Those two stars.
How far apart are they?
Not in centimetres. They are not painted on a wall — one of them may be a hundred times further away than the other.
Then how?
In angles. All the way round the horizon is 360 degrees, and from the horizon to straight overhead is 90.
Hold your arm out. That is your ruler.
My paw is smaller than yours, though.
And your arm is shorter. The two cancel out, so it comes to about the same angle for almost everybody.
It has to be a straight arm. Bend your elbow and every number below is wrong.
| What you hold up | Roughly | Something that size |
|---|---|---|
| Little fingernail | 1° | two full Moons side by side |
| Three fingers together | 5° | the two pointer stars of the Plough (the Big Dipper) |
| Closed fist | 10° | the bowl of the Plough |
| Hand spread wide | 20° | Orion, from shoulder to foot |
| Horizon to overhead | 90° | nine fists stacked up |
From the northern half of the world, these three are up there most of the year.
Start with the Moon. It is enormous.
Hold your little finger up in front of it.
Your fingernail wins. The Moon is half a degree across, and it hides behind half a fingernail. Every giant Moon you have seen in a photograph was taken with a long lens.
…That cannot be right. I have SEEN it.
Everybody says that. Go and do it tonight, and you will believe your own hand instead of me.
What about something bigger?
Find the Plough, the seven stars some people call the Big Dipper. The two stars at the end of its bowl are the ones that point at the Pole Star, Polaris, and three fingers should just cover the gap between them.
Then hold up your closed fist against the bowl itself. Your fist should fit it almost exactly.
And if my fist is too small?
Then you have measured your own hand against the sky. Write down what yours is really worth and use that number from now on.
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Space