Light-second (ls)

AstronomicalCurrent unit

The distance light travels in a vacuum in one second: 299,792,458 meters. The Moon is about 1.28 light-seconds from Earth.

1 light-second = 299,792,458 meters (exactly).

About the light-second

Because the speed of light is now fixed at 299,792,458 meters per second, a light-second is exactly that many meters, with no uncertainty at all. Astronomers and radio engineers treat it as a convenient yardstick for distances that are small on the scale of the solar system but large on the scale of a laboratory. The abbreviation ls appears in tables of astronomical units, and it is not an official SI unit.

The Moon is the standard example: its average distance from Earth is about 1.282 light-seconds, which is why a radio exchange with a lunar mission has a noticeable round-trip lag of roughly two and a half seconds. The Earth's mean diameter is only about 0.0425 light-seconds, while the Sun's diameter is about 4.643. One astronomical unit is about 499 light-seconds. Sixty light-seconds make a light-minute, 3,600 make a light-hour, and 31,557,600 make a light-year, based on the Julian year.

Sources: Wikipedia: Light-second; Wikipedia: Light-minute

Light-second conversion chart

1 light-second expressed in every other length unit:

More Astronomical units

• astronomical unit (length)
• light-minute (length)
• light-hour (length)
• light-year (length)
• parsec (length)
• megaparsec (length)
• Earth mass (weight & mass)
• solar mass (weight & mass)
• Julian year (time)
• tropical year (time)
• sidereal year (time)
• synodic month (time)
• sidereal day (time)
• aeon (time)
• milliarcsecond (angle)
• zodiac sign (angle)

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