arcsecond
/ ARK-sek-und /
When two stars sit very close together in the sky, or when a nearby star shifts ever so slightly because the Earth has moved, the angles involved are far too tiny to express in degrees. Astronomers need a much finer unit for these whisker-thin angles, and that unit is the arcsecond.
An arcsecond is one 3600th of a degree: there are 60 arcminutes in a degree, and 60 arcseconds in an arcminute, so 1'' = 1/3600 of a degree. To feel how small that is, one arcsecond is roughly the angular size of a small coin seen from about 4 kilometres away, or a human hair viewed from about 10 metres. The full Moon, half a degree across, spans about 1800 arcseconds — so a single arcsecond is about one two-thousandth of the Moon's width. Finer still, astronomers use milliarcseconds (thousandths) and microarcseconds (millionths) for the most precise measurements.
The arcsecond is the working unit of high-precision astronomy. It defines the parsec (the distance giving a one-arcsecond parallax), sets the scale of a telescope's resolving power and of atmospheric blurring (good ground-based 'seeing' is about one arcsecond), and is the size of the parallax wobbles that anchor the distance ladder. Pushing measurements down to milli- and microarcseconds is precisely what let modern surveys map the Milky Way in three dimensions.
Even the nearest star beyond the Sun, Proxima Centauri, shifts by only about 0.77 arcseconds of parallax across a year — less than half the width of one of these letters seen from across a room. Measuring such a tiny angle is the first rung of the distance ladder.
One arcsecond is a minuscule angle — yet it sets the scale of cosmic distance.
The symbol for arcsecond is a double prime ('') and for arcminute a single prime ('); these are not the same as quotation marks, and not the same as the seconds and minutes of clock time, despite sharing the words.