Measuring angles in the night sky

Many of the “What’s Up?” postings will make reference to angular separations of objects in the night sky. For example, you may see a listing such as “Spica 1.8 degrees North of the Moon” or maybe “Venus 3 degrees North of the Moon” or even “Venus is 28 degrees above the horizon”. So how does one measure or approximate these angular distances? Very easily, actually. Just use your fist and fingers; like this:-

Hold your fist at arms length and:-

• Extend your little finger; it’s width is approximately 1 degree.
• Extend your three middle fingers (without the little finger); thats about 5 degrees.
• A clenched fist (thumb to little finger) is about 10 degrees.
• From the tip of the little finger to the tip of the thumb, an extended hand with fingers and thumb splayed subtends about 20 degrees.

Thats it! Those measurements are approximations but are accurate enough to find the objects listed in the “What’s Up?” postings.

Try this:-

The full moon subtends an angle of slightly more than 1/2 degree. On the next full moon hold your hand at arms length and place your little finger over the moon. How much of the moon does your little finger cover? Try this on one of those nights when the full moon is near the horizon and looks really huge! What do you see? Are you surprised?

During a total eclipse of the sun the moon will completely cover the sun so it stands to reason that the sun also subtends an angle of roughly 1/2 degree in the sky. If it were safe to do so you could hold out the tip of your little finger and totally cover the sun! Cool, eh!

Have fun!