Professional astronomers use specialized software not normally available to students to determine the rotation periods of asteroids from fragmented light curve data. This paper describes a simple yet accurate method based on Microsoft Excel[R] that enables students to find periods in asteroid light curve and other discontinuous time series data of the sort that they might obtain from robotic telescopes like the Faulkes Telescopes. The method is based on least squares fitting of a Fourier sum model. An initial estimate of the rotation period is made using a composite light curve obtained by superposition of the separate data sets. This initial estimate is then refined by observing how the variance of residual errors changes as the period is varied and the model moves in and out of phase with individual sets of observations. Examples are given relating to the rotation periods of six asteroids based on light curve data of varying quality from excellent to poor. (Contains 16 figures and 2 tables. Brief details of how the method is implemented is appended.)