A canonical unit is a unit of measurement agreed upon as default in a certain context.

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.

The Canonical Distance Unit ${\displaystyle \;{\text{[CDU]}}\;}$ is defined to be the mean radius of the reference orbit.

The Canonical Time Unit ${\displaystyle \;{\text{[CTU]}}\;}$ is defined by the gravitational parameter ${\displaystyle \;\mu \;}$:

where

In canonical units, the gravitational parameter is given by:

Any triplet of numbers, ${\displaystyle \,M\,,}$ ${\displaystyle \,{\text{[CDU]}}\,,}$ and ${\displaystyle \,{\text{[CTU]}}\,,}$ that satisfy the equation above is a “canonical” set.