Total Lunar Eclipse

In the early pre-dawn morning hours of Tuesday, April 15th, most of the Western Hemisphere will witness a total lunar eclipse.  Totality (when the Earth completely blocks the Sun as seen from the Moon) begins at 3:07 am EDT and ends at 4:25 am EDT.  Those in other time zones can adjust as needed.  The official NASA eclipse information is here.

Solar and lunar eclipses occur when the Sun, the Earth, and the Moon line up so that one body is blocking another as seen from the third body.  A lunar eclipse such as Tuesday night’s requires this sort of alignment.

The terms penumbra and umbra can be confusing, but here is what they mean.  Within the umbra, the Sun’s disk is completely obscured from view by the Earth.  Within the penumbra, only part of the Sun’s disk is obscured.  A total lunar eclipse occurs when the Moon passes into the umbral shadow of the Earth.

Since the Earth is much larger than the Sun as seen from the Moon, the Moon can spend a fairly long time passing through Earth’s umbral shadow.  Tuesday morning’s total eclipse lasts well over an hour, and that’s not even counting the time when the partial eclipse begins (1:58 am EDT) and ends (5:33 am EDT).  The exact size of the shadow depends on the distance between the Earth and the Moon at the time of eclipse, but the difference between the maximum and minimum shadow sizes is not that large.

Total solar eclipses are an entirely different story.  The necessary alignment is this.

As seen from the Earth, the Sun and the Moon are very nearly the same apparent size.  Another way of saying this is that the umbral shadow of the Moon is quite small when seen from the Earth.  Total solar eclipses do not last long (the longest possible is around seven minutes), and totality occurs only along a narrow strip of Earth’s surface.  One generally has to travel to see a few fleeting moments of the Sun’s being blocked out during the day.  But speaking as one who has done so, the sight is well worth the effort.

The good news is that lunar eclipses are much more accessible.  And if you don’t want to stay up that late or get up that early next week, you can mark your calendar for the night of September 28th, 2015.  A total lunar eclipse will begin that night at 10:11 pm EDT, and end at 12:27 am EDT the next morning.

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Posted in Sky Phenomena, The Moon

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