in table \[tab:M37\_rotation1\].

In figure \[fig:MainBandLC\] we show phased light curves for a random sample of stars that fall along the main period-color band (see §4). Figure \[fig:RapidRotLC\] shows phased light curves for a random sample of rapid rotators ($P < 2~{\rm days}$). Finally, in figure \[fig:LongPeriodRotsLCS\] we show the light curves of 4 of the 5 long-period stars in the clean sample with $P > 15~{\rm days}$ and $r < 18.5~{\rm mag}$. Note that the fifth star is rejected as a non-cluster member based on its RV.

Period and Color Uncertainties
------------------------------

As we will discuss in §8.2, the spread in rotation periods for stars of a given mass puts a powerful constraint on theories of stellar angular momentum evolution. To determine whether or not the observed period spread is real it is important to have accurate estimates for both the period and color uncertainties.

### Period Uncertainties {#sec:perunc}

Uncertainties in the period can be divided into two classes: errors due to aliasing or choosing the wrong harmonic of the true period and random errors. Aliasing generally yields a discrete set of peaks in the periodogram of a light curve resulting in an uncertainty in choosing the peak that 