Christmas Full Moon 2015

You might have seen that there will be a Full Moon this Christmas Day, this first time this has happened since 1977.

It’s been described on social media as the “perfect” Full Moon. Quite what that means I’m not sure, but there is one factor that makes this month’s Full Moon stand out against the others; it will be this year’s highest Full Moon. But this is true of the Full Moon that happens nearest the winter solstice every year, so there’s nothing particularly “perfect” about this one.

A Full Moon occurs when the Moon is opposite the Sun in our sky, and we see the entire lit hemisphere of the Moon, making it appear full and round.

After it’s full, the Moon will appear to shrink to a gibbous moon (less than full but more the half), a half moon (called the last quarter moon), then a crescent, then a new moon, before growing again through crescent, half, gibbous and back to full. This pattern repeats itself every 29 days, which is approximately one month.

But because it isn’t exactly a month, and because our months vary in length between 28 and 31 days, the Full Moon doesn’t occur on the same numbered day every month, it drifts, seemingly at random.

For example the last decade’s December Full Moons happened on 15, 5, 24, 12, 2 AND 31*, 21, 12, 28, 17, 6 December.

(* In 2009 there were two full moons in December; the second is colloquially known as a blue moon.)

As the 29-day pattern of phases drifts around the 31 days in December it’s not surprising that the occurrence of Christmas Day full moons is around once every thirty years.

The last time this happened was 38 years ago in 1977; the next time it will happen is 19 years in the future, in 2034; an average of every 29 years.

Observing the Moon

If Santa was generous enough to bring you a telescope for Christmas, the Moon is one of the very best places to start. But you don’t want to observe it when it’s full.

This is mainly due to the overall lack of contrast and shadow on the Moon’s surface when it’s lit head-on by the Sun.

The best views of the Moon can be had when you can clearly see the dividing line between light and dark. This line is called the terminator, and when you observe it through a telescope the Moon appears in 3D, with shadows inside craters and beside mountains that really give you a feel for the structure of the Moon’s surface.

In addition the Full Moon is so bright that it drowns out the light of lots of faint astronomical objects, meaning that you would ideally wait till the moon is New, or a think crescent, before venturing out with your telescope to hunt for galaxies and nebulae.

So for stargazers this Christmas Day Full Moon is very far from “perfect”…