February sky at night - click on the image below to see a larger version

Seeing the most spectacular of this month’s celestial events will need a certain amount of dedication — the only total lunar eclipse visible from the UK this year happens in the early hours of February 21. A lunar eclipse lacks the once-in-a-lifetime excitement of a solar eclipse but it is an intriguing sight none the less. It also has the advantage of being easy to observe; as long as the skies are clear you can’t fail to catch the show.

A lunar eclipse occurs when the Sun, Moon and Earth all lie in a straight line, with the Moon in the shadow of the Earth. Although the Moon completes an orbit around the Earth every month, the angle of its orbit makes means that everything lines up only once or twice a year.

This time round the Moon begins to enter the Earth’s shadow just after midnight, at 00.35. At this point only a slight darkening, easily missed if you do not know to expect a change, will be visible. Observers on the lunar surface would see the Earth begin to cover the Sun, but plenty of light is still reaching the Moon. Just over an hour later, at 01.43, the Moon enters the umbral region, where the Sun is completely blocked by the Earth. From this until the total part of the eclipse begins, at 03.01, an increasingly large dark bite will appear to have been taken out of the Moon.

During the 51 minutes of totality no light can pass directly from the Sun to the Moon, and so you might expect it to disappear from view completely. However, some sunlight is bent by the Earth’s atmosphere, and we see the Moon during totality dimly lit by this refracted light.

The appearance of the eclisped Moon thus depends on the state of the Earth’s atmosphere. In the past particularly dark eclipses have followed large volcanic eruptions. The Moon in eclipse is most often described as being a deep, blood-red colour, but last year’s lunar eclipse, in March, was at the opposite end of the scale, with the Moon appearing a salmon-pink colour. I predict more of the same this time round, helped by the Moon’s track being near the edge of the shadow rather than through the centre, but the only way to be sure is to go out and look.

With the total phase over, the colour will quickly fade and the sequence of events that led to totality will be seen in reverse; the Moon leaves the umbra at 05.09 and the eclipses finally finishes at 06.17, just in time for those going early to work.

You can use the Moon as a pointer to this month’s pre-eminent planet, Saturn. On eclipse night the Moon will be just below a pair of bright stars. The one on the right is Regulus, the brightest star in the zodiacal constellation of Leo, and the one on the left is Saturn, joining Mars in the evening sky. Although the red planet is now well past its best and fading fast, it is unmistakable just after sunset as the only really red object in the sky.

Another fine sight this month is the thin crescent of the extremely young Moon on February 8. Wait until after the Sun has set (17.00 from London; slightly earlier if you are further north), and then scan the horizon in the west-southwest to try and spot it; binoculars will help, but be extremely careful that the Sun is out of the sky before you resort to any kind of optical aid. You have two hours between sun- and moonset, and around 18.00 might give you the best chance of catching the Moon. If you are lucky, you might see the “old Moon in the young Moon’s arms”, with the rest of the Moon’s disk shining faintly next to the brilliant crescent.

Most obvious when the Moon is a thin crescent, this is the phenomenon of earthshine. Light reflected from the Earth’s surface hits the dark part of the Moon’s surface and indirectly lights it. While normally just a curiosity, earthshine was used in one of my favourite recent observations to test our ability to detect alien life. Astronomers have been remarkably successful in detecting planets around stars other than our own Sun, and would like one day to be able to observe their atmospheres in search of the signs of life.

To practice, they have been looking at Earthshine and trying to detect the signature of life on Earth in the reflected light. The presence of large quantities of oxygen in the atmosphere is a telltale sign, and vegetation covering the land’s surface should also show up in the spectrum. The good news from the observations? There is definitely life on Earth, although it is not yet clear from their results whether it is intelligent or not.