At dawn, two things rise in the sky. First, is the Sun, the glowing ball of hydrogen plasma around which Earth orbits. Second, is a new Moon. From our perspective, the Moon is between us and the Sun and is showing its near side to us fully shadowed. It will set at sunset, slightly after the sun. As is traverses the sky, the Moon moves west at a lunar-diameter per hour. Over the course of a day it will have lagged behind the Sun by 6 degrees. After 27 days, the Moon will be back to its original position. The Sun however, is not in the same position. As Earth orbits the Sun, even in as little as a month, it moves further around the Sun and we are looking at it at a different angle. So the waning crescent Moon takes a few more days to make it back to the new moon phase. The Moon has now completed one orbit of Earth. No other body in the solar system moves in this fashion.
Naturally, the Moon is a satellite of Earth. Or so it would appear.
Moon one day after its New phase, with planet Jupiter. Image reversed by my telescope.
In the turmoil of the early solar system, many rocky objects were flying around and crashing into each other (I say rocky because the gas giants formed early on and stayed more or less undisturbed, physically at least). The larger bodies were molten balls of matter that grew larger when they absorbed another body; the larger the body, the stronger the gravity and thus the more matter it can attract. At one point, the proto-Earth was more or less the size it is now and was attracting larger objects.
Enter: the Moon. Or, rather, a Mars-sized body posthumously named Theia. The prevailing lunar origin theory is that Theia hit Earth with sufficient force to tear a sizable chunk of molten Earth out into space, where Theia left a good bit of its own material strewn in orbit. In roughly a month, the bits of Theia and the bits of Earth came together in an orbit around Earth, squeezed together until spherical, and became the Moon. An offshoot of this theory has models of a second, smaller moon being formed which was then swallowed by the larger moon, but for the sake of this article I’ll spare you the details.
Theia and early Earth via Researchgate.net
During the main impact, much of the material of both proto-Earth and Theia intermingled. The current chemical makeup of the Earth-Moon system is generally the same. In particular there is an identical abundance of an oxygen isotope that is present in both bodies which could only be caused by this intermingling. It can then be said that the bodies are two halves of an isotopically homogeneous whole. The impact also sent the iron-nickel core of Theia into the Earth, where it combined with Earth’s core (This follows from Earth’s substantial density and gravity, whereas the Moon has a low density and gravity suggesting it is mostly rock).
Relative to its primary, the Moon is the largest satellite. By mass, it is one-eightieth the size of Earth. By width, one-fourth. The relative sizes are so large that the barycenter (gravitational focal point of the orbit) for the system is about half of a radius from Earth’s center, roughly within the mantle. In this way, it could be said that the Earth-Moon system is less of a primary and satellite and more of a major planet and its smaller binary companion.
Jupiter for example, is one-ten thousandth to even its largest satellite, Ganymede, which is in fact the largest of the non-planets. Another example is that of Pluto and Charon. The dwarf planet and its largest satellite are even more evenly matched than Earth and the Moon. Charon is half the width and about an eighth of Pluto’s mass. The barycenter for this system is above Pluto’s surface by about one diameter. Because both orbit a point outside either body, the Plutonian system is often said to be a binary planetary system; both are in co-orbit around the Sun.
The orbit of Pluto and Charon via giphy.com
Earlier, I said that the Moon doesn’t orbit Earth. In fact, it more so orbits the Sun, and with some (moderately) simple math, you can prove it. Isaac Newton formulated a law of gravitation which states the force of gravity on an object is proportional to the product of the two masses divided by the square of their distance. Because the mass is divided by a squared radius, as the distance doubles, the gravitational attraction between the two objects drops by a quarter of its original value.
Using Earthly measures, the Sun is 333,400 times as massive. The distance between the Sun and Earth (and the Moon) is approximately 400 times as far as the Earth-Moon distance. Plugging these into Newton’s equation for the force of the Sun on the Moon and separately for the Earth on the Moon gives a ratio of 2 to 1 in the favor of the Sun.
Even though the Sun is many times further, its incredible mass pulls doubly on the Moon than does the nearby Earth. As the Moon moves about Earth, from the perspective of the Sun, it never stops or moves backwards. From above Earth’s orbit, it appears that the Earth and Moon are co-orbital, tracing out the same path around the Sun.
Mathematics aside, there is another way to show this motion. Jupiter’s major moons (with the slight exception of Callisto) orbit in a plane centered on Jupiter’s equator. At no point do these satellites stray from this equatorial orbit. The Moon, however, orbits along the ecliptic plane. This is the plane in which Earth’s orbit (and the orbits of other planets) is situated, and it is the line on which the Sun moves through the sky. If the Moon were a true satellite of Earth, it would be held fast to Earth’s equator, and eclipses would be confined to equinoxes (recall that an equinox is when the Sun is directly overhead at noon along the equator). Instead, eclipses can happen anywhere and at any time. The next solar eclipse, which is when the Moon moves in front of the Sun and casts a shadow on Earth, is coast-to-coast in North America on August 21, well before the fall equinox. Unless you live on the extreme corners of the U.S., it should be within a day’s drive. I highly recommend traveling to watch this rare (for our region at least) astronomical event. Your next chance won’t be till 2024.
Paths of totality for the next few eclipses. Athens Astronomical Society will be in Nashville, Tennessee.
