – missing the shadows created by the sun. With a full moon, the moon’s light is actually sunlight reflected off the Moon’s surface. The moon reflects about 10 – 12%% of the Sun’s light.

During an eclipse, one celestial body passes in between of a light source and another celestial body, casting a shadow. During a solar eclipse, like how the United States experienced last August, the moon passes directly in between of the sun and the Earth. Because the sun creates light, a shadow appeared on the Earth. During a lunar eclipse, the Earth passes directly between the sun and the moon. The is now in the Earth’s shadow and can no longer reflect the sun’s light.

Photo by Kevin Rheese

The shadows cast onto the objects have monikers: the Penumbra and Umbra. With a lunar eclipse, when the moon begins to move into the Earth’s shadow, it’s in the penumbra. Here, the moon appears red. Why? The components of the Earth’s atmosphere filter out colors in the sunlight to cause the moon to appear red.

This phenomenon, called Rayleigh Scattering, also causes our colorful sunrises and sunsets, and is why the sky is blue. After the penumbra, the moon enters the umbra shadow—or completely cut off from sunlight. From there it exits the umbra, returns to the penumbra which then completes the lunar eclipse. There are two kinds of lunar eclipses: partial and total. A partial lunar eclipse is when the moon falls only partly through the Earth’s shadow. A total lunar eclipse, or Blood Moon, is when the moon completely passes through the Earth’s shadow.

Photo by Geoff Livingston

With the Solar Eclipse, as seen last August, the moon moves in front of the Sun blacking it out. The moon blocks the sunlight from reaching the Earth, casting a shadow on the Earth. Again, there are partial solar eclipses and total solar eclipses. A partial solar eclipse is when the sun, moon, and Earth are not exactly line up. A total solar eclipse, seen form a statistically small location on the Earth, is when the moon completely crosses in front of the sun and obscures the sun’s light. The sky becomes very dark, as if night. During the total solar eclipse is the only time the sun’s corona is observable, as pictured below.

If you’re waiting for the next lunar eclipse, give it another 7 months. On January 21, 2019, there is the next total solar eclipse lasting 1 hour. Coincidentally, there will be a partial eclipse on the 50th anniversary of the first moon landing mission (Apollo 11), July 16, 2019.

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