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14 Science, mathematics, and computing

14.7 Astronomy

14.7.1 Conventions

Most current astronomy texts follow those recommendations for style set out by the International Astronomical Union (IAU), though other styles may be found, especially in older works. Ensure consistency within a given work, preferring modern to older styles except when reproducing earlier texts.

Capitalize Earth, Moon, and Sun in astronomical contexts but not in general text; the definite article is omitted before Earth in technical texts:

On Earth, valleys occur both on dry land and beneath the sea; there are also valleys on the Moon, Mars, and Venus.

The members organise turtle walks everyday and during full moon and new moon days, intensify the patrols.

Common sense seems to be disappearing off the face of the earth.

Use lower case for suns and moons. Capitalize Galaxy and Solar System when it refers to the Milky Way and our planetary system, but galactic and solar is lower case in all other contexts. While minor planet is preferable to asteroid in technical usage, asteroid belt is an accepted astronomical term.

14.7.2 Units

One-letter abbreviations for time units (second, minute, hour, day, year) are acceptable (s, m, h, d, y) if the context makes them clear. Time units can be written on the line with spaces (5h 35m 45s) or superscripted and closed up (5h35m45s); aim for consistency. Dates are commonly written in the form 1954 March 31. A year stated in the form 2000.0 represents an epoch; the ‘.0’ is significant and should not be deleted. Angles are often written with the angle symbol immediately before the decimal point (0″.001). Conventional notation is also acceptable but be consistent.

Note light year is a unit of distance, not time, is abbreviated with points, l.y., and is used in non-specialist texts. The distance unit parsec, symbol pc, is preferred in academic texts. SI prefixes (see 14.1.4) can be used with parsec (e.g. 5 Mpc), unlike the light year. Within the Solar System, distances are measured in astronomical units, symbol AU.

14.7.3 Stellar nomenclature

Galaxies, nebulae, and bright star clusters can be designated by names (Crab Nebula, Beehive Cluster, Sombrero Galaxy) or by numbers in a catalogue such as that of Messier (M1, M104) or the New General Catalogue (NGC 1952, NGC 4594). Note the prefix M is closed up, whereas a space is inserted after NGC.

The eighty-eight constellations have been assigned official names by the IAU. Many bright stars within these constellations have traditional names (Sirius, Canopus, Castor, and Pollux), though astronomers tend to favour the Bayer letter system, in which Greek letters are allotted in alphabetical order by brightness as seen from Earth. The letter can be written in full or using the Greek character, and is used in combination with a three-letter roman abbreviation (no point) assigned by the IAU, so in the constellation Centaurus, for example, the brightest star is written as Alpha Centauri, Centauri, or Cen. The genitive form of the constellation name is used (Orionis not Orion), both capitalized when in full form (Beta Crucis). An alternative designation is the Flamsteed system, which uses a number and the constellation abbreviation (61 Cyg, 58 Ori).

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