History of Light year

	Revision 14 . . November 23, 2001 11:19 am by (logged).109.250.xxx [light year is defined in terms of Julian year, not time Earth takes to orbit sun]
	Revision 13 . . November 23, 2001 12:31 am by AxelBoldt
	Revision 12 . . November 23, 2001 12:25 am by AxelBoldt
	Revision 11 . . November 23, 2001 12:22 am by Gareth Owen
	Revision 10 . . (edit) September 19, 2001 7:40 pm by Zundark [AU -> astronomical unit]

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A light year is the distance light travels in one year, or more specifically, the distance that a photon would travel, in free space and infinitely far away from any gravitational? or [magnetic field]?s, in the time it takes the Earth to complete on orbit of the Sun. It is approximately equal to 9.46 * 1015 meters ("10 million million kilometers"), because the speed of light in vacuum is 299,792,458 meters per second.

A light year is the distance light travels in one year, or more specifically, the distance that a photon would travel, in free space and infinitely far away from any gravitational? or [magnetic field]?s, in one Julian year (365.25 days of 86400 SI seconds each). It is approximately equal to 9.46 * 1015 meters ("10 million million kilometers"), because the speed of light in vacuum is 299,792,458 meters per second.