February’s Deep Penumbral Lunar Eclipse
North American observers can watch the Moon flirt with Earth’s shadow on the evening of February 10th.
No daytime shadow you normally see has a sharp edge. Instead, it’s always surrounded by a fuzzy perimeter—called the penumbra—created because the Sun is not a point source. A penumbra’s width, as seen from the shadow caster, is always equal to the angular width of the light source in the opposite direction.
A penumbral lunar eclipse not quite as deep as the one we’re about to see occurred over the Far East on November 28, 2012 Hoin Kong Space Museum
That’s why the penumbra of Earth’s shadow is almost exactly as wide as the Moon. Both the Sun and Moon appear very nearly the same width, about ½° across, as seen from Earth. And on February 10th, North Americans can watch the Moon plunge deeply into that dusky zone without actually touching the core of Earth’s shadow, the interestingly dark-red umbra, as happens during a partial or total lunar eclipse.
So a penumbral lunar eclipse is essentially a tease. But February’s will be about the best one possible. The Moon’s northern limb will miss Earth’s umbra by only about 100 miles (160 km), or 3% of the Moon’s diameter. Although no bit of the Moon will go dark, the penumbral shading will be very plain to see.
Eastern North America, and all of Central and South America, will have a fine view of these shadowy happenings. In these locations twilight will be deepening or entirely over, and the full Moon will be shining well up in the eastern sky, by the time of maximum shading. Northeasterners can watch the whole progression from start to finish.
Seen from the central part of North America, the eclipse will be at maximum around or soon after moonrise and sunset, with the Moon still low in a bright sky. For those of you in the West, the Moon rises and the Sun sets after the eclipse has peaked. But even here you may be able to witness the subtle anomaly on the Moon fading away.
|Moon enters Penumbra||22:32||5:32 p.m.||—||—||—|
|Peumbra first visible?||23:14||6:14 p.m.||5:14 p.m.||—||—|
|Mid-eclipse||0:44*||7:44 p.m.||6:44 p.m.||5:44 p.m.||—|
|Penumbra last visible?||2:14*||9:14 p.m.||8:14 p.m.||7:14 p.m.||6:14 p.m.|
|Moon leaves penumbra||2:55*||9:55 p.m.||8:55 p.m.||7:55 p.m.||6:55 p.m.|
Seeing is Believing
The outer part of Earth’s penumbra is so pale that you can’t detect it. You won’t see anything happening until the Moon’s edge has slid at least halfway in. For this event, that means about 90 minutes before mid-eclipse (which will come at 0:44 Universal Time on February 11th).
The shading will begin to show on the Moon’s left side, as seen in early evening from mid-northern latitudes. In North America, only from the northeastern U.S. and eastern Canada will the Moon be well up in time to watch for this. A lot will depend on how uniform any thin cloudiness may be, and on how much time and care you take to judge the reality of subtle impressions.
During the lunar eclipse on February 10–11, 2017, the Moon skirts just outside Earth’s dark umbra. Look for deep penumbral shading on the north half of the lunar disk.
Sky & Telescope diagram
The passing minutes will gradually confirm your first correct judgments. By the time mid-eclipse approaches, the lopsidedness of the Moon’s illumination will be totally obvious. The shading will appear to become ever more concentrated toward the northern limb, because an umbra’s drop-off in brightness is always sharpest near its inner edge.
And then, as the Moon rises higher and any remaining twilight fades down, the process runs in reverse. This is the part of the eclipse that most Americans will be able to see best. How long can you definitely hold a trace of shading with your eyes, and with a telescope? Will 90 minutes after mid-eclipse really be the last time anything is visible?