On Friday, Feb 10th, we’ll see the first Lunar Eclipse of the year…sort of. This one is a “deep penumbral eclipse”. So what the heck is that?
Shadows come in two parts, and these parts are more distinct when the light source is big, like the Sun. The inner, darkest part of the shadow is called the umbra. The outer, less distinct part is called the penumbra.
These two shadow regions are easiest to visualize by thinking of two light sources, instead of one big one. In fact you’ve probably seen this phenomenon many times, walking across a lit parking lot at night, maybe, where there are multiple lights overhead. Let’s say there are two streetlights. Each of those lights casts a complete shadow as you walk along. However, even in those places where you cast a shadow from light number one, light number two can illuminate the ground. This creates a “dim” but not “dark” outline of your body. Only in those places where you block light from both lights one and two, where your two shadows overlap, will the shadow be its darkest. That region is considerably smaller than your shadow would be if you only had a single point light source.
The same is true with planetary bodies – the sun is HUGE, and while the Earth casts some form of shadow wherever it blocks any light (penumbra), it only casts a full shadow where it blocks ALL the light and its many individual shadows overlap (umbra). From Earth, it just so happens that the Moon and Sun are about the same apparent size, so total solar eclipses are rare and special. From the Moon, though, the Earth is a great deal bigger than the Sun. Total solar eclipses from the Moon, what we see as Lunar Eclipses, are fairly common.
What will happen this Friday is essentially a partial solar eclipse, as viewed from the Moon. Earth will partially block the sun, almost completely, but some little crescent of sunlight will still reach the Moon. The Moon will pass through the Earth’s penumbra, but not its umbra.
Because part of the Sun is always visible from the Moon, there is no clear shadow “edge” in this case. Just a gradual dimming that may not be apparent until some time after the eclipse has begun. On the East Coast of the US, look for a distinct darkening between 7 and 8:30pm. As an aside, the full moon is “big” this month, as it occurs only three days after apogee – that point where the Moon is closest to Earth in its orbit. So even if the eclipse lacks the full umbral darkening, the Moon should be pretty, regardless!
Get Out There!