One light year (abbreviated ly) is the distance that light travels in one Earth year. It is approximately equal to 9.46 ×1012 kilometers, or 5.88 ×1012 miles. Light years are common units of measurement used in starship navigation. (TNG: "The Naked Now")
In the Treaty of Armens (2255), it is stipulated, Third party assistance may be requested from a Federation Starfleet vessel or a Sheliak Corporate spacecraft if the distance from the vessel to the respective homeworld is greater than five thousand (5.0 10E3) lightyears. UFP Standards Measurement Bureau Units. Assistance may also be assisted if the vessel is less than 1000 lightyears from a standard UFP subspace relay booster station. (TNG: "The Ensigns of Command")
When Commander Benjamin Sisko welcomed the first guest from the Gamma Quadrant in the Alpha Quadrant, a Tosk, he told him that he traveled almost ninety thousand light years through the wormhole. (DS9: "Captive Pursuit")
The exact length of a light year depends on the exact length used for one "Earth year". The IAU uses a Julian year of 365.25 days, while other sources may use a Gregorian year of 365.2425 days, or another year altogether.
|Source||year (days)||light year (kilometers)||light year (miles)|
|365.242199||9.460 528 4×1012||5.878 499 81×1012|
‡ Note that while Yahoo separately reports a year length of 365.24220 days, its rounding of the light year length to five digits works out to a year length of ~365.2411 days.