The astronomical unit (AU) is a measure of distance equal to the distance between the Earth and the Sun, approximately 93 million miles (150 million km). The AU has been estimated for centuries, with modern measurements being the most accurate. The current definition is the distance from the center of the Sun at which a particle would take one Gaussian year to complete its orbit, approximately 92,955,807 miles (149,597,870,691 km). The AU is useful for understanding the relative distances within our solar system, with planets and celestial bodies being measured in AU.
An astronomical unit (AU) is a measure of distance often used in astronomy, equal to the distance between the Earth and the Sun. In terms of the most common units of measure, one AU is equal to approximately 93 million miles (150 million km), or the distance traveled by light in just over eight minutes. The symbol AU is most often used to represent the astronomical unit, although less commonly you may see AU used instead.
People have estimated the distance between the Earth and the Sun for a long time. Many Greeks provided measurements, often incorrect by quite huge margins. The Greek Eusebius came up with a measure that is surprisingly close to the modern measure of an astronomical unit. In one of his works he estimated it at 804 million stadia. Stadia, a Greek unit of measurement, are approximately 605 to 625 feet (85 to 90 m), making his estimate somewhere between 92 to 95 million miles (149 to 153 million km).
At the end of the 17th century, the AU was officially estimated to be about 87 million miles (140 million km) using the position of Mars at two different points in Earth’s orbit. At the end of the 18th century a method was devised using Venus as a point of measurement, during its transit over the face of the sun. This method yielded a much more accurate figure. In the early 20th century an asteroid passed close to Earth and an even more accurate figure was calculated for the astronomical unit.
By the mid-20th century and well into the 21st century, improvements in various measurement technologies allowed much more precise measurements to be made, further refining the AU. Using space probes and satellites, modern definitions have been created, with a much greater degree of accuracy than any in the past. In 1976 the current definition of the astronomical unit was updated, to obtain a more sophisticated measure. Perhaps the most precise 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 that sounds confusing, think of it roughly as saying that an astronomical unit is the distance from the center of the Earth to the center of the Sun.
The actual figure for one AU produced by this definition and more modern measurements is approximately 92,955,807 miles (149,597,870,691 km). This figure was adopted in 1996 and is considered accurate to within about 10 feet (about 3m).
The astronomical unit can be useful not only for astronomers, but also for ordinary people trying to understand the relative distances involved within our solar systems. While the distances between planets may seem too great to ever 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 obviously 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 5 AU from the Sun, Pluto is a whopping 40 to 50 AU away. And if that sounds like a long way, consider that the closest star to our solar system is 268,000 AU away.
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