Measure Distances : A Handy Guide to Measuring the Sky

 A “Handy” Way to Measure Distances

Hold your hand at arm’s length and close one eye.

  • Make a fist, with the back of your hand facing you. The width of your fist will approximately be 10 degrees. This means that any two objects that are on the opposite ends of your fist will be 10 degrees apart. The North Star (Polaris) and Dubhe, one of the northern pointers of the Big Dipper are 3 fists apart. This means that angular distance or angular separation between the two stars is 30°.
  • Open up your fist, stretch your little finger and thumb as far as you can and curl down the rest of your fingers. The tip of your little finger and your thumb will span about 25°. The Big Dipper spans around 25°.
  • The tip-to-tip span between your index finger and your little finger is 15°.
  • Your three middle fingers will span about 5°.
  • Your little finger at an arms length is about 1° wide.

It is important to note that such measurements are approximate – not everyone has the same sized hand.

With these simple measuring rules in your hand, you can not only understand basic stargazing jargon but also tell other budding stargazers where to look for a specific celestial object in the sky.

Angular Size

Angular size or angular diameter of a celestial object is the angular separation between opposite edges of the object. The Sun and the Moon are the only objects in the sky whose angular size is visible to the naked eye.

Remember to never look at the Sun directly without any eye protection!

The angular diameter of a full Moon is about 30', while the angular diameter of the Sun is around 32'.


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