In astrodynamics, canonical units are defined in terms of some important object’s orbit that serves as a reference.
In this system, a reference mass, for example the Sun’s, is designated as 1 “canonical mass unit” and the mean distance from the orbiting object to the reference object is considered the “canonical distance unit”.
Canonical units are useful when the precise distances and masses of objects in space are not available.
Moreover, by designating the mass of some chosen central or primary object to be “1 canonical mass unit” and the mean distance of the reference object to another object in question to be “1 canonical distance unit”, many calculations can be simplified.
is defined to be the mean radius of the reference orbit.
: where In canonical units, the gravitational parameter is given by: Any triplet of numbers,
The quantity of the time unit [CTU] can be solved in another unit system (e.g. the metric system) if the mass and radius of the central body have been determined.
For Earth-orbiting satellites, approximate unit conversions are as follows: The astronomical unit (AU) is the canonical distance unit for the orbit around the Sun of the combined Earth-Moon system (based on the formerly best-known value).