In astrodynamics, A canonical unit is a unit of measurement defined in terms of an object's reference orbit. In this system, a reference mass, for example the sun, is assumed to be 1 "mass unit" and the mean distance from the orbiting object to the reference object is considered the "distance unit".
Canonical units are useful when the precise distances and masses of objects in space are not available. However, by setting the mass of a given object to be 1 mass unit and the mean distance of the reference object to another object in question, many calculations can be simplified.
The distance unit is defined to be the mean radius of the reference orbit.
The time unit is defined by the gravitational parameter :
For Canonical Units, the gravitational parameter is defined as:
The Time unit (TU) can be converted to another unit system for a more useful qualitative solution using the following equation:
An Astronomical units is canonical unit based on the orbit of the Earth around the Sun
Roger R. Bate, Donald D. Mueller, Jerry E. White. Fundamentals of Astrodynamics.