Skywatch Line for Friday, September 25, through Sunday, September 27, written by Alan French

This is Dudley Observatory’s Skywatch Line for Friday, September 25, through Sunday, September 27, written by Alan French.

This is an exciting weekend for amateur astronomers and casual sky watchers – we have a total lunar eclipse on Sunday night. The eclipse is well timed for the eastern United States, with the eclipse visible from start to finish. 

When the Moon is full it is on the opposite side of our Earth from the Sun. Earth casts a shadow into space, and usually the Moon passes above or below the shadow and we simply enjoy a lovely full Moon. When the Moon moves through the shadow, we have a lunar eclipse. On Sunday night the Moon will move completely into the darkest part of the shadow, so we’ll have a total lunar eclipse. 

The Earth’s shadow is composed of two parts. The lighter, outer penumbra is where Earth only blocks part of the Sun’s light. The darker, inner umbra Is where the Earth blocks all of the Sun’s direct light. Light, however, is bent or refracted as it travels through our atmosphere. It is also scattered, with the blue end of the spectrum scattered the most. The result is that the light bent into the darker part of our planet’s shadow is mostly red, so a totally eclipsed Moon may appear reddish. How colorful and how bright the eclipsed Moon appears depends on how many clouds are around the edge of the Earth, as seen from the Moon, and how many particulates are in our atmosphere. (Forest fires, volcanoes, and man-made pollution can darken an eclipse.) The Moon is illuminated by the sunrises and sunsets around the Earth. 

The Moon will enter the penumbra, the brighter part of the shadow, at 8:12 pm, but it will be some time before the subtle penumbral shading will be visible on the Moon. You might spot it around 8:40 pm. 

The Moon will move into the umbra at 9:07 pm and this darkening will be almost immediately obvious. By 10:11 pm the lunar orb will be entirely in the umbra and the eclipse will be total. The Moon will be in the southeast. The Moon will be deepest into the umbra at 10:47 pm and will start leaving the umbra at 11:23 pm, ending totality. It will slip completely out of the umbra at 12:27 am Monday morning, and the last subtle penumbral shading will be visible around 12:55 am. At 1:23 am the Moon will be completely out of the penumbra and the eclipse will be over. 

As the Moon moves fully into and through the umbra look for color, perhaps red or copper. Also notice the darkness of the eclipsed Moon. The northern part of the Moon is deeper into the umbra, so it will probably look darker. 

No special equipment is needed to watch and eclipse. It is lovely by eye. It can be interesting to watch with any modest optical aid too – binoculars or a small, lower power telescope. 

 If you’d like to join other people and have telescopic views of the eclipse, consider going to miSci for Dudley Observatory’s Night Sky Adventure: Total Lunar Eclipse. There is a modest admission charge.