### 1. INTRODUCTION

In cosmology (or to be more specific, *cosmography*, the
measurement of the Universe) there are many ways to specify the
distance between two points, because in the expanding Universe, the
distances between comoving objects are constantly changing, and
Earth-bound observers look back in time as they look out in distance.
The unifying aspect is that all distance measures somehow measure the
separation between events on radial null trajectories, ie,
trajectories of photons which terminate at the observer.

In this note, formulae for many different cosmological distance
measures are provided. I treat the concept of ``distance measure''
very liberally, so, for instance, the lookback time and comoving
volume are both considered distance measures. The bibliography of
source material can be consulted for many of the derivations; this is
merely a ``cheat sheet.'' *C* routines (KR) which compute all of
these distance measures are available from the author upon request.
Comments and corrections are highly appreciated, as are
acknowledgments in research that makes use of this summary or code.