A cautionary note on conversions between galactic and equatorial coordinate systems

The conversion between equatorial (α, δ) and new galactic (l, b) coordinate systems involves the straightforward application of an IAU definition and some basic spherical trigonometry. Unfortunately, the common reference books (e.g., Allen 1973; Kraus 1966) do not quote the definition correctly and frequently copied computer algorithms (e.g., Ball 1969; Manchester and Gordon 1970), having incorporated this error, lead to an inconsistent forward-backward conversion and to errors of as much as one arc minute. We present here the source of these errors and accurate parameters for use in general conversion routines.