The first known person to measure the distance to the sun was Greek astronomer Aristarchus of Samos, who lived from about 310 B.C. (Image credit: NASA) History of the astronomical unit Our solar system in astronomical unitsĪrtists rendering of the Kuiper Belt and Oort Cloud. The current definition addresses this problem by basing the distance on the speed of light in a vacuum, which always remains constant. Another is related to Einstein's theory of general relativity, which posits that space-time is relative to the observer's location. Why the change? One reason was that the previous method of calculating an AU depended on knowing the mass of the sun, but that measurement is always changing as the sun converts its mass into energy, Nature reported. The IAU, the international group that defines astronomical constants, decided to make the measurement simpler in August of that year. Changes to the definitionīefore 2012, the definition of an astronomical unit was not defined as a constant and depended on several factors. It takes light 8.3 minutes to travel between Earth and the sun but 3.26 years to travel one parsec. One parsec is about 19 trillion miles (31 trillion km). A parsec is a unit used to measure vast distances in interstellar space, such as distances between stars and galaxies, and is partially defined using an AU. What's the difference between an astronomical unit and a parsec?Īn astronomical unit is the distance between Earth and the sun and measures distances on the scale of star systems. Mercury, the closest planet to the sun, is about 0.39 AU from our star, while Neptune, the farthest planet from the sun, is 30.06 AU away from it. What are examples of astronomical units?Įarth, by definition, is 1 AU from the sun. One astronomical unit is exactly 149,597,870,700 meters (92,955,807 miles or 149,597,871 km), as defined by the International Astronomical Union. Astronomical Unit FAQs What is the length of one astronomical unit? The next-nearest star to the sun, Proxima Centauri, is about 1.3 parsecs, 4.25 light-years or 268,770 AU away. Technically, a parsec is defined as the distance at which 1 AU subtends a one-arcsecond angle (see diagram to the right). The parsec is a more technical measurement that is derived from an astronomical unit and is used mainly by scientists. Another unit is the parsec, which is equal to about 3.26 light-years, according to NASA.
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