Canonical units
A canonical unit is a unit of measurement agreed upon as default in a certain context. In astrodynamics. 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. Overview. The "Canonical Distance Unit" formula_1 is defined to be the mean radius of the reference orbit. The "Canonical Time Unit" formula_2 is defined by the "gravitational parameter" formula_3: where In canonical units, the gravitational parameter is given by: Any triplet of numbers, formula_8 formula_9 and formula_10 that satisfy the equation above is a “canonical” set. The quantity of the time unit can be solved in  another unit system (e.g. the metric system) if the mass and radius of the central body have been determined. Using the above equation and applying dimensional analysis, set the two equations expressing formula_3 equal to each other: formula_12 The time unit () can be converted to another unit system for a more useful qualitative solution using the following equation: formula_13 For Earth-orbiting satellites, approximate unit conversions are as follows: Astronomical Unit. The astronomical unit () is the canonical distance unit for the orbit around the Sun of the combined Earth-Moon system (based on the formerly best-known value). The corresponding  time unit is the (sidereal) year)), and the mass is the total mass of the Sun ().