Lunar Eclipse 28 September 2015: The Blood Supermoon

Stargazers in the UK are ideally placed to see a rare astronomical event next week, a Total Lunar Eclipse. While not as dramatic as a Total Solar Eclipse, a lunar eclipse is well worth watching for, as the Moon turns deep red at totality.

A Blood-red Lunar Eclipse

Unlike a Total Solar Eclipse, where totality lasts only a few minutes, a total eclipse of the Moon last several hours. In the morning hours of Monday 28 September the lunar eclipse begins at 0111BST and ends at 0622BST as the Moon sets. During the very early and late hours of the lunar eclipse you will see part of the full Moon’s disk darken, but it’s only when the Moon enters totality that it will turn red. This dramatic event will happen between 0311 and 0423BST.

This month’s lunar eclipse is made even rarer by the fact that the full Moon on 28 Sep is what’s called a Supermoon. This means that the Moon is closer than normal to Earth, and will appear slightly larger and brighter in the sky. But don’t believe the hype: it will be only a few % closer and so your eye will not be able to detect the difference between this Supermoon and any other Full Moon – except this time it’ll be blood red due to the eclipse! (The Moon may actually look bigger to you if you catch it low on the horizon, but that’s due to the Moon Illusion).

The brilliant XKCD sums up the Supermoon hype nicely

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 cones: 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.

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 5 hours 11 minutes, and 72 minutes of this is spent entirely within the umbra, i.e. in total eclipse.

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 UK is ideally placed to view this total lunar eclipse, although you will have to stay up very late, or get up very early. The Moon is in the sky for the entirety of the eclipse. Observers in western Europe, NW Africa, E North America, and South America will all see the full eclipse from beginning to end.

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