Astronomical Unit of Length
The astronomical unit of length is 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 dimensions of k2 are those of the constant of gravitation (G), i.e., L3M−1T−2. The term “unit distance” is also used for the length A while, in general usage, it is usually referred to simply as the “astronomical unit”, symbol AU, au or ua.
An equivalent definition of the astronomical unit is the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.017 202 098 95 radians per day. It is approximately equal to the mean Earth–Sun distance.
The speed of light in IAU is the defined value c0 = 299 792 458 m/s of the SI units. In terms of this speed, the astronomical unit of length has the presently accepted value: 1 ua = c0τA = 1.495 978 707 00 × 1011±3 m., where τA is the transit time of light across the atomic unit. The atomic unit of length is determined by the condition that the measured data in the ephemeris match observations, and that in turn decides the transit time τA.
Read more about this topic: Astronomical System Of Units
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