https://www.timeanddate.com/astronomy/measuring-the-sky-by-hand.html

To get started, hold your hands at arm’s length in front of your face.

Now raise your little finger. The width of the tip of your little finger at arm’s length is about one degree. So if you are looking for an object that is one degree away from Jupiter, for example, place your little finger next to Jupiter and you get the approximate distance to the object.

Raise your three middle fingers to measure 5 degrees.

Clench your fist or hold your hand like a policeman stopping traffic to measure 10 degrees.

Now stretch your thumb and little finger as wide as you can (one full hand-span) to measure about 20 degrees.