Tonight I've looking at an optical phenomenon presenting as rather large ring around the moon. I'm thinking it's probably a 22 degree halo, but it seems kind of big... maybe 46.

Anyway, I realised I had no good way of estimating the angle between the moon and the ring. Does anyone know any good rules of thumb for judging visual angles without instrumentation?

I found this, but perhaps there are other ways? Maybe there are some tricks to using it more effectively.


1 Answer 1


Hold your left arm out to the horizon. Hold your right arm vertically upwards. That's 90 degrees. Now bring your left arm up to halfway between the horizon and your right arm. Your arms are now 45 degrees apart.

Now bring your right arm halfway towards your left arm. Your arms are now 22.5 degrees apart.

Compare the angle between where your fingers are pointing with the angle between the objects in the sky to see whether it is a 22 degree halo.

  • $\begingroup$ That's a creative suggestion. Not sure I can execute it (and then move my arms to the right position) even remotely accurately. One can rule out the 46 degree halo with only one halving, or see if the diameter is 92 without any halving at all. $\endgroup$
    – Lucas
    Commented Jun 29, 2015 at 0:47
  • $\begingroup$ @Lucas True. A 46 degree halo would, as you say, have a diameter greater than the angle from horizon to vertical - should be fairly easy to see if this is the case without looking like you're practicing your semaphore. $\endgroup$
    – IanF1
    Commented Jun 29, 2015 at 0:52
  • 1
    $\begingroup$ Luckily, it's dark ;) $\endgroup$
    – Lucas
    Commented Jun 29, 2015 at 0:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.