[Above: Photo of a lunar eclipse from September 2015, taken just as the Moon started toslipinto the darker part of Earth's shadow. Tonight's eclipse should look similar. Credit: Phil Plait]

Tonight Friday, February 10, 2017 about half the planet will be treated to a lunar eclipse. But not an ordinary one: It will be a deep penumbral eclipse, and youll have to be diligent to see it.

OK, so what does that mean?

Well, for one thing, the Moon wont get as dark as it does in a total lunar eclipse. But it will get noticeably duskier, especially on one side, and that should be pretty neat to see. Also, itll be visible mostly to folks in Europe and Africa, but those of us in the U.S. get a view of it as well, right after sunset (see below for specific times).

An eclipse is when an astronomical object blocks the light from another astronomical object. As the Moon orbits the Earth, it sometimes slides into the Earths shadow in space. When it does, the Earth blocks the Sun, the Moon grows dark, and we get a lunar eclipse.

But things get a bit complicated. Because theres nothing between the Earth and Sun, the Earths shadow is always pointing away from the Sun in space. But it actually casts two shadows; a dark, narrow cone-shaped one called the umbra (Latin for shadow) and a wider, less dark one surrounding the umbra called the penumbra (almost shadow). This is due to the geometry of the Sun being a disk in the sky and not a dot. I explain how all this works in my episode of Crash Course Astronomy: Eclipses:

The first part is about solar eclipses (the lunar eclipse part starts around 6:45), but explains why we have an umbra and penumbra. NASA also has a nice video showing the geometry of eclipses; it wasnt made for this particular eclipse, but it does show the orbital tilts to scale very nicely:

Each lunar eclipse is different because of the tilt of the Moons orbit. Sometimes it passes into the umbral shadow, and we see a nice, dark eclipse. But sometimes the Moons orbital tilt only lets it dip its toe, so to speak, into the penumbra. If it stays near the outer edge the dimming is so minor you might not even notice!

But thats why tonights eclipse (remember, thats why I started this article in the first place) is so interesting: The Moon misses the umbra, but only by a tiny bit. Thats why this is called a deep penumbral eclipse; it passes deeply into the secondary shadow of the Earth, but not the really dark one. As the folks at Sky and Telescope note, the Moon misses the umbra by a mere 160 km! Mind you, the Moon is 3470 km across, so this is a pretty near miss. In fact, the penumbra is just narrower than the Moon itself, and the Moon is never completely inside the penumbra at any one time during this eclipse.

So, when should you go out to see it? The Moon first starts to slide into the penumbra at 22:34 UTC, or 5:34 p.m. Eastern US time. If you live on the east coast of the US thats just after the Moon rises. Those of us farther west wont see this part of the eclipse, because it wont have risen yet!

The deepest part of the eclipse occurs about two hours later at 00:45 UTC technically, in Greenwich (where UTC is officially marked) the next day, February 11, but in the US its still Friday night at 7:45 p.m. Eastern time. For my location, in Colorado, the Moon rises just minutes later, so for me it will already be in the deepest part. If youre on the west coast, you wont even see this until its already over. That happens at 02:53 UT (9:53 p.m. Eastern).

So, the farther east you are, the better. Anyone in Europe and Africa will see the whole thing, but itll be late at night. India and China will only be able to see the start of the eclipse; itll set before the Moon reaches the deepest area of the penumbra.

And what will you see? Thats hard to say. If were lucky, at the greatest point in the eclipse the northern part of the Moon will look darker than the southern side, but neither will be dark dark. More dusky, probably. It should certainly be noticeable, though. Still, it wont get blood red like it does during a total lunar eclipse, so dont expect that. This will be more subtle.

Im not even sure Ill be able to tell from where I am; the Moon will rise fully eclipsed and then get brighter over the next two hours. It kinda does that anyway as it rises and clears the murk near the horizon. Ill take a look anyway, because every eclipse is different, so you never know. Thats part of the fun!

The next total lunar eclipse is just a year from now, on January 31, 2018. That one does favor the west coast, so yall will get your chance. Still, it happens late at night so you might want to nap first.

And, of course, we have a major solar eclipse coming: on August 21, 2017 the path of that will sweep across the continental US in what may be the most viewed eclipse in history. And duh: Ill have a lot more about that coming soon. Stay tuned. Until then, enjoy tonights show!

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Will you be able to see tonight's deep penumbral lunar eclipse? - Blastr