What unit of length is defined as the mean distance between the earth and the sun (approximately 149,597,871 km)?
Astronomical system of units planets. Earth mass () is the unit of mass equal to that of the Earth. 1 = . Earth mass is often used to describe masses of rocky terrestrial planets. It is also used to describe Neptune-mass planets. One Earth mass is times a Jupiter mass. 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 when the units of measurement are the astronomical units of length, mass and
Astronomical unit barycentric dynamical time, TDB). This is because the distance between Earth and the Sun is not fixed (it varies between and ) and, when Earth is closer to the Sun (perihelion), the Sun's gravitational field is stronger and Earth is moving faster along its orbital path. As the metre is defined in terms of the second and the speed of light is constant for all observers, the terrestrial metre appears to change in length compared to the "planetary metre" on a periodic basis. The metre is defined to be a unit of proper length, but the SI definition does not