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Total Lunar Eclipse; a chance for skywatchers to ‘see red’

 

In the morning of 28 September 2015, skywatchers across the UK will be able to see a total eclipse of the Moon. This spectacular event is easy to see and also fairly rare, with the next chance to catch one from the UK not until January 2019. A total solar eclipse occurs when the Moon passes exactly between the Earth and the Sun, causing the light from the Sun to be blocked out entirely. A total lunar eclipse occurs when the Earth passes exactly between the Sun and the Moon. The Sun is behind the Earth, and the Moon moves into the Earth's shadow. The Moon doesn't create its own light, but reflects the Sun's rays, so when the Moon is in the shadow of the Earth the Sun's rays are blocked and it enters eclipse.

solar lunar eclipse diagramA solar eclipse occurs in the day time at new moon, when the moon is between the Earth and the sun, while a lunar eclipse occurs at night when the Earth passes between the Sun and the Moon. CC Wikipedia.

The Moon first darkens slowly as it moves into the penumbra of the Earth, the lighter part of the shadow. When the Moon is completely within the umbra, the darker part of the shadow, it takes on a red hue that varies in colour.

Sometimes the eclipsed Moon is a deep red colour, almost disappearing from view, and sometimes it can be quite bright. The colour is due to Rayleigh scattering – where the Sun's blue light is scattered off molecules in the Earth's atmosphere – which also happens at sunsets. The Sun's red light is scattered much less by air, and is bent by the Earth's atmosphere in a process called refraction, travelling all the way through it to light up the Moon's surface.


There are different types of lunar eclipse but a total eclipse is the most spectacular and is the only type that causes the Moon to appear red.

red moonPhotograph of the total lunar eclipse in 2007. Courtesy of William Stewart. http://www.nemetode.org/. @nemetode

 

Lunar eclipses occur several times a year and take place as often as solar eclipses, if all the different types are taken into account. Whereas though you can only see a total solar eclipse if you are in the narrow path of the Moon's shadow, lunar eclipses are visible wherever the Moon is above the horizon at the time, so each one can be seen from a large area of the Earth. For that reason they are much more common from any given location.


Lunar eclipses always happen at a full Moon as this is when it moves behind the Earth and into line with the Earth and Sun. But whilst there is a full Moon every month we do not see a total lunar eclipse each time because the plane of the Moon's orbit around the Earth is slightly inclined to the plane of the Earth's orbit around the Sun. Eclipses only take place when the Moon, Earth and Sun line up in three dimensions.

lunar eclipse stagesThis excellent graphic shows what to expect from the stages of the lunar eclipse as viewed from the UK on the 28th September. Courtesy of @Mars_Stu. cumbriansky.wordpress.com.

 

The coming total lunar eclipse will be visible across the whole of the UK between 1.11am and 6.22am (BST) on the 28th of September. The full eclipse (totality) begins at 3.11am, with mid-eclipse at 3.47am and ends at 4.23am – during this hour the Moon will appear red.

 

In the UK the Moon is above the horizon for the whole eclipse so, weather permitting, it should be visible in its entirety. The eclipse lasts 5 hours 11 minutes from start to finish, and the total eclipse (when the Moon appears red) will last 1 hour 12 minutes.


Lunar eclipses are very easy to witness as no special equipment or safety precautions are required. Solar eclipses are dangerous because observing the Sun directly can damage your eyesight, but the light from a lunar eclipse is much fainter and so is completely safe.

lunar eclipse 2007
This photograph shows the progression of a total lunar eclipse, shot in March 2007. Courtesy of Peter Simpson. @postcardcv. blueskybirds.co.uk.

 

 

 

 

 

 

In the early hours of the 28th of September all you have to do is dress warmly and go outside! If you can see the full moon you will be able to observe the eclipse as it happens. If you want a close up view of the Moon as it turns red, a pair of binoculars is helpful. Because though lunar eclipses are easy to watch with the unaided eye, you can just go outside and enjoy the view!

 

Useful links

A global visibility plot can be found here:

http://astro.ukho.gov.uk/eclipse/1412015/

A nice animation of how the total lunar eclipse will look from the UK:

http://www.timeanddate.com/eclipse/in/uk/london