Penumbral Lunar Eclipse as seen from the Moon. Credit: JAXA
There are three distinct pieces(or parts if you prefer) to a shadow: the umbra, penumbra, and antumbra. These three terms are usually only associated with the shadow cast by celestial bodies, but could apply in general.
Umbra
The umbra is the darkest part of a shadow where all light is cut off. The completely dark portion of the shadow cast by a celestial body during an eclipse. It can also refer to the darkest region of a sunspot.
Penumbra
The penumbra is the region where only a portion of the light source is obscured by the occluding body. A partial eclipse. It is the region where some or all of the light source is obscured. NASA’s Navigation and Ancillary Information Facility states that a body in the umbra is also in the penumbra.
Antumbra
The antumbra is the region where the occluding body appears entirely contained within the disc of the light source. If an observer in the antumbra moves closer to the light source, the apparent size of the occluding body increases until it causes a full umbra. An observer in this region experiences an annular eclipse(occurs when two objects are exactly in line, but the apparent size of one is smaller than that of the other, so one appears as a very bright ring(annulus) surrounding the outline of the other).
The presence of these parts of a shadow all depend on the point of observation. You may see an antumbra from point A, but from point B(with B being closer to the light source), you may see a full umbra. Try placing a tennis ball in front of a normal household flashlight. View it from 5 meters away then move closer and watch as the light source disappears.
We have written many articles about penumbra for Universe Today. Here’s an article about Moon Eclipse, and here’s an article about what happens during a lunar eclipse.
If you’d like more info on the Eclipse, check out NASA Science Article about Eclipse. And here’s a link to the Glossary of Eclipse Terms by NASA.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.
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