April 24, 2013 at 10:02 PM ET
Looking for a darkening moon? Thursday's partial lunar eclipse will be particularly subtle, and it won't be visible at all from North America — but you can still catch the show, such as it is, on the Web.
Lunar eclipses occur when Earth's shadow blots out part of the full moon's disk. When the shadow covers the whole disk, the moon takes on an eerie reddish glow. The effect is much less pronounced during a partial eclipse. And NASA's eclipse expert, Fred Espenak, says Thursday's eclipse will be "barely partial": Earth's umbral shadow will reach less than 1.5 percent across the moon at the most.
That means the partial phase will last just 27 minutes, from 3:54 to 4:21 p.m. ET. That's the shortest duration for a partial lunar eclipse since 1958. But there's more to the event than those 27 minutes: Before and after the partial phase, the moon passes through a semi-shaded region of space during what's known as the eclipse's penumbral phase. When you add that in, the darkening of the moon lasts more than four hours.
Unfortunately for North Americans who want to watch the subtle spectacle with their own eyes, it's an inconvenient four hours — lasting from 2:03 to 6:11 p.m. ET, when the sun is in the sky and the moon isn't. Europeans and Africans, Asians and Australians are in a much better position.
This map shows how much of the eclipse is visible from where:
Thursday's event is the only partial lunar eclipse of 2013. Two other moon-darkenings, on May 25 and Oct. 18, only get as far as the penumbral phase. There'll be solar eclipses in May and November of this year — but if you're partial to lunar eclipses, this is as good as it gets until next April.
If you're outside the eclipse zone, or if the skies are cloudy, you can turn to the Web:
Even if you miss out on the live feeds, it's a good bet that SpaceWeather.com and Space.com will have pictures of the eclipse afterward. If you snap a nice photo of the darkening moon, please share it with us via NBC News' FirstPerson photo upload website.
More about lunar eclipses:
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.