The speed of light is always constant. This is the foundation of the Special Theory of Relativity for uniform motion. (The General Theory of Relativity is a relative theory about accelerated motion). Based on this principle of the constancy of light velocity, the speed of light came to be used as the ultimate standard for length and time. The Special Theory of Relativity also led to the discovery that energy (E) and mass (m) are equivalent, which formulated E = mc2.

The speed of light (c = 2.99792458×108 m/sec.) is equivalent to 299,792.458 km/sec. Light thus travels at nearly 300,000km/sec. Since the earth and moon are 380,000 km apart, light from the Moon takes about 1.3 seconds to reach us. And since the sun is 150 million km from the earth, the light we see right now left the sun about 8 minutes ago. A light year (the distance light travels in a year) is 9.5 trillion km.