By everyday standards, light travels very rapidly - approximately 300,000 km each second, in vacuum or air. This is roughly a million times faster than sound, and fast enough to circle the Earth more than 7 times in one second Such a rapid speed is very hard to measure without specialized techniques, and in ancient times the speed of light was the subject of speculation. The first effective measurements of the speed of light were made in the seventeenth century, and were progressively refined. Today, time intervals can be measured extremely precisely, to the point where the metre is now defined officially as the distance light travels in "vacuum" in 1⁄299,792,458 of a second. As a consequence, according to NIST: "… the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m/s."

**Speed of light in different units**

metres per second : 299,792,458 (exact)

km per hour : 1,079,252,848.8 (exact)

miles per hour : ≈ 670,616,629.3844

miles per second : ≈ 186,282.39705122

**Approximate length of time for light to travel...**

One foot :0.98 nanoseconds

One metre : 3.3 nanoseconds

One km : 3.3 microseconds

One mile : 5.4 microseconds

Around Earth's equator : 0.13 seconds

From Earth to geostationary orbit and back : 0.24 seconds

From Earth to the moon : 1.3 seconds

From Earth to the sun : 8.3 minutes

To Earth from Alpha Centauri : 4.4 years

From edge to edge of the Milky Way : 100,000 years

**Practical effect of the finite speed of light**

The speed of light plays an important part in many modern sciences and technologies. Radar systems measure the distance to a target by measuring the time taken for an echo of the light pulse to return. Similarly, a global positioning system (GPS) receiver measures its distance to satellites based on how long it takes for a radio signal to arrive from the satellite. The distances to the moon, planets, and spacecraft are determined by measuring the round-trip travel time.

Another effect of the finite speed of light is stellar aberration. Suppose we look at a star with a telescope idealized as a narrow tube. The light enters the tube from a star at angle θ and travels at speed c taking a time h/c to reach the bottom of the tube, where our eye detects the light. Suppose observations are made from Earth, which is moving with a speed v. During the transit of the light, the tube moves a distance vh/c. Consequently, for the photon to reach the bottom of the tube, the tube must be inclined at an angle φ different from θ , resulting in an apparent position of the star at angle φ.

In astronomy beyond the solar system, distances are often measured in light-years, the distance light travels in a year.

In electronic systems, despite their small size, the speed of light can become a limiting factor in their maximum speed of operation.

As light propagates down the telescope, the telescope moves requiring a tilt to the telescope that depends on the speed of light. The apparent angle of the star φ differs from its true angle θ, a phenomenon called stellar aberration

The blue glow in this "swimming pool" nuclear reactor is Čerenkov radiation, emitted as a result of electrons traveling faster than the speed of light in water.