What does arcsecond mean?
An arcsecond is a very small unit of angular measurement, equal to one sixtieth of an arcminute and therefore one three-thousand-six-hundredth of a degree. It is denoted by the double-prime symbol ″ and belongs to the sexagesimal system inherited from ancient Babylonian astronomy, which divides circles into degrees, minutes, and seconds. Arcseconds are indispensable wherever tiny angles matter: astronomers use them to describe stellar parallax, the apparent separation of binary stars, and the resolving power of telescopes; surveyors and geodesists rely on them for precise measurements of the Earth's surface; and optical engineers quote lens and mirror quality in fractions of an arcsecond. Because modern instruments are so sensitive, related units such as the milliarcsecond and microarcsecond are common in professional literature. Though unfamiliar to most people outside science, the arcsecond is fundamental to how we map the sky and measure our planet with precision.
nounA unit of angular measurement equal to one sixtieth of an arcminute, or 1/3,600 of a degree, used chiefly in astronomy, navigation, and geodesy.
- A unit of angular measurement equal to one sixtieth of an arcminute (1/3,600 of a degree), symbol ″, used especially in astronomy, surveying, and optics.
"The Hubble Space Telescope can resolve details as small as a few hundredths of an arcsecond."
"Proxima Centauri shows a parallax of roughly 0.77 arcseconds as Earth orbits the Sun."
"Even under excellent seeing conditions, atmospheric turbulence blurs star images to about one arcsecond."
Regular plural; used when expressing quantities greater than one, e.g. '0.5 arcseconds' or 'several arcseconds'.
"The two stars are separated by just over two arcseconds."
One arcsecond is the angle a US quarter coin would subtend from about five kilometres away — and astronomers routinely measure angles thousands of times smaller than that.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of arcsecond
The word combines 'arc', from Latin 'arcus' meaning bow or curve, with 'second', from Latin 'secundus' meaning following or next in order. In the sexagesimal division of angles, the 'second' part was so named because it was the second subdivision of the degree, after the minute — a usage that also gave its name to the unit of time. The compound 'arcsecond' arose in scientific English to distinguish this angular unit from the temporal second, becoming standard terminology in astronomy and geodesy.
How arcsecond is actually used
A technical term used almost exclusively in scientific contexts — astronomy, geodesy, surveying, and optical engineering. The standard symbol is a double prime (″), not quotation marks, and it is never abbreviated 'as'. Values are often given in milliarcseconds or microarcseconds for very small angles.
Easily confused with arcsecond
An arcsecond is a unit of angle (1/3,600 of a degree), whereas a plain second is a unit of time.