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The physical group size equivalent to m minutes of arc can be calculated as follows: group size = tan( m / 60 ) × distance. In the example previously given, for 1 minute of arc, and substituting 3,600 inches for 100 yards, 3,600 tan( 1 / 60 ) ≈ 1.047 inches. In metric units 1 MOA at 100 metres ≈ 2.908 centimetres.
The micrometre (SI symbol: μm) is a unit of length in the metric system equal to 10 −6 metres ( 1 / 1 000 000 m = 0. 000 001 m). To help compare different orders of magnitude , this section lists some items with lengths between 10 −6 and 10 −5 m (between 1 and 10 micrometers , or μm).
Metric prefixes; Text Symbol Factor or; yotta Y 10 24: 1 000 000 000 000 000 000 000 000: zetta Z 10 21: 1 000 000 000 000 000 000 000: exa E 10 18: 1 000 000 000 000 000 000: peta P 10 15: 1 000 000 000 000 000: tera T
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol ′, is a unit of angular measurement equal to 1 / 60 of one degree. [1] Since one degree is 1 / 360 of a turn, or complete rotation, one arcminute is 1 / 21 600 of a turn.
Conversions between units in the metric system are defined by their prefixes (for example, 1 kilogram = 1000 grams, 1 milligram = 0.001 grams) and are thus not listed in this article. Exceptions are made if the unit is commonly known by another name (for example, 1 micron = 10 −6 metre).
The use of the parsec as a unit of distance follows naturally from Bessel's method, because the distance in parsecs can be computed simply as the reciprocal of the parallax angle in arcseconds (i.e.: if the parallax angle is 1 arcsecond, the object is 1 pc from the Sun; if the parallax angle is 0.5 arcseconds, the object is 2 pc away; etc.).
For a dense cluster with mass M c ≈ 10 × 10 15 M ☉ at a distance of 1 Gigaparsec (1 Gpc) this radius could be as large as 100 arcsec (called macrolensing). For a Gravitational microlensing event (with masses of order 1 M ☉ ) search for at galactic distances (say D ~ 3 kpc ), the typical Einstein radius would be of order milli-arcseconds.
However, there are numerous other coordinate systems that use the degree (and its subdivisions arcminute and arcsecond), such as Galactic coordinates, though it is also common to express galactic longitude and latitude in ordinary decimal fractions of degrees, the coordinates of an object may be expressed as "l = 48.85 and b = -1.96", meaning ...