The Earthmoon Binary Planet

Simple accounts of the Solar System often start by saying that the Earth orbits the Sun, and as it does so the Moon revolves around the Earth. While this is a reasonable first step, it is not quite true. Many stars are said to be binary: pairs of stellar bodies locked together in mutual gravitational embrace, each orbiting the center of mass of the duo. Similarly the Earth—Moon system can be thought of as being a binary planet.

The mass of the Moon is about one part in 81 that of the Earth. There are larger natural satellites elsewhere in the Solar System, such as Jupiter's Ganymede and Callisto, Saturn's Titan, and Neptune's Triton, but they are smaller in proportion to the mass of the associated planet. The only exception is Pluto and its moon Charon, discovered in 1978; Charon is about one-eighth the mass of Pluto, so that system certainly comprises a binary planet, although they are both tiny.

In the case of the Earth—Moon system, one should really say that the pair orbits their combined center of mass, which is termed the barycenter. In turn the barycenter orbits the Sun. The barycenter is on the line joining the middles of Earth and Moon, and the relevant calculation places it about 2,900 miles from the core of our planet. Because the terrestrial radius is about 3,964 miles, the barycenter is within the Earth, as shown in Figure A-1. As the Moon orbits, the Earth also swivels around this point, as indicated in the diagram.

Generally we are not aware of any wobble in our movement, but by the same token we tend not to notice that we are speeding along on our path around the Sun at near 18.5 miles per second (almost 67,000 mph). This velocity varies between about 18.2 miles per second in early July and 18.8 in January. Similarly, on

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