ELi on Earth: Total Eclipse of the Moon Sunday, Sept. 27

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Monday, September 21, 2015, 7:00 am
Robert Victor

Image: "Eclipse 2010 cropped" by John Vermette 

UPDATE! We have a special offer for ELi readers. Be one of the first five people to identify yourself to the person in the ELi t-shirt at the parking ramp gathering on Sunday, September 27, and get a free ELi mug!

Sky watchers in the eastern U.S. have a chance to enjoy a total eclipse of the Moon on the evening of Sunday, September 27.

The event starts as a partial eclipse as the Moon enters the Earth’s umbra, or dark central core of Earth’s shadow, Sunday evening at 9:07 p.m. EDT. Not long after the partial eclipse gets underway, the circular edge of Earth’s shadow will become evident.

The Earth’s diameter is nearly four times the Moon’s. But the Earth’s inner shadow cone, for this eclipse, converges at the Moon’s distance to a dark circle not quite three times as large as the lunar disk. That’s still big enough for the Moon to fit inside, with lots of room to spare. The Moon will spend 72 minutes in total eclipse, completely enveloped in the umbra.

In the opening partial stages of the eclipse, as more of the Moon becomes immersed in shadow, the reddish color of the shadow will become evident. The reddish color is a result of sunlight that has passed through the Earth’s atmosphere and been refracted, or bent, into the Earth’s shadow cone. When light passes through Earth’s atmosphere, most of the bluer light (of shorter wavelengths) gets scattered by molecules in the air, and most of the red light passes through.

Total eclipse begins at 10:11 p.m. EDT Sunday evening, when the Moon first becomes completely immersed in the umbra. Even so, the Moon may have a bright edge to it, at the lower right. The shadow’s outer edge is usually much brighter than the center, which receives sunlight that has passed deep within Earth’s atmosphere on its way to the Moon.

Deepest eclipse occurs at 10:47 p.m. EDT, when the northern edge of the Moon comes closest to the center of Earth’s shadow.

Total eclipse ends at 11:23 p.m., when the Moon’s lower left limb begins to come out of Earth’s shadow.

The Moon’s withdrawal from the umbra is complete, and the concluding partial eclipse is over at 12:27 a.m. EDT.

For perhaps half an hour before 9:07 p.m. and after 12:27 a.m., a barely dusky shading – the penumbra, or region of partial shadow – may be noticed on a portion of the Moon’s disk.

The brightness and color of the Moon during a total eclipse varies widely from one eclipse to another, depending on atmospheric conditions over places on Earth where the Sun is rising or setting at time of eclipse. Sunlight must pass through these zones in order to reach the Moon during total eclipse, and presence of clouds in our lower atmosphere can block much of the sunlight and darken Earth’s shadow. For instance, the great volcanic eruptions of 1963, 1982, and 1991 were each followed by exceptionally dark total lunar eclipses due to the haze they produced.

This eclipse is special for several reasons. First, the eclipse occurs during convenient evening hours for the contiguous 48 United States, Mexico, and southern Canada.

It is also the last total lunar eclipse for some time. The next total lunar eclipse that will be easily seen in Michigan will be in the middle of the night on January 20-21, 2019.

This is also the fourth and last event of a tetrad, four consecutive total lunar eclipses at intervals of 6 lunar months. The next tetrad will take place in 2032-2033, but only the final member of that series, on October 8, 2033, will be seen in Michigan, just before dawn.

This current eclipse coincides with the closest approach of the Moon during this year, sometimes referred to as the “Supermoon.”

Interestingly, this is both the faintest Full Moon of the year -- around mid-eclipse, at 10:47 p.m. EDT -- and the brightest -- around the time the Moon enters and leaves the penumbra at 8:12 p.m. and 1:23 a.m. The reason for the brightening when the Moon is just outside Earth’s shadow is the “opposition effect” where the Moon reflects light most effectively back toward the Earth. Stand between a reflective road sign and a light source and note the bright halo around the shadow of your head, and you’ll see the same effect!

Early in the evening, before the eclipse begins, watch for the Full Moon rising a few minutes before sunset. From a vantage point with unobstructed views toward east and west, it will be possible to observe the Sun and the Full Moon simultaneously. From East Lansing, this viewing opportunity occurs at 7:23 p.m.

If you’re in the area, join us on the top level of the parking ramp behind Abrams Planetarium by 7:15 p.m. if skies are clear, and we’ll observe the rising Moon and setting Sun. Take the elevator from the southwest corner of the ramp next to the CATA Transit Center behind Abrams Planetarium. Get off at the top level (4) and join us!

At 7:30 p.m., rain or shine the Abrams Planetarium is offering a free sky preview of the eclipse and the wonderful planetary gatherings in October’s morning sky.

If the sky is clear, a viewing of the lunar eclipse in front of the Abrams Planetarium will follow starting at 9:00 p.m. to track the progress of the eclipse until at least 11:30 p.m. 

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