On Thursday, May 15, backyard astronomers have their first chance of two chances this year to see one of the most easily identified celestial beauties, a total lunar eclipse.
A lunar eclipse occurs when the Earth is directly between the Sun and Moon. It always occurs during a full Moon, but because the Moon's orbit of Earth is tilted about 5 degrees from the Earth's orbit of the Sun (aka the ecliptic), an eclipse only occurs when the two orbits cross. When that happens, the Moon passes through the Earth's shadow, and there is an eclipse.
Actually, there are two parts to the shadow. The darkest inner circle is called the umbra. There is also a less-dim ring surrounding the umbra, called the penumbra. The penumbral portion of the eclipse is subtle and difficult to see, even with a telescope. The umbral portion, though, is quite easy to view. When only part of the moon passes through the umbra, there is a partial eclipse. This week, though, the entire moon will move through the umbra, causing a total eclipse.
As the Moon enters the umbra, Earth's shadow will cut into it, causing the lit crescent to become smaller and smaller, but still brightly lit by the Sun. Surprisingly, when the shadow overtakes the Moon, the Moon doesn't disappear in darkness. Instead, the full Moon turns red - perhaps copper, orange, or rust. This happens because some sunlight passes through Earth's atmosphere and is refracted - bent - so it illuminates the Moon. The atmosphere absorbs most of the blue in the light, causing the reddish tint during totality.
The entire eclipse will be visible east of a line running roughly from North Dakota to New Mexico. Further west, the eclipse will already have begun by the time the Moon rises, but a portion of the total phase will be visible, as will the exit. The Moon will rise in the southeast quadrant of the sky shortly after 8:00 PM. The partial eclipse begins at 10:03 PM EDT (9:03 CDT, 8:03 MDT.) Totality begins at 11:14 EDT (10:14 CDT, 9:14 MDT, and 8:14 PDT - just after Moonrise on the west coast) and will last about 52 minutes. When totality ends, there will be another partial phase of one hour and eleven minutes, until the Moon has moved fully out of Earth's shadow.
The eclipse is visible from city locations, but if you can view it somewhere without man-made lighting, you will also get to see just how bright a full Moon is. Longtime city dwellers do not always realize that a full Moon is bright enough to cast shadows, or that the light of a bright Moon washes out the view of dimmer stars and other celestial objects. If you have the opportunity to view the eclipse from a dark site, notice what you can see during totality, and compare it to what is visible when the Moon is fully lit by the Sun. And if you have binoculars, take them along so you can get an up-close view of the action.
This is also a good opportunity to look for Jupiter and Saturn. The ecliptic runs roughly west- northwest from the Moon. Jupiter is almost overhead, and Saturn is just above the west- northwest horizon. Saturn will be leaving the night sky soon. If you have a telescope, or know someone with a telescope, look at Saturn while it is still tilted to show off the rings to their best advantage.
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