A light year is the distance light travels in one year in space (vacuum). So light-year is a unit of length, not time, though the term “year” is present in light-year. The symbol of the light-year is “ly.”
One light-year is approximately equals to 9 trillion kilometers. That is a 9 with 12 zeros behind it!
However, a light-year is not an ordinary unit of length. We use light years in astronomy for the measurement of vast celestial distances. Here celestial distance means distance between stars, galaxies, nebulae etc.
As we know, light is the fastest-moving object in the universe, and more importantly, its speed is constant in space (vacuum), which is 300,000 kilometers per second or 186,000 miles per second. To use the speed of light as a unit of distance we have to multiply it with the unit of time from where the term “light-year” comes.
Why do we use light-years?
If we measure celestial distances in terms of ordinary units of length like kilometers or miles, then it gives us a gigantic number that is very difficult to remember. That’s why astronomers use light as a unit of distance.
For example, the distance of our nearest neighboring star, Proxima Centauri, is 40,205,000,000,000 kilometers, which is very difficult to remember. However, in terms of light-year, it is only 4.25 light-years.
However, the major advantage of using light-years is that it tells us how far back in time we are watching. Because light takes time to reach us from celestial objects.
If a celestial object is located 1 light-year away from us, then light takes 1 year to reach us from that celestial object. In other words, we see the celestial object as it was exactly one year ago.
Light-time is an extensive form of a light-year
Light-time is a more general form for a light-year. A light-time is the distance light travels within a specific period of time.
We usually use light-second, light-minute, light-hour, and light-day for the measurement of interplanetary distances, and we use light-year, kilolight-year, megalight-year, and gigalight-year for the measurement of interstellar and intergalactic distances.
One light-second is equal to 300,000 kilometers
A light-second is the distance light travels in one second in space, and it is equal to 300,000 kilometers or 186,000 miles.
Moonlight takes about 1.3 seconds to reach us, so the distance of the earth from the moon is 1.3 light-seconds.
One light-minute is equal to 18,000,000 kilometers
A light-minute is the distance light travels in one minute in space, and it is equal to 60 * 300,000 kilometers, or 18,000,000 kilometers, or 11,160,000 miles.
Sunlight takes about 8.3 minutes to reach us, so the distance of the earth from the sun is 8.3 light-minutes.
Similarly, sunlight takes 43.2 minutes to reach Jupiter, so the distance of Jupiter from the sun is 43.2 light-minutes.
One light-hour is equal to 1.08 billion kilometers
A light-hour is the distance light travels in one hour in space, and it is equal to 60*60*300,000 kilometers, or 1.08 billion kilometers, or 671 million miles.
Sunlight takes about 4.1 hours to reach Neptune, so the distance of Neptune from the sun is 4.1 light-hours.
One light-day is equal to 25.9 billion kilometers
A light-day is the distance light travels in one day in space, and it is equal to 24*60*60*300,000 kilometers or 25.9 billion kilometers, or 16.1 billion miles.
Sunlight takes about 29 days to reach the inner wall of Oort Cloud, so the minimum distance of Oort Cloud from the sun is 29 light-days.
One light-year is equal to 9.46 trillion kilometers
A light-year is the distance light travels in one year in space, and it is equal to 365*24*60*60*300,000 kilometers or 9.46 trillion kilometers or 5.88 trillion miles.
Light from Proxima Centauri (our nearest neighboring star) takes about 4.25 years to reach us, so the distance of the earth from the star Proxima Centauri is 4.25 light-years.
Similarly, light from the Andromeda galaxy (our nearest neighboring large galaxy) takes about 2.5 million years to reach us, so the distance of the earth from the Andromeda galaxy is 2.5 million light-years, or 2.5 megalight-years. In other words, we see the Andromeda galaxy as it was 2.5 million years ago.
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