Angles

13.10 - Understand that an angle of one degree (°) comprises 60 minutes of arc (arcmin) (60’) and that each arcminute is comprised of 60 seconds of arc (arcsec) (60”)

Astronomers measure diameters of objects and the amount of sky that separates them from others in 'degrees of arc'. So imagine the sky is 360° around the horizon. A maritime navigator might use degrees on a boat to measure the difference between the horizon and the sun. What's smaller than a degree? Why an arc minute of course. This is one sixtieth of a degree. What if we want to measure smaller than that? Well we have arc seconds which are one sixtieth of an arc minute. That means there are 3600 arc seconds in a degree.

We can measure the approximate differences in degrees with out outstretched hands against various asterisms and constellations

Want to show off? Milliarseconds and Microarcseconds are even smaller. So small we can ignore them for now.

By the way, sometimes these words are combined e.g. arcminute and sometimes separate e.g. arc minute. Both appear to be correct so I use both all over the place.

 

Animation
Summary
Table describing the detail of degrees, arc minutes and arc seconds
Unit Value Symbol
Degree
1
360
of a circle
 °
Arcminute
1
60
of a degree
 ′
Arcsecond
1
60
of an arcminute
 ″
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