How long is a light year ?

A light year refers to the distance light travels in a given year through empty space. Since light moves through vacuum at around 300,000 kilometers per second, reaching up to 9 trillion kilometers per year, terms like kilometers and miles are not sufficient to describe great distances.

To determine great distances of planets and other parts of the universe in terms of light years, astronomers need to learn how fast light travels. For instance, the light from a star named Alpha Centauri as you see today takes around 4.2 years for it to reach the earth. As such, when you observe that star today, you are seeing it exactly as it was 4.2 years ago.

Until now, there is no star discovered that is exactly one light year away from the earth. The closest neighboring stars are Proxima Centauri (4.2. light years away) and double star Alpha Centauri A & Alpha Centauri B (at 4.2 light years distance).

Aside from using light years, scientists and astronomers use terms like light minutes and light seconds. The distance from the earth to the sun, also known as 1 AU (astronomical unit), is 8.3 light minutes. Meaning, it would take 8.3 minutes for the sun’s light to reach the earth or as astronomers would say it, “the sun is 8.3 light minutes away from the earth”.

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