Astronomical Unit

Introduction

The astronomical unit (AU) is a unit of length often used in astronomy to represent astronomical distances within our solar system. It is approximately equal to the mean distance from the Earth to the Sun, making it a convenient scale for distances involved in planetary motion.

Question: If the Earth were the size of a marble, how big would the sun be?

Definining the Astronomical Unit

The astronomical unit was redefined in 2012 by the International Astronomical Union (IAU) as exactly:

Prior to this, the AU was commonly understood to represent the average distance from the Earth to the Sun. The redefinition was prompted by the desire for a precise and unambiguous value that could be used for more accurate calculations and measurements.

Use of the Astronomical Unit in Astronomy

In astronomy, the AU is particularly useful for describing distances within our solar system. For example, the average distance from the Earth to Mars might be given as 1.52 AU, and the distance from the Sun to Jupiter as 5.20 AU. By using the AU, these large distances can be expressed in a more manageable and intuitive way.

AU and Other Units

For larger interstellar and intergalactic distances, astronomers typically use other units such as the light-year (the distance light travels in one year) or the parsec (approximately 3.26 light-years). These units are much larger than the AU and are more convenient for describing the vast distances between stars and galaxies.

The AU, light-year, and parsec are all part of the astronomer’s toolbox, each suited to different scales of distance in the universe.

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