Arcsecond
From Open Encyclopedia
A second of arc or arcsecond is a unit of angular measurement which comprises one-sixtieth of an arcminute, or 1/3600 of a degree of arc or 1/1296000 ≈ 7.7×10-7 of a circle. It is the angular diameter of an object of 1 unit diameter at a distance of 360×60×60/(2π) ≈ 206,265 units, such as (approximately) 1 cm at 2.1 km, or 1 astronomical unit at 1 parsec, which defines the parsec.
Correspondingly, 1 radian ≈ 206,265 arcseconds.
The symbol for marking the arcsecond is the double prime (″) (U+2033, ″). One arcsecond would be 1″ (or 1"). The double prime symbol is also used to denote the inch: this can, in certain contexts, cause confusion.
It can be abbreviated as arcsec, but should then not be confused with the inverse trigonometric function arc secant, which has the same abbreviation.
Other abbreviations occasionally used are a double prime with an arch over the double prime symbol (<math>\hat {}</math>), or asec, but these are not as common as those mentioned above.
Uses
Astronomy uses the arcsecond. It is worth noting that degrees (and therefore arcseconds) are used to measure declination, or angular distance north or south of the celestial equator, not right ascension. Also the arcsecond is used to give values of seeing, sizes of objects on the sky, field of view of telescopes, or practically any angular distance.
Caveat Emptor
There are a couple things to keep in mind regarding arcminutes, and therefore arcseconds.
- Astronomers typically measure Right Ascension position in hours, minutes, and seconds. RA minutes and seconds are not to be confused with Declination arcminutes and arcseconds, as they are of a different size. Angular sizes are measured in degrees regardless of whether measuring in an East-West, North-South, or other direction.
- In right ascension and longitude, the size of the degree (and therefore arcmin and arcsec) changes with how far you are from the equator. As opposed to a degree of latitude, which always corresponds to about 111 km (69 mi), a degree of longitude corresponds to a distance from 0 to 111 km: it is 111 km times the cosine of the latitude, when the distance is laid out on a circle of constant latitude; if the shortest distance, on a great circle were used, the distance would be even a little less.
See also
da:Buesekund de:Bogensekunde es:Segundo sexagesimal fr:Seconde d'arc ko:초 (각도) nl:Boogseconde ja:秒 (角度) no:Buesekund pt:Segundo de arco sk:Uhlová sekunda sv:bågsekund fi:Kulmasekunti zh:角秒
