It’s always a good night when I hear the wolves howl. The sound enhances the wild, ancient quality of the night sky, the grandest wilderness of them all. Wolves, which howl more frequently during the wintertime breeding season, give their name to January’s Full Wolf Moon, which occurs Friday, Jan. 10. That night, the moon will rise shortly after sunset in the constellation Gemini the Twins and shine until sunrise the next morning.
It’s easy to love a full moon. There’s just something about that little circle in the sky that graciously lights up the long night. Although the moon reflects only 12 percent of the light it receives from the sun (the rest is absorbed by its dark rocks), it appears positively brilliant in contrast to the night sky. To get a sense of the moon’s true brightness observe it near full in the daytime. Sunlit clouds are much brighter!
Full moons always lie directly opposite the sun in the sky with Earth in the middle. When the sun sets in the west, the moon rises in the east. In December-January, the sun occupies the constellation Sagittarius and remains low in the sky. But because the moon lies opposite the sun, it will shine from Gemini and ride high. If you look around midnight you’ll see the full moon nearly overhead. During the early winter months we get about 15 hours of full moonlight, the same as the amount of sunlight we receive in early summer.
This month’s Wolf Moon is special for another reason — it will be eclipsed! But only by Earth’s outer shadow called the penumbra. During a partial or total lunar eclipse, the moon first passes through the penumbra and then into the much darker umbra. For Friday’s eclipse, the Sun-Earth-Moon lineup isn’t precise, so the moon will miss the “bullseye” and only pass through the penumbra.
Unfortunately, this eclipse won’t be visible from the United States and most of Central and South America, but you can watch it live online at Gianluca Masi’s Virtual Telescope website starting at 11:00 a.m. CST (noon EST, 10 a.m. MDT and 9 a.m. PDT) Jan. 10. Penumbral eclipses aren’t as dramatic because there’s no obvious dark “bite” along the moon’s edge. But the penumbral shading will still be easy for observant skywatchers to see starting about about 15-20 minutes into the eclipse.
The entire world will have to content itself with watered-down shadows this year — there are four penumbral eclipses and exactly zero partial or totals.
During all lunar eclipses the Earth’s shadow has a soft, fuzzy edge for the same reason a tree casts a fuzzy shadow. Because the sun is an extended disk rather than a point of light, light from one side of the sun spills into areas that the other side of the sun can’t reach and vice versa. This “spillage” softens and diffuses the shadow’s edge, creating a penumbra.
The penumbra isn’t black because it’s a mix of shadow and sunlight. The umbra or inner shadow is dark in comparison because no sunlight reaches it. Only point-like light sources like Venus can create sharp-edged shadows.
A few nights ago, I took a walk under the bright gibbous moon. Bare trees cast shadows across the snow. While I marveled at how black and opaque the shadows looked compared to the daytime variety, it hit me — they all had soft edges the same as Earth’s shadow cast on the moon and for the same reason. Like the sun, the moon is an extended disk (not a point). An object like a tree that blocks the moon’s light casts a shadow with a dark center (umbra) surrounded by a penumbra.
Go out the next clear, moonlit night and see for yourself. Imagine the moon as the sun, the tree trunk as the Earth and the shadow eclipsing a slice of the snowy ground. Go crazy. Take a ball with you and swing it in and out of the tree’s shadow to create your own eclipse!