Home > General Astronomy > Arcminutes and Arcseconds  Arcminutes
and Arcseconds  In a previous post I described how to measure
degrees of arc in the sky, but a degree is quite a large
distance when measuring angle, and so is usefully split into
smaller angles, the arcminute being 1/60 of a degree, and the
arcsecond being 1/60 of that, or 1/3600 of a degree.  One
arcminute is written 1′, and one arcsecond is written 1″ (i.e.
1° = 60′ = 3600″)  So exactly how big are these tiny angles?
We can use high school trigonometry to work out what size of
object covers (subtends) one arcminute:  working out angle  How
big is one arcminute?  In the above diagram, a is our angle,
1/60 of a degree, d is our arm’s length distance (say 60cm or
0.6m), and therefore h/2 = d x tan a = 0.6m x tan (1/60) =
0.000175 m, and so h = 0.00035m, or 0.35mm.  Therefore an
arcminute is the angle subtended by a piece of medium thickness
card (0.35mm, or 350 micrometer, card is often used to make
birthday cards) held at arm’s length. That’s