The comoving distance (line of sight) D_C between two nearby objects in the Universe is the distance between them which remains constant with epoch if the two objects are moving with the Hubble flow. It can be thought of as the distance with the expansion of the Universe factored out, i.e. the comoving distance between fixed objects remains constant, and is useful in cosmological calculations.

The comoving distance between two events at the same redshift or distance but separated on the sky by some small angle ΔΘ is D_M ΔΘ and the comoving distance (transverse) D_M is simply related to the line-of-sight comoving distance D_C by the product of the Hubble distance D_H with trigonometric functions sinh (for a closed universe) and sin (for an open universe) to account for the curvature of spacetime. For a flat universe D_M = D_C.