It's orbit is actually slightly elliptical. Perfectly circular orbits are almost impossible due to gravitational perturbations between the planets. The 'eccentricity' of a perfect circle is 0.00 and for the Moon's orbit, this ratio between its 'major' and 'minor' axis difference ( major-minor/major) is only 0.05...5%. Since the average Earth-Moon distance is about 384,000 km, this means that there is only a difference of about 0.05x384,000 = 19,200 km between its major and minor axis.
As I said, exactly circular orbits would be expected to be the exception when you consider that there are an infinite number of other 'non-circular' orbits compared to only one kind of pure circular orbit ( with an eccentricity of exactly 0.0000). In fact, in practical terms, what one calls 'circular' is actually a matter of how accurate you mean. In an exact definition, the lunar orbit is close to circular, but is actually elliptical.
Physically-speaking, elliptical orbits are noted for the changing speed of the orbiting body as it comes close too, and far from, its primary body. When you have satellites close-in like the Moon or the satellites of Jupiter and Saturn, the primary body can cause tidal dissipation in the satellite by actually deforming the satellite. This causes a breaking effect on the satellite, rendering the orbit more circular as time goes on. It also causes the satellites rotation and revolution periods to get locked into synchrony. Circular orbits are prevalent among the closest satellites orbiting a planet, with more elliptical ones in the outer satellites where this tidal effect is less effective.
Copyright 1997 Dr. Sten Odenwald
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