In \cite{Lomb}, Lomb developed a nonlinear regression approach to estimating the frequency of a noisy sinusoid when the measurement times were not equispaced, and a method for correcting the times so that the resulting regression sum of squares appeared very similar to the usual periodogram.
Scargle \cite{Scarg} suggested that the usual periodogram be discarded, and replaced by the new version, which has become known as the Lomb-Scargle periodogram. In this paper, we extend Lomb's development to include a `DC' term. We show why it is important to include this term, especially when the times are irregular or the frequency low.