Observation How to Easily Measure Angles on the Sky

An easy way to measure angles on the sky (e.g. when you learn sky naked-eye, or when you want to narrow a position) is to use the following tricks:

• closed fist, at arm's length is 10° on the sky
• closed fist, thumb stretched, at arm's length, is 15°
• hand, fingers stretched, at arm's length, is 20° between thumb and last finger

For smaller scales:

• fingertip, at arm's length, is 1° on the sky. This, by the way, means that a fingertip may hide Sun -don't try! Sun is dangerous! check at our dedicated tutorial- or the Moon. These two bodies indeed are just ½° on the sky whatever larger their brightness may make them look like. This, by the way, also allows another trick: Moon has a diameter of ½° and this may be used for quick and small angles assessments, or during a Moon or a Sun eclipse
• thumb, at arm's length, is 2.5° on the sky

Celestial hints:

• the Great Dipper's width is 25° from the end of the handle to the last side of the bucket
• the Orion's Belt is 3° wide
• the distance between Pollux and Castor, of Gemini, the Twins, is 5°
• note: one hour of R.A. (right ascension) is worth 15°

Moon has a apparent diameter of ½ degree! picture courtesy site 'Amateur Astronomy'

Here is the occasion to recall some other basic values:

• the celestial sphere is rotating about itself in 24 hours. For a given celestial object, that translates into a motion of 15 degree per hour. This is true for planets and Moon included, the apparent motion of which may be neglected most of the time
• the celestial sphere, each month, is moving of about 1h 30 in right ascension westwards
• the Moon, each day, is moving 12.5 degree against the starry background as it moves by about its own diameter each hour. The Moon, at mid-latitudes, is rising by about 50 minute later than the previous day as that value is varying between 70 and 30 minute along the year (with a maximum reached by January, and minimum in September)
• the Sun, each day, is moving by 0.98 degree against the background of the sky as the Sun is rotating in about 27 days and a quarter about itself

Much useful!