Astronomy Jargon 101: Umbra

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll be cool in the shade of today’s topic: umbra!

Umbra is just the Latin word for “shadow,” which is a good thing because it’s the word used to describe the shadow cast by one astronomical object on another.  The most common example is seen during solar eclipses, when the Moon blocks the disk of the Sun on the sky, or during lunar eclipses, when the Earth casts a shadow on the Moon.

During solar eclipses, the Moon’s shadow is relatively small, and if you’re in the right spot at the right time you can see a sky with the sun completely blocked out. When this happens, dusk seems to set from all directions on the horizon at once, the stars come out, and nocturnal creatures get very confused.

During lunar eclipses, the Earth’s shadow is much larger than the Moon itself. Because of this, many stargazers across the globe get to enjoy the sight of the Moon plunged into darkness, usually developing a deep red color.

If the bright objects in the Universe were simple points of light, then the umbra would be relatively straightforward. It would be no more complicated than the silhouette you create when playing shadow puppets on the wall, for example.

But bright objects in the Universe tend to be large, and so the light comes from a big area. It’s difficult to completely block all the light from the entire area, and so sometimes the umbra is only partial. For example, during a solar eclipse the Moon may only cover a portion of the Sun, leaving you in partial shadow. The term for this is penumbra, which is taken from the Latin term for “almost shadow.”

Another example of a partial eclipse, lacking a fully developed umbra, is the antumbra. This happens when the object casting the shadow is too small to completely block the light source. When this happens during a solar eclipse, the Sun appears as a thin ring around the Moon, resulting in an annular eclipse.