A light-year is an astronomical unit of distance equal to 9.5 trillion kilometers (5.9 trillion miles), defined as the distance light travels in an Earth year.
The universe is big and the distances between the objects in it can be surprisingly large too. A light-year is defined as the distance light can travel in one Earth year. It can be calculated by multiplying the speed of light (3 x 108 m/s or 300,000,000 m/s) by the length of a year (365.25 days or 3.15 x 107 seconds). Light years to meters: 9.45 x 1015 m, or 9.45 trillion kilometers!
An advantage to using the light-year as a unit of distance is that it makes us realise that light doesn’t move instantaneously. If a galaxy is a million light-years away, it means that the light has taken a million years to reach Earth. This means that when we observe a galaxy that is a million light years away, we are actually looking at the galaxy a million years ago.
For distances within a star system, the light-year is normally too big a distance to use. For example, the distance from the Sun to Earth is 0.00000156 light-years. The unit is mostly suited to intergalactic distances. Astronomers prefer to use the astronomical distance unit the parsec, but the light-year has survived as a popular unit. The term 'light-year' has been mentioned in many popular science fiction works, such as Star Trek.
This term is often confused as a unit of time because it includes the word 'year'. Sometimes people will write things like “technology is light-years ahead”; this can add to confusion about the term. A light-year is a unit of distance, like miles or kilometers—only on a much larger scale.
A light year is the distance that light travels in one year, which is about 5.88 trillion miles (9.46 trillion kilometers). Astronomers use it to measure vast distances in space because stars and galaxies are so far away that regular units like kilometers or miles would be impractical.
Scientists calculate a light year by multiplying the speed of light (about 299,792 kilometers per second) by the number of seconds in a year. This gives the total distance light travels in one year, which is approximately 9.46 trillion kilometers.
Using miles or kilometers for space distances would result in extremely large and hard-to-understand numbers. Light years make it easier to compare and communicate the vast separations between stars and galaxies.
The closest star to Earth after the Sun, Proxima Centauri, is about 4.24 light years away. The Andromeda Galaxy is roughly 2.5 million light years from Earth.
A year is a measure of time (365 days), while a light year is a measure of distance—the length light travels in one year. They are different concepts used for different purposes.