Astronomical Units and Light Years

A light year is a very large distance used to measure immense distances, often between galaxies. The distance of a light year is equivalent to how fast light travels in a year. It is about 9.46 trillion kilometres. Even though a light year is very large, the universe is much bigger and therefore distances can be billions of light years. First used by astronomer Friedrich Wilhelm Bessel in 1838.

Light years are used to measure immense distances, often between galaxies. Astronomical units are slightly smaller than light years and help measure places generally in our solar system. Light hours, light minutes and light seconds are also used to measure distances in the solar system.

An astronomical unit (AU) is a unit to measure distance. An AU is about 149,597,871 km or about 149.6 million kilometres. In 1976, scientists decided to officially define a light year as the distance between the Earth and the Sun. However, a problem arose, as that distance changes as the Earth’s orbit is not perfectly circular. Therefore, in 2012, an AU was officially defined as 149,597,871 km. Gian Domenico Cassini is the person who started the idea of the AU.