Astronomical Unit

Astronomical Unit

Astronomical unit is the unit of length which is used by astronomers to measure shorter distances or sizes of terrestrial bodies in space. For extremely long distances we use light years but for defining distances comparable to the size of our solar system, we need a unit which is smaller than a light year but much bigger than kilometers. Its symbol is ‘au’.

For solving this purpose International Astronomical Union defined one astronomical unit as the length in 1976, for which the value of Gaussian Gravitational Constant becomes 0.01720209895. This is the value when astronomical units of length and mass are used for measurement. Another definition is that the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with an angular velocity of 0.01720209895 radians per day is called one astronomical unit. One more definition is that when we are to describe the position of an object in the solar system, the length at which, the product GM which is called heliocentric gravitational constant becomes equal to (0.01720209895)2 au3/d2. This unit of length was defined to measure length of celestial bodies.

This distance is roughly equal to the distance been the Sun and Earth. Before 1976, astronomical unit was defined as the length of semi major axis which was a less accurate measurement. For solving this problem, these more precise methods were adopted in 1976. In the IERS numerical system, speed of light in vacuum is 299792458000 m/s if astronomical units are measured in SI units. The time taken by light to travel one au is equal to 499.0047838061±0.00000001 seconds. With the help of this data, the length of 1 au in meters comes to be equal to 149,597,870,700±3 m.

Since the definition of 1 au as the measure of Gravitational constant was difficult to maintain therefore, in 2012, 1 au was defined as 149,597,870,700±3 m, leaving no confusion in the measurements. This distance is roughly the radius of the locus of earth around the Sun.
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