Dark Sky Diary

The first total eclipse of the Moon of 2011 occurs this Wednesday evening, 15 June 2011, and it will be the longest lunar eclipse in over a decade. However the views from the UK (and Europe) will be constrained by the fact that the Moon will be below the horizon for much of the eclipse, and will rise fully eclipsed, or in some cases even coming out of eclipse. It’s still worth having a look though: just try to find somewhere with a very low and clear SE horizon, as this will be the direction in which the Moon will rise, and it will be in eclipse only while it is VERY low (only a few degrees above the horizon).

Lunar Eclipse, December 21, 2010, by Jiyang Chen

A lunar eclipse occurs when the Moon, in its orbit around the Earth, passes into the Earth’s shadow, as cast by the Sun. You might imagine that this would happen once every lunar orbit, or once a month. That it does not is due to the fact that the Moon’s orbit around the Earth is tilted by around 5 degrees compared with the Earth’s orbit around the Sun. So in most orbits the Moon passes above or below the Earth’s shadow.

However, once in a while (there are at least two lunar eclipses each year) the orbital planes will align so that the Moon passes through the Earth’s shadow, sometimes just grazing it, in which case we get a partial lunar eclipse, and at other times passing right through the shadow, when we get a total lunar eclipse.

The Earth’s shadow has two distinct regions, forming two concentric circles [correction, the shadow is two cones, not circles – need to think in 3D! (see comments)]: the inner, darker, part of the shadow is called the umbra, and objects within this part of the shadow receive no direct light from the Sun. The outer, lighter, part of the shadow is called the penumbra, and objects within this part of the shadow can receive direct light from the Sun, but part of the Sun’s disk will be obscured by the Earth, and so less light than normal falls on the object.

The umbra and the penumbra

For the lunar eclipse of 15 June 2011 the Moon will pass very deeply into the darker umbra, making this an especially dark – and long – eclipse.

There are several distinct phases of a lunar eclipse, as the Moon travels through the penumbra and umbra. For this lunar eclipse the total time during which the Moon is at least partially in the Earth’s shadow is 219 minutes, and 100 minutes of this is spent entirely within the umbra, i.e. in total eclipse.

The phases of the 15 June 2011 total lunar eclipse (via NASA's eclipse website)

These phases are given the names: P1, the time when the Moon’s disk enter the penumbra; U1, the time when the Moon’s disk enters the umbra; U2, the time when the entirety of the Moon’s disk is within the umbra; U3, the last time when the entirety of the Moon’s disk is within the umbra; U4, the last time when part of the Moon’s disk is within the umbra; and P4, the last time when part of the Moon’s disk is within the penumbra.

The timings for these instances are well known. In the following table are: the timings for P1, U1, U2, U3, U4, and P4; the time of greatest eclipse (i.e. where the Moon is closest to the centre of the umbra); and the local times of moonrise for a variety of places around the UK

UT = Universal Time = GMT
BST = British Summer Time = GMT+1

As you can see, the UK is far from ideally placed to view this total lunar eclipse, but the further south and east you are the better your chances of seeing something. The Moon will rise well past U2 across the UK, and everywhere except the Channel Islands and SE England it will rise well past greatest eclipse. Indeed in the north of Scotland the Moon will rise after the total eclipse phase ends (i.e. past U3).

Observers in the Channel Islands and in the SE of England will have around 50 minutes of total eclipse to observe although the Moon will still only be a few degrees above the horizon at U3), and here in Glasgow I’ll have about 4 minutes between moonrise and U3!

While in total eclipse no direct sunlight will fall on the Moon, but we will still be able to see it illuminated a dull red colour. How can this be, if there is no sunlight shining on it to light it up? It is due to the fact that the Sun’s light is refracted, or bent, through the Earth’s atmosphere. The red light from the Sun’s spectrum is refracted the most, and so it is this light that will illuminate the Moon during a lunar eclipse. In effect, the light you will see on the Moon is the combined light from all the sunrises and sunsets on Earth, being focused onto the Moon by the lensing effect of the Earth’s atmosphere.

A detailed information sheet for this eclipse (and others) is available (pdf) on the NASA Eclipse website.