Monday, 21 September 2015

Lunar Eclipse - 27th/28th September 2015

Lunar Eclipse - 27th/28th September 2015

Overnight on 27th/28th September, there is a total lunar eclipse. This is the first total lunar eclipse visible in its entirety from the UK since 2008 and the next one won’t be until 21st January 2019. Because the Moon’s orbit is elliptical rather than circular, at certain times of the month it is closer to Earth than others. The point where it is furthest away is called apogee, the point when it is at its closest is called perigee. This total lunar eclipse is special because it occurs when the Moon is a perigee.

When the Moon is at perigee, its diameter is about 14% larger, and it appears about 30% brighter. The picture below shows the relative sizes of the Full Moon at apogee and perigee (Source: Wikipedia)

Perigee Full Moons are quite common, but to have an eclipse coincide with one is much rarer. The last time this happened was 1982, and it won't happen again until 2033!

There are two kinds of eclipse, lunar and solar. Both are dependent on the Earth, Sun and Moon aligning in a particular way, with one body being affected by the shadow cast from another body. This special alignment is called syzygy.

Lunar Eclipses

A total lunar eclipse (sometimes called a “Blood Moon” occurs when the Moon enters into the shadow of the Earth which is cast by the Sun shining behind it.  Because of the relative size of the Earth compared to the Moon, lunar eclipses last several hours from first contact to last contact.  

The diagram above shows the alignment of the Sun, Earth and Moon during a lunar eclipse.  Because this exact alignment is required, a lunar eclipse can only occur during a Full Moon. As a total lunar eclipse begins, the Moon first passes into the penumbra. This is called first contact. At this stage the Moon looks slightly darker or greyer.  The next stage is second contact, when the Moon enters into the Umbra, and at this point it will begin to look like a chunk of the Moon is missing as the well defined umbral shadow of Earth passes across the lunar surface. This shadow will slowly get bigger until the Moon passes into the totality phase of the eclipse.  Due to the refraction of light from the sun by Earth’s atmosphere, the Moon usually appears red in colour during this period, but atmospheric conditions at that time will determine exactly how the Moon will look during totality, ranging from copper, burgundy or orange. This totality phase lasts around 1 hour 40 minutes. Next comes third contact, when the Moon begins to leave the umbra, and the large umbral shadow will begin to get smaller as the Moon passes through the other side of the penumbral shadow.  Finally comes fourth contact, when the Moon leaves the penumbra and its colour returns to normal.  During a total lunar eclipse, totality lasts around 1 hour 40 minutes, but the whole event lasts several hours.  A lunar eclipse is visible from anywhere on Earth that is facing the Moon at that time.

If you are planning to observe the lunar eclipse, here is a summary of the timings of the event from the UK:

1:11am BST - The Moon first moves into the penumbral shadow, when the Moon is around 40 degrees above the Southern Horizon. If conditions are good then half an hour later you will be able to detect a reduction in brightness on the western limb.  2:07am BST - The Moon will then touch the umbra. At this stage the curved, well defined shadow of the Earth’s edge will begin to move across the western limb. 3:11am BST – The total eclipse begins 3:47am BST – Maximum eclipse occurs when the Moon is 30 degrees above the south western horizon. Depending on the local conditions, the Moon will appear red/brown 4:23am BST – The Moon’s western limb will begin to emerge from the umbra 5:27am BST – The Moon will completely emerge from the umbra 6:23am BST – The Moon will completely emerge from the penumbra

There is a Full Moon every month, so why don’t we get a lunar eclipse every month? This is because the Moon’s orbit around the Earth is tilted at a slight angle, so the correct alignment only occurs approximately twice a year. But that does not mean it will be visible from your location. The exact time of Full Moon also varies every month so from any given location it may occur during daylight hours when the Moon has already set. In order to see a full lunar eclipse, you need several factors to come together. The Moon must be exactly Full during night time hours as well as having the correct syzygy. And of course you need the clouds to stay away!

It does mean staying up all night, but if we have clear skies at the weekend, this should prove to be a great event so make sure you tell your friends!