Anomalous Moon

There is something anomalous about the Moon. The ancients had an inkling of this when they were trying to devise their calendars, and even now, lunar-based calendars can never be precise. Part of the anomaly stems from its size relative to its primary, and part from its orbit around the Earth.

There are at least 139 known natural satellites in the solar system, although that number keeps increasing as more small satellites of the outer planets are discovered. Our Moon, with its diameter of 3,476 kilometers, is the fifth largest of these satellites in absolute terms. Three of Jupiter’s moons discovered by Galileo are bigger than the Moon, as is one of Saturn’s moons, Titan. Of the four inner planets, only Earth has a satellite of noteworthy size: Mercury and Venus have none, while Mars has two tiny moons that appear to be captured asteroids. None of the moons of Uranus comes close to the size of our Moon, and even the largest satellite of Neptune, Triton, is much smaller.

The Moon is also much closer in size to its parent planet than any other moon in the solar system. After our Moon, the next largest satellite relative to its primary is Titan, but Saturn is 4,220 times more massive than Titan, whereas Earth has only 81 times more mass than its Moon. Most satellites are less than one ten-thousandth of the mass of their primary. In this sense, the Earth and its Moon seem more like double planets.

The Moon also has another peculiarity that is unique within the solar system.

It is not quite correct to say that the Moon revolves around the Earth. It would be more accurate to say that both the Earth and Moon revolve about their common center of gravity, a point called the barycenter. If both bodies were equal in every aspect, their barycenter would lie exactly between them. Since the Moon’s mass is only 2% that of the Earth’s, the barycenter tends more toward the Earth’s direction; in fact, it is actually located around 4,700 km from the center of the Earth.

Due to the Sun’s stronger gravity, the Moon constantly tends to fall into the Sun although it is in a permanent orbital tango with the Earth at the same time. The Moon’s anomalous orbit results from its distance relative to Earth, which is far greater than most other planetary satellites.

The 384,401 km average distance of the Moon from Earth is sixty times the Earth’s radius. This is more than double the distance between Jupiter and its outermost moon, Callisto, almost triple the distance between Uranus and its outermost moon, Oberon, and four times greater than the distance from Neptune to its large moon, Triton.

In fact, of the major satellites in the solar system, only that other curious moon Iapetus, the outermost of Saturn’s major satellites, is as far away as our Moon in relation to its primary. Iapetus is the exception in being more than twice as far from Saturn as any other of the planet’s major satellites.

These are some of the things about our Moon that make it so special. No wonder it continues to evoke in us a sense of mystery over how it happened to be up there, and a sense of continued challenge over what humankind can do in worlds other than our own.