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The source of the gravitational quirkiness is a number of huge mascons (short for "mass concentrations") buried under the surfaces of lunar maria or "seas." Formed by colossal asteroid impacts billions of years ago, mascons make the moon the most gravitationally lumpy major body in the solar system. The anomaly is so great—half a percent—that it actually would be measurable to astronauts on the lunar surface. A plumb bob held at the edge of a mascon would hang about a third of a degree off vertical, pointing toward the central mass. Moreover, an astronaut in full spacesuit and life-support gear whose lunar weight was exactly 50 pounds at the edge of the mascon would weigh 50 pounds and 4 ounces when standing in the mascon's center.