One of the earliest was the so-called binary accretion model, which concluded that the Moon accreted from material in orbit around the Earth left over from its formation. Another, the fission model, was developed by George Darwin (son of Charles Darwin), who noted that, as the Moon is gradually receding from the Earth at a rate of about 4 cm per year, so at one point in the distant past it must have been part of the Earth, but was flung outward by the momentum of Earth's then–much faster rotation. This hypothesis is also supported by the fact that the Moon's density, while less than Earth's, is about equal to that of Earth's rocky mantle, suggesting that, unlike the Earth, it lacks a dense iron core.