What is An Astronomical Unit (AU)?

Astronomical Unit
Astronomical Unit Diagram Courtesy of https://commons.wikimedia.org/wiki/User:Huritisho Creative Commons License

An astronomical unit, which is often represented by the acronym AU, represents the mean distance between the Earth and our sun. This means that an AU is can be measured as approximately 93 million miles, or 150 million kilometers.

The time that it takes light to traverse this 93 million mile gap between the Sun and the Earth is approximately 8 light-minutes. A more exact measurement of an astronomical unit is 92,955,807 miles.

The astronomical unit was created as a method to be used primarily for measuring distances within the Solar System or around other stars.

Average distances in AU of each planet from the sun:

  • Mercury: 0.4 AU
  • Venus: 0.7 AU
  • Earth: 1 AU
  • Mars: 1.5 AU
  • Jupiter: 5.2 AU
  • Saturn: 9.6 AU
  • Uranus: 19.2 AU
  • Neptune: 30.0 AU

It is worth noting that there are approximately 63,240 AU in 1 light year.

The history of measuring the distance between the Earth and the Sun goes all the way back to ancient times. The first people to estimate the distance betwen the Sun and the Earth were all the way back in the 3rd century BCE. Archimedes and Aristarchus were the first to provide estimates; although their estimates were off by large margins of the value used today as the AU.

It wasn’t until 1672, when Italian-born French astronomer Gian Domenico Cassini made a reasonably close estimate of the AU using a determination of the parallax displacement of the planet Mars; and moreover, the distance to Earth. Further refinements would continue until 1976, when the International Astronomical Union defined the astronomical unit as the distance from the Sun at which a massless particle in a circular orbit would have a period of one year.

If you’re curious for more information on planetary bodies in our solar system, check out these articles.

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