Speed of Light
The speed of light is a central physical constant and plays a key role in relativity, information, and observation in the cosmos.
physics.constants
Metrics
Exact Value
Exactly defined since 1983 (SI definition of the meter).
Light-Year
Distance light travels in one Julian year.
Light Travel Time Earth–Moon
Mean Earth–Moon distance approx. 384,400 km.
Light Travel Time Earth–Sun
Mean Earth–Sun distance approx. 149.6 million km (1 AU).
Relations
Precise timekeeping and relativity play a role in navigation systems (context relation).
Explanations
The speed of light is exactly 299,792,458 meters per second and represents the fastest speed that exists in the universe. At this speed, light travels about 9.461 trillion kilometers in one year – a distance astronomers call a 'light-year'. Even at this enormous speed, light takes about 1.3 seconds to travel from Earth to the Moon and about 8.3 minutes from Earth to the Sun.
Sources
CODATA recommended value. Exact by definition since 1983 (SI).
Related Cards
Provenance
Status
seed
Review
none
Last Updated
2026-02-16