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Definitions and measurements
arcminute (' or min)
a unit of angular measure, also called the minute of arc, equal to 60
arcseconds and to 1/60 degree. There are 21 600 arcminutes in a circle.
arcsecond (" or as or sec or s)
a unit of angular measure, also called the second of arc, equal to 1/60
arcminute. One arcsecond is a very small angle indeed: there are 1 296 000
seconds in a circle. The SI defines s as the symbol for the time unit and
recommends " as the symbol for the arcsecond. The symbol as has become
common in astronomy, where very small angles are stated in milliarcseconds
(mas).
astronomical unit (ua or au or AU)
a unit of distance used by astronomers to measure distances in the Solar
System. One astronomical unit equals the "average" distance from the center
of the Earth to the center of the Sun (mathematically, it is the length of the
semimajor axis of the Earth's elliptical orbit, which is the ordinary average of
the Earth's minimum and maximum distances from the Sun). The currently
accepted value, adopted in 1996, is 149 597 870 691 meters (1.495 978 706 91 x
108 kilometers or about 92 955 807 miles), with an uncertainty of about 30
meters. The astronomical unit is a convenient yardstick for measuring the
distances between objects in the Solar System. Astronomers find it
particularly convenient to use astronomical units in solving the equations of
planetary motion. Because these equations are the same regardless of the
unit used for distance, the predictions they generate will remain correct even
if future astronomers determine a slightly different length for the distance
between the Earth and Sun. This unit is accepted for use with SI units. The
official symbol for the unit is ua, but the symbol au is common in
English-speaking countries. Note that AU is also used as the symbol for the
absorbance unit (see above).
light year (ly)
a unit of distance used in astronomy. One light year is the distance that light
travels in one year through a vacuum (and, of course, most of the Universe is
close to being a vacuum). The "year" used for this calculation is usually the
Julian year of exactly 365.25 days or 31 557 600 seconds; sometimes the mean
tropical year of 31 556 925.9747 seconds is used. Since light travels at 299 792
458 meters per second, using the Julian year one light year equals 9.460 730
473 x 1015 meters, or 9.460 730 473 trillion kilometers. One light year is
approximately 5.880 trillion miles.
arcminute (' or min)
a unit of angular measure, also called the minute of arc, equal to 60
arcseconds and to 1/60 degree. There are 21 600 arcminutes in a circle.
arcsecond (" or as or sec or s)
a unit of angular measure, also called the second of arc, equal to 1/60
arcminute. One arcsecond is a very small angle indeed: there are 1 296 000
seconds in a circle. The SI defines s as the symbol for the time unit and
recommends " as the symbol for the arcsecond. The symbol as has become
common in astronomy, where very small angles are stated in milliarcseconds
(mas).
astronomical unit (ua or au or AU)
a unit of distance used by astronomers to measure distances in the Solar
System. One astronomical unit equals the "average" distance from the center
of the Earth to the center of the Sun (mathematically, it is the length of the
semimajor axis of the Earth's elliptical orbit, which is the ordinary average of
the Earth's minimum and maximum distances from the Sun). The currently
accepted value, adopted in 1996, is 149 597 870 691 meters (1.495 978 706 91 x
108 kilometers or about 92 955 807 miles), with an uncertainty of about 30
meters. The astronomical unit is a convenient yardstick for measuring the
distances between objects in the Solar System. Astronomers find it
particularly convenient to use astronomical units in solving the equations of
planetary motion. Because these equations are the same regardless of the
unit used for distance, the predictions they generate will remain correct even
if future astronomers determine a slightly different length for the distance
between the Earth and Sun. This unit is accepted for use with SI units. The
official symbol for the unit is ua, but the symbol au is common in
English-speaking countries. Note that AU is also used as the symbol for the
absorbance unit (see above).
light year (ly)
a unit of distance used in astronomy. One light year is the distance that light
travels in one year through a vacuum (and, of course, most of the Universe is
close to being a vacuum). The "year" used for this calculation is usually the
Julian year of exactly 365.25 days or 31 557 600 seconds; sometimes the mean
tropical year of 31 556 925.9747 seconds is used. Since light travels at 299 792
458 meters per second, using the Julian year one light year equals 9.460 730
473 x 1015 meters, or 9.460 730 473 trillion kilometers. One light year is
approximately 5.880 trillion miles.