Light-second

It is defined as the distance that light travels in free space in one second, and is equal to exactly 299792458 m (approximately 983571055 ft or 186282 miles).

[1] Communications signals on Earth rarely travel at precisely the speed of light in free space.

The light-second is a convenient unit for measuring distances in the inner Solar System, since it corresponds very closely to the radiometric data used to determine them.

The value of the astronomical unit (roughly the distance between Earth and the Sun) in light-seconds is a fundamental measurement for the calculation of modern ephemerides (tables of planetary positions).

It is usually quoted as "light-time for unit distance" in tables of astronomical constants, and its currently accepted value is 499.004786385(20) s.[3][4] Multiples of the light-second can be defined, although apart from the light-year, they are more used in popular science publications than in research works.

The yellow shell indicating one light-day distance from the Sun compares in size with the positions of Voyager 1 and Pioneer 10 (red and green arrows respectively). It is larger than the heliosphere's termination shock (blue shell) but smaller than Comet Hale-Bopp 's orbit (faint orange ellipse below). Click on the image for a larger view and links to other scales.
The faint yellow sphere centred on the Sun has a radius of one light-minute. For comparison, sizes of Rigel (the blue star in the top left) and Aldebaran (the red star in the top right) are shown to scale. The large yellow ellipse represents Mercury's orbit.