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Size Comparison: Light Lengths

Comparison of distances in kilometers and miles and units of light, such as light seconds, light years. Light always travels the same distance in a vacuum in a certain amount of time. The corresponding units are used for long distances, i.e. for space. The astronomical unit is the average distance between the Earth and the Sun, about 8 light minutes. For kilometers and miles, you can choose a prefix like trillions for better representation due to the large numbers. Please enter one value to calculate all others.

Kilometers:	thousandmillionbilliontrillionquadrillionquintillionsextillion
Miles:	thousandmillionbilliontrillionquadrillionquintillionsextillion
Light-seconds:
Light-minutes:
Light-hours:
Light-days:
Light-years:
Distance Earth-Moon:
Astronomical units:
Parsec:
Round to   0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15   decimal places

Example: the closest star to us apart from the Sun, the Alpha Centauri system, is about 4.34 light years away. That's just over 41 trillion kilometers, written out as 41059570251000 km.

The light-year is certainly the most prominent unit of length for expressing astronomical distances. For distances within our solar system or other such systems, the astronomical unit is a common measure. The average distance between the Earth and the Moon is less frequently used as a size for comparison. The parallax second, or parsec, is often used for larger astronomical distances and is common in scientific contexts. Units of light shorter than the light-year are less common. Perhaps the most frequently encountered of these are the light-second and the light-minute, since the Moon is just over one light-second and the Sun just over eight light-minutes away from us.

Besides simply stating distances, the length of light also allows for a vivid consideration of time durations. Since information can only travel at the speed of light in the universe, light travel times show how old the light from distant objects is when it arrives on Earth. Observations of distant stars, nebulae, or galaxies therefore provide insights into earlier states of the universe. The greater the distance, the further back in time we are looking.

Last updated on 01/10/2026. Author: Jürgen Kummer

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