The Beginners Series – Measuring and Finding Stuff

Being able to describe where to find stuff in the sky is a very handy way of learning the night sky. You don’t need an expensive piece of equipment, just your arm and the hand at the end of it.

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award-winning photographer Alex Conu

The first time I got a telescope and pointed it at the sky I used it to view the Moon, why? Because the Moon was the easiest thing to find – it’s very bright and there’s only one of them. Plus it looked super amazing seeing the mountains and the craters, the telescope gave me the opportunity to see detail on another world (moon). It was an amazing experience to see such an usual landscape pop out of the eyepiece. That was the Moon, very easy to find, can’t really miss it, so what about the stuff that’s a bit harder to find. We looked at a coordinate system a couple of days ago which is how the sky is organised into a system where things can have their location described consistently. Now the trick is to use it!

Lucky us humans have hands that make for very useful astronomical instruments. Our hands are remarkably consistent when held at arms length and used to measure angular distances in the sky. It’s because all of our arms are different lengths and generally if you have small hands, you probably have short arms so your hands have roughly the same apparent size as everyone else when they hold their hands at arms length. This is quite useful because a little finger is about 1 degree in width, which is about twice as big as the full Moon. The full Moon is about 0.5 degrees across.

A bad photo of my little finger – 1 degree

If you stretch your little finger as far from your thumb as you can then the distance from the tip of your little finger to the tip of your thumb is about 20 degrees. If you do the same thing with your index finger and little finger then the distance from little finger tip to index finger tip is about 15 degrees. So three of those up from the horizon is about 45 degrees. It the same horizontally as well.

20 degrees – one day I will learn how to focus the camera
15 degrees

If you clench your fist at arms length and hold the back of your fist towards you then that distance is about 10 degrees.

10 degrees

If you put your three middle fingers together then the combined width of them when you arm is fully stretched is about 5 degrees.

5 degrees

So these techniques are quite handy when thinking abut how long things take to cross the sky. We know that it takes 24 hours for the world to spin around 360 degrees so that means that the sky will track along at 15 degrees an hour (the stretched index finger tip to the little finger tip at the end of a fully stretched out arm).

So if you want to know what time the sun is going to set, and you roughly know where it is going to set, you can measure the distance. Because you know that index to little finger tip is one hour and one little finger width is 4 minutes (it takes the 4 minutes for the sky to move 1 degree) you can work out that a fist is 40 minutes and a thumb tip to little finger tip is 80 minutes. Knowing all of this you can then figure out what time the sun will set. A good skill if you get lost, you can do this for any star, Moon or the planets as well.

Now you know how to measure where stuff is in the sky so when you are using your planetarium software on your fancy smart phone (with the red filter on so you don’t wreck your night vision) and it says something is 23 degrees above the horizon – you’ll be able to easily find it. Go forth and try it out!