# What is an Astronomical Unit (AU)? (with photos)

A galaxy.

An astronomical unit (AU) is a distance measurement often used in astronomy, equal to the distance between the Earth and the Sun. In terms of the most common units of measurement, one AU equals about 93 million miles (150 million km), or the distance light travels in just over eight minutes. The AU symbol is most often used to represent the astronomical unit, although less commonly you might see UA being used instead.

An astronomical unit (AU) is the distance between the Earth and the Sun.

People have been estimating the distance between the Earth and the Sun for a long time. Many Greeks came up with measurements, often wrong by quite huge margins. The Greek Eusebius came up with a measurement that is surprisingly close to the modern measurement of an astronomical unit. In one of his works, he estimated at 804 million stadiums. Stadiums, a Greek unit of measurement, are approximately 605 to 625 feet (85-90m), making their estimate somewhere between 92 and 95 million miles (149-153 million km).

Jupiter is 5AU from the Sun.

In the late 17th century, the AU was officially estimated to be about 87 million miles (140 million km) using the location of Mars at two different points in Earth’s orbit. In the late 18th century, a method was devised using Venus as a measuring point during its transit across the face of the sun. This method produced a much more accurate figure. At the beginning of the 20th century, an asteroid passed close to Earth and an even more precise number for the astronomical unit was calculated.

In the 18th century, Venus was used to devise a very accurate measurement method.

In the mid-20th century and into the 21st century, improvements in various measurement technologies allowed much more accurate measurements to be performed, further refining the AU. Using space probes and satellites, modern definitions were created, with a degree of accuracy far greater than any in the past. In 1976, the actual definition of the astronomical unit was updated to obtain a more sophisticated measurement. Perhaps the most accurate definition could be given as the distance from the exact center of the Sun at which a particle would take one Gaussian year (365.2568983 days) to complete its orbit. If this sounds confusing, think of it as roughly the same as saying that an astronomical unit is the distance from the center of the Earth to the center of the Sun.

The actual number for an AU produced by this definition, and by more modern measurements, is approximately 92,955,807 miles (149,597,870,691km). This number was adopted in 1996 and is considered accurate to about 10 feet (approximately 3m).

The astronomical unit can be useful not only for astronomers but also for normal people trying to understand the relative distances involved in our own solar systems. While the distances between the planets may seem too great to understand when given in miles or kilometers, when given in astronomical units it becomes much easier to see the relationships between them. For example, while the Earth is naturally 1AU away from the Sun, the Moon is only 0.0025AU away from the Earth. And while Jupiter, which we think is quite far away, is just over 5AU from the Sun, Pluto is a whopping 40 to 50AU away. And if that seems like a long way off, consider that the closest star to our solar system is 268,000 AU away.