Penumbral lunar eclipse dims the moon’s glow


Last night, the Moon — which just two weeks ago lined up perfectly with the Sun to cast its shadow over parts of northeast Australia and the south Pacific — was now around on the other side of the planet, giving Earth a chance to return the favour.

However, unlike the lunar eclipses we normally see, where the Moon turns red, this particular lunar eclipse probably slipped by most people's attention — even if you were in an area that was able to see it — because the light from the moon only dimmed slightly.

Was the Earth less substantial, thus casting less of a shadow? Did the Sun get weaker? Were we on the verge of discovering dark matter and it sucked away some of Earth's shadow to get darker and stay hidden?

[ Related: U.S. planned to nuke the Moon to win Cold War ]

Well, no. The difference is that this was a 'penumbral' lunar eclipse.

When a shadow is cast, the size of the light source relative to the size of the object casting the shadow determines both the size and shape of the shadow. If you have a very small light source, you will only see one shadow, which is called the 'umbra'. When you have a large light source, such as the Sun, you will see two types of shadow cast by a planet, such as the Earth — the 'umbra' and the 'penumbra'. The umbra is the dark cone-shaped shadow where the light from the Sun is completely blocked by the planet. The penumbra is a more diffuse shadow that surrounds the umbra, where the light from the Sun is only partially blocked.

For a typical lunar eclipse, the Moon first gets dimmer as it enters the Earth's penumbra, turns a reddish hew while it passes through at least part of its umbra, and remains dim until it exits out the other side of its penumbra.

In a penumbral lunar eclipse, the Moon completely misses the Earth's umbra, thus does not take on the characteristic reddish colour that we normally associate with lunar eclipses. It passes through only the Earth's penumbra, and thus only grows a little bit dimmer.

[ Related: Massive impact may have shaped the face of the Moon ]

Even though the solar eclipse two weeks ago was a total solar eclipse, this particular lunar eclipse missed out on being a total penumbral lunar eclipse by what appears to be just a hair's breadth. The next total penumbral lunar eclipse won't be until August of 2053, but for now, after two penumbral lunar eclipses and a partial lunar eclipse in 2013, we'll get to enjoy four total lunar eclipses in a row — two in 2014 and two in 2015!