The speed of light

In short: In a vacuum light travels at about 299,792 kilometres per second. According to relativity nothing with mass can reach or exceed this speed.

In more detail: In materials light is slower, in water about 225,000 km/s, in glass around 200,000 km/s.

In detail

• Light-second and light-year: A light-year is the distance light covers in a year, about 9.46 trillion kilometres.
• Sunlight: It takes about 8 minutes and 20 seconds to reach Earth. So we see the Sun as it was a good 8 minutes ago.
• Moon: Light from the Moon arrives after about 1.3 seconds.
• Stars: Looking into space is looking into the past.
• Signal delay: For satellites and space probes the speed of light makes conversations sluggish (echo-und-schallgeschwindigkeit, licht-und-farben).