AbstractAnalysis of 1968 New York City birth records supports folklore that the moon influences human reproduction. Births vary systematically over a period of 29.53 days, the length of the lunar cycle, with peak fertility at third quarter. The effect is small, but is present in four independent time series of births (male, female, black, white) and remains when weekly variation has been removed. A photic explanation is developed which links rhythmic variation in lunar illumination to the timing of ovulation.