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”…

New Telescope for Christmas?

Are you lucky enough to have been given a shiny new telescope for Christmas? If so you have joined the ranks of thousands of other stargazers around the world, and you’re no doubt eager to get outside and use your new toy.

But a new telescope can be quite a complicated and daunting piece of hardware, so let’s go through the basics, to help you on your way.

1. Read the instructions!
Like most complicated pieces of equipment your telescope should have a user manual or a set of instructions. If not, try looking online. These instructions will help you assemble your telescope, and become familiar with all of the various parts, including the finder scope, eyepieces, focus mechanism, and motion controls.

2. Align the finder scope in the daytime.
The small telescope that sits on the main tube of your telescope is called the finder scope. You can use this to locate an object in the sky, and if the finder- and main telescopes are aligned (facing in exactly the same direction) then that object will be in the centre of the field of view of your main scope too. Aligning the finder scope is a bit fiddly though, so do it in the daytime before you observe. Point your main telescope at a specific distant object, like a far away tree, or chimney pot, or transmitter mast. (WARNING: don’t point it anywhere near the Sun). Once that object’s in the very centre of your main scope field of view, look through your finder scope. The chances are it’s not in the crosshairs here, so adjust the position of your finder scope until it is. This is usually done using small screws that physically move the finder scope around till it’s aligned with the main scope. Now it’s ready to use tonight.

3. Pick the correct eyepieces.
Your scope probably came with a couple of eyepieces. These should be marked with their focal length, in mm. The higher this number the lower the magnification. So a 25mm eyepiece will provide smaller images than a 10mm eyepiece. But magnification isn’t everything. In most cases you should start with your least powerful eyepiece, which gives you the largest field of view. Once you’ve found your target you can substitute a more powerful eyepiece in, to get a larger image. It’ll be larger, but dimmer. After all you’re spreading the same amount of light over a larger image. You’ll also notice any wobbles in the telescope much more when you’re using a higher power eyepiece. So your low-power eyepiece will give you brighter, clearer images, even if they’re much smaller.

4. Learn how to move around the sky.
All telescopes are different, and the way that you move them from one object to the other varies. In general though they will all have hand-screws that you can tighten and loosen to lock the telescope in position, or to move it. There may also be dials or screws to give fine adjustments to a telescope’s positioning. On the other hand, some telescopes – called dobsonians – are just moved by physically nudging the telescope tube. However yours moves, you’ll be doing this in the dark, so practice, practice, practice until it’s second nature to you. Some motorised telescopes find and track the stars and planets for you, but these are a bit trickier to set up properly, so read the instructions.

5. Choose the right targets.
Don’t go hunting down very faint elusive objects on your first night out; stick to the brighter ones that are easier to find. On Christmas night and for a couple of days after, the Moon and Jupiter are up in the evening. Even a small telescope will give great views of the Moon (although you’ll get a better view when it’s not quite so full, and you can observe the line between light and dark, called the terminator line) and will let you see Jupiter’s moons, looking like four tiny specks next to the bright planet Jupiter.

Whatever you look at with your new telescope, enjoy it, and remember that patience is a virtue. You’re the proud owner of a great stargazing tool, but you need to practice to get the hang of using it properly.

Let everyone know what you’ve been observing, and how you’ve found your new telescope, in the comments below.