Early Theories

Scientists' interest in the origin of the moon in the mid- and late 1900's stood in sharp contrast to attitudes about the moon during the early days of astronomy. The Italian astronomer and physicist Galileo Galilei first gazed at the moon in 1609 with the newly invented telescope, but he offered no theories about the moon's history, nor did most of the other astronomers who came after him.

It was only in 1879 that the British mathematician George H. Darwin, son of the great naturalist Charles Darwin, proposed one of the first theories of lunar origin. According to Darwin's “fission” theory, the material that became the moon was spun off from the very young, fast-spinning Earth, which was still in a molten state. Darwin's theory had little influence on astronomers for almost 100 years, in large part because they continued to be uninterested in the moon's origin.

Scientists finally started to investigate the question in the 1950's. By 1952, a “capture” theory of the moon's origin, proposed by an American chemist, Harold C. Urey, had become the prevailing view. Urey believed that the moon was a primordial (ancient) body that formed at the dawn of the solar system and was later captured by the Earth and pulled into its present orbit.

Urey's views had great influence on the planning of the National Aeronautics and Space Administration's (NASA) Apollo program, the series of manned flights to the moon that took place between 1969 and 1972. Many scientists believed that the moon rock samples that Apollo astronauts were to bring back would reveal the minerals and compounds—incorporated into the primordial, virtually unchanging moon—that were present when the solar system formed. They also expected the samples to help prove or disprove the capture theory. If the moon formed far away from the Earth, they reasoned, it might have a different chemical composition than the Earth.

But long before the lunar samples were gathered, some scientists had doubts about the capture theory. If the moon had formed far away in the solar system, mathematical calculations showed, it would have had to follow a long, looping orbit to reach the Earth, which it would have zipped past at high speed. It would not be possible, the scientists calculated, for a moon captured from such an orbit to end up in its present position, with the Earth and moon spinning at their current rates, once every 24 hours and once every 28 Earth days, respectively.

Some scientists proposed in the early 1960's that the moon formed at roughly the same time and at the same distance from the sun as the Earth, though not alongside it. The infant moon would have been orbiting the sun at nearly the same speed as the Earth and would sometimes move slowly past it. The moon would then have been easy to capture. If this scenario was true, the moon should be made of the same materials as the Earth.