Light years and distances in space

In short: A light year is not a time but a distance: how far light travels in one year. That is roughly 9.5 trillion kilometres.

In more detail: Distances in space are so huge that kilometres become impractical. Astronomers therefore use larger units such as the astronomical unit, the light year and the parsec.

In detail

• Astronomical unit: It equals the average distance between the Earth and the Sun, about 150 million kilometres. It is well suited to distances within the Solar System.
• Light from the Sun and Moon: Sunlight takes about eight minutes to reach the Earth; light reflected by the Moon takes just over a second.
• Nearest star: Proxima Centauri, the nearest star after the Sun, is about 4.2 light years away. With today’s space probes the journey would take tens of thousands of years.
• Milky Way: Our galaxy is about 100,000 light years across. The Sun lies roughly halfway between the centre and the edge.
• Andromeda: The Andromeda Galaxy is around 2.5 million light years away and is one of the most distant objects that can still be seen with the naked eye under a very dark sky.
• Parsec: One parsec is about 3.26 light years. The unit comes from the parallax method, which measures how a star’s apparent position shifts as the Earth orbits the Sun.
• Other methods: For distant galaxies, astronomers use stars and stellar explosions of known brightness as well as the redshift of their light.
• Looking into the past: Because light takes time to travel, we see distant objects as they were when the light set out. Looking deep into space is therefore always looking back in time.
• Further reading: The speed of light, galaxies and the Solar System (lichtgeschwindigkeit, galaxien-und-milchstrasse, sonnensystem).