Astronomical distances
Click the Sun or a planet to see distance and facts. Orbits are shown on a logarithmic scale.
Klik på Solen eller en planet for afstand og fakta
Unit converter
Convert between kilometers, AU, light-years, parsec and more — type a value or select a known distance.
Klik på en enhed for at bruge den som udgangspunkt
Kendte afstande
Behind the numbers
What is a light-year?
A light-year is the distance light travels in exactly one year — approximately 9.46 × 10¹² km (nearly 10 trillion kilometers). It is not a unit of time, even though "year" appears in the name. Light travels at 299,792 km/s, covering a staggering distance in a year. For comparison, Proxima Centauri, our nearest star, is 4.24 light-years away.
What is a parsec?
A parsec (pc) is defined using the parallax method: the distance from which Earth's orbit around the Sun would appear to subtend an angle of 2 arcseconds — i.e. one arcsecond of parallax. 1 parsec equals approximately 3.26 light-years or 3.09 × 10¹³ km. Astronomers prefer parsecs over light-years because parsecs reflect the actual measurement method used to determine stellar distances.
What is an Astronomical Unit (AU)?
An Astronomical Unit (AU) is exactly 149,597,870.7 km — Earth's mean distance from the Sun. It is a convenient unit within the solar system. Mars is about 1.5 AU from the Sun, Jupiter about 5.2 AU and Neptune about 30 AU. Beyond the solar system, AU is too small to be practical, and astronomers switch to light-years or parsecs.
Megaparsec and the scale of the universe
For distances between galaxies and galaxy clusters, astronomers use megaparsecs (Mpc) — one million parsecs. The Andromeda Galaxy is about 0.78 Mpc away. The radius of the observable universe is approximately 14,250 Mpc (46.5 billion light-years). The large figure is because the universe has expanded since the Big Bang, so the light from the most distant galaxies has traveled a shorter path than the current distance.
Speed of light and communication
Signals (radio, laser light) travel at the speed of light — approximately 300,000 km/s. From the Sun to Earth takes 8.3 minutes. From the Sun to Neptune about 4 hours. A message to Proxima Centauri and back would take over 8 years. This sets fundamental limits on what future space travel and communication can achieve.
The Milky Way in numbers
The Milky Way is approximately 100,000 light-years (≈ 30 kpc) in diameter and about 1,000 light-years thick in the disk. Our solar system is located about 26,000 light-years (≈ 8 kpc) from the center. The Milky Way contains an estimated 100–400 billion stars. Although these numbers sound enormous, the Milky Way is a medium-sized galaxy — Andromeda is nearly twice as large.