If you keep a close eye on the moon on Friday night, you'll be able to watch as it slips through the outer part of Earth's shadow, producing a partial — or penumbral — lunar eclipse.
The best places to see the eclipse from are anywhere in the lightest regions on the map below. Those areas will see it from start to finish, between 9:51 p.m. Friday and 1:50 a.m. Saturday, Greenwich Mean Time (click here to convert to your local time).
The areas shaded light grey will only catch part of the eclipse. The regions on the map to the right of the brightest area will see it as the moon is setting, and the regions to the left of the brightest area will see it as the moon is rising. Anyone in the darker grey region won't be able to see this particular eclipse. If it happens to cloud over during your best time to see the eclipse, websites like the Slooh Space Camera will have a free live-feed available for everyone to watch.
It's interesting to note that eclipses (both lunar and solar) would be common, and probably rather dull, if if weren't for two things — the Moon's orbit is tilted and Earth's shadow has more than one part.
Earth's shadow is always pointing directly away from the sun, and perfectly centred on a line called 'the ecliptic'. However, because the moon's orbit around the Earth is tilted slightly compared to the ecliptic, when the moon is full — thus directly on the other side of the Earth from the sun — it can be above or below Earth's shadow, partially in it, or fully inside it. Anytime any part of the moon touches Earth's shadow, it's counted as a lunar eclipse.
The colour of a lunar eclipse depends on what part of the Earth's shadow the moon moves through. If the angle of the moon for any particular full moon has it slipping through the edge of the shadow, called the penumbra, the moon just grows dimmer and then brightens again when it passes back out of the shadow. However, if it's angle takes it through the dark 'core' of the shadow, called the umbra, the moon first dims as it enters the penumbra, then turns a dusky red colour as it's completely swallowed up by the umbra, then it turns back to the familiar grey and brightens again when it completely passes out of the shadow again.
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Friday's penumbral eclipse will just have the moon dipping partially into the upper edge of Earth's shadow. So, it will grow dimmer, but not by as much as it would if the penumbra fell across the entire face of the moon.
If this eclipse doesn't end up being particularly spectacular, it's still heralding the arrival of a total solar eclipse in about two and a half weeks. On Sunday, November 3rd, anyone in Africa or on the Atlantic Ocean will be able to watch (carefully, of course) as the moon passes across the disk of the sun. Unfortunately for anyone outside of that, the angle isn't right for us to see much, if any of this particular eclipse, but astronomers and skywatchers are sure to have various live-feeds up on the web for all of us catch a glimpse of it.
(Images courtesy: Reuters/HM Nautical Almanac Office)
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