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The astronomical unit (symbol: au, or AU) is a unit of length defined to be exactly equal to 149,597,870,700 m. Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its modern redefinition in 2012.
The solar radius is usually defined as the radius to the layer in the Sun 's photosphere where the optical depth equals 2/3: [1] 695,700 kilometres (432,300 miles) is approximately 10 times the average radius of Jupiter, 109 times the radius of the Earth, and 1/215th of an astronomical unit, the approximate distance between Earth and the Sun.
The astronomical unit of length is now defined as exactly 149 597 870 700 meters. It is approximately equal to the mean Earth–Sun distance. It was formerly defined as that length for which the Gaussian gravitational constant (k) takes the value 0.017 202 098 95 when the units of measurement are the astronomical units of length, mass and time.
The separation of Earth from the Sun can be denoted R E and the mean distance can be denoted R 0, approximately 1 astronomical unit (AU). The solar constant is denoted S 0. The solar flux density (insolation) onto a plane tangent to the sphere of the Earth, but above the bulk of the atmosphere (elevation 100 km or greater) is:
Earth orbit (yellow) compared to a circle (gray) Earth orbits the Sun at an average distance of 149.60 million km (8.317 light minutes, 92.96 million mi) [1] in a counterclockwise direction as viewed from above the Northern Hemisphere. One complete orbit takes 365.256 days (1 sidereal year ), during which time Earth has traveled 940 million km ...
This is because the solar constant is evaluated at a fixed distance of 1 Astronomical Unit (au) while the solar irradiance will be affected by the eccentricity of the Earth's orbit. Its distance to the Sun varies annually between 147.1·10 6 km at perihelion and 152.1·10 6 km at aphelion.
The scale of the Solar System is sufficiently large that astronomers use a custom unit to express distances. The astronomical unit, abbreviated AU, is equal to 150,000,000 km; 93,000,000 mi. This is what the distance from the Earth to the Sun would be if the planet's orbit were perfectly circular. Formation and evolution
The lunar distance is on average approximately 385,000 km (239,000 mi), or 1.28 light-seconds; this is roughly 30 times Earth's diameter or 9.5 times Earth's circumference. Around 389 lunar distances make up an AU astronomical unit (roughly the distance from Earth to the Sun). Lunar distance is commonly used to express the distance to near ...