While variations in the diurnal temperature range (DTR) have been linked to atmospheric enhancement of greenhouse gases, it is useful to identify all contributing factors potentially acting upon the DTR. In this study, an area-weighted record of daily temperatures from the United States Historical Climate Network is used to examine the influence of lunar phase variations on the DTR. For the period from 1950 to 1995, a statistically significantly higher DTR occurs near the full moon (~10.23 ¿C) while a lower DTR occurs near the new moon (~10.13 ¿C). This synodic monthly difference appears to be primarily influenced by warmer maximum daily temperatures occurring near the full moon. This finding allows evaluation of two potential extraterrestrial factors influencing terrestrial temperatures as a function of lunar phase: a) the barycenter effect which is defined as variations in terrestrial and lunar position around their mutual gravitational centroid over the course of synodic month, and b) reflected solar/infrared emission off the lunar surface over the course of a synodic month. While both the barycenter effect and the reflected solar and infrared emission off the lunar surface would influence the minimum temperatures, we suggest that the barycenter effect is likely the dominating extraterrestrial mechanism in explaining lunar phase variations in the diurnal temperature range because its additional influence on daily maximum temperatures. ¿ 1999 American Geophysical Union