# Speed of light

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## Light-hour

A light hour (also written light-hour) is a unit of length. It is the distance travelled by light in vacuum in one hour. Based on the current definition of the metre a light-hour is equal to 1,079,252,848,800 metres (~1.08 Tm). To give an example, Pluto's orbit's semi-major axis is about 5.473 light-hours.

## Light-year

A light-year is equal to approximately 9,460,528,404,879 km (about 9.461 Pm) 5,878,482,164,161 statute miles[1] 63,239.7263 AU The actual, exact length of the light-year depends on the length of the reference year used in the calculation, and there is no wide consensus on the reference to be used. The figures above are based on a reference year of 31,556,925.9747 seconds, but other reference years are often used, such that the light-year is not an appropriate unit to use when extremely high precision is required. The light-year is often used to measure distances to stars. In astronomy, the preferred unit of measurement for such distances is the parsec which is defined as the distance at which an object will generate one arcsecond of parallax when the observing object moved one astronomical unit perpendicular to the line of sight to the observer. This is equal to approximately 3.26 light years. The parsec is preferred because it can be more easily derived from, and inter-compared with, observational data. However, outside scientific circles, the term light-year is more widely used by the general public. Units related to the light year are the light-minute and light-second, the distance light travels in a vacuum in one minute and one second, respectively. Since the speed of light is defined as 299,792,458 metres per second, a light-second is exactly 299,792,458 m in length and a light-minute is exactly 17,987,547,480 m. In contrast to the light-year, the lengths of the light-minute and light-second are fixed with 100% precision.