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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 (AU; equal to 150,000,000 km; 93,000,000 mi) is what the distance from the Earth to the Sun would be if the planet's orbit were perfectly circular. For comparison, the radius of the Sun is 0.0047 AU (700,000 km; 400,000 mi).
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:
The calculated stellar distance will be in the same measurement unit as used in Distance earth-sun (e.g. if Distance earth-sun = 1 au, unit for Distance star is in astronomical units; if Distance earth-sun = 1.5813 × 10 −5 ly, unit for Distance star is in light-years).
Reflected sunlight from the Moon's surface takes 1.2–1.3 seconds to travel the distance to the Earth's surface (travelling roughly 350 000 to 400 000 kilometres). 10 −6: 1.58 × 10 −5 ly: One astronomical unit (the distance from the Sun to the Earth). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance.
The Earth's axial tilt changes slowly over thousands of years but its current value of about ε = 23°44' is nearly constant, so the change in solar declination during one year is nearly the same as during the next year. At the solstices, the angle between the rays of the Sun and the plane of the Earth's equator reaches its maximum value of 23 ...