Astronomical unit

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Astronomical unit

An astronomical unit (AU, au, a.u., or ua) is a unit of length, roughly the distance from Earth to the Sun. It is used primarily for measuring distances within the Solar System or around other stars. The astronomical unit is approximately equal to 149,597,870.7 kilometers (92,955,807.3 miles).

Definition

The astronomical unit was originally conceived as the average distance between the Earth and the Sun. However, due to the elliptical nature of Earth's orbit, this distance varies over the course of a year. The current definition, adopted by the International Astronomical Union (IAU) in 2012, is based on the length of the semi-major axis of Earth's orbit around the Sun.

Historical Context

The concept of the astronomical unit dates back to the early astronomical observations of the Ancient Greeks, who attempted to measure the distance between the Earth and the Sun. The first reasonably accurate measurement was made by the astronomer Giovanni Cassini in 1672, using the parallax method with observations of Mars.

Modern Measurement

Modern measurements of the astronomical unit are made using radar reflections off planets and other celestial bodies, as well as observations of the motion of spacecraft within the Solar System. These methods have allowed for a highly precise determination of the AU.

Usage

The astronomical unit is commonly used to express distances within the Solar System, such as the average distance from the Sun to Mercury, Venus, Mars, Jupiter, and other planets. It is also used in the study of exoplanetary systems to describe the distances of planets from their host stars.

Related Units

Other units of distance used in astronomy include the light-year, which is the distance light travels in one year, and the parsec, which is equivalent to about 3.26 light-years.

See Also

• Light-year
• Parsec
• Solar System
• International Astronomical Union
• Giovanni Cassini