Penumbral Eclipse

¡SkyCaramba! Weekly astronomy blog for the week ending October 19, 2013

The moon will pass through the northern part of the earth’s outer shadow, or penumbra, on October 18 or 19, depending on your time zone. This will not be a total eclipse. The moon won’t even pass partially into to Earth’s umbra where it would have to go to be totally eclipsed. It may take on a red-orange color though. Some penumbral eclipses have been greenish-blue.

The moon will enter the penumbra on October 18 at 21:50 Universal Time. That’s when you can start looking for subtle color changes that slowly sweep across the lunar disk from the southeast edge.

The eclipse colors should be most visible at 23:50 when the moon is as deep into the earth’s shadow as it’s going to get for this event. You should notice that the northern and northwestern edges of the moon are still brightly lit. They will not be in the shadow at all.

By 1:50 on the 19th, the shadow will finish sweeping across the moon and exit the disk on the southwest side.

Any time you look for something on the moon using the cardinal directions, remember the directions are a little different than we’re used to thinking of them for things on Earth. We’re used to north being up and west being to the left. A person in the northern hemisphere looking at the moon is usually going to find north is up and east is left. In the southern hemisphere, south is usually going to be up.

Why does the moon become reddish-orange during an eclipse? The same reason sunrises and sunsets are reddish-orange. The colors on that end of the spectrum pass through Earth’s atmosphere with less refraction than those on the blue end. The blue end scatters most easily, causing the sky to be blue most of the day. When the sun is low in the sky, however, there’s so much more atmosphere for the sun to shine through that even less of the blue gets through.

That reddish-orange light makes it to the moon’s surface and lights it up in an unusual color, much like a stage light with a colored filter can light up a play scene in an unusual color for a dramatic effect. Sometimes, however, greenish or bluish light makes it through. Astronomers don’t always agree as to the possible cause. It may be certain kinds of volcanic ash or an especially stormy atmosphere that causes the unusual refraction.

Africa, Europe, and the Atlantic Ocean will be good places to view the entire eclipse from. Most of Asia and the Indian Ocean will see the first part. The Americas will see the last part. If you don’t plan to be in a place where you can see all or any of it, you can probably find live astronomy camera feeds on the Internet.


Images on this page from NASA.