Does The Moon Orbit The Earth Or The Sun?

For your viewing pleasure, here’s another look at our planet and satellite from a distant vantage point. The composite image was made by NASA’s OSIRIS-REx’s spacecraft on Oct. 2, 2017 and released this week. The spacecraft was about 3 million miles (5 million km) from Earth at the time and en route to its target, the asteroid Bennu, a carbon-rich mass of rock that might contain organic materials. Three images taken through separate color filters were combined and color-corrected to make the composite, and the Moon was “stretched” (brightened) to make it more easily visible.

Earth and moon have been buddies since the moon’s creation in a cataclysmic impact some 30-40 million years after the formation of the Earth. It’s hypothesized that a Mars-sized asteroid struck the early Earth, sending billions of tons of debris into space which later coalesced into the moon. Four billion years later, we’re grateful for both its beauty and practicality in lighting the way at night.

Someone asked me recently whether the moon goes around the Earth or around the sun. The answer is “yes” to both questions. Two bodies in space orbit around their center of mass called the barycenter. Because Earth possesses the lion’s share of the mass in the Earth-moon system, the moon orbits around the Earth and not the other way around. Despite the giant appearance of the full moon, our satellite is only 1.2% as massive as the Earth.

Together, the Earth and moon orbit the sun, which is 333,000 times more massive than Earth. The moon also feels a gravitational tug from the sun separate from that of Earth — one of the effects of which is to alter the shape of its orbit around the Earth — so it’s also fair to say that the moon orbits the sun though not in the immediate way it does Earth.

Viewed from above the plane of the solar system, the moon’s orbit looks like a thread weaving in and out of Earth’s orbital path over the course of time. That’s because over the course of 27 days it moves from inside Earth’s orbit at new moon to outside of it at full. At the true scale of the solar system, the thread is almost concentric to Earth’s orbit.

You might be surprised to learn that the Earth-moon barycenter isn’t in the planet’s core but 2,902 miles (4,671 km) from its center in the deep mantle. If the moon were a lot smaller, the center of mass would be deeper inside the core; if much larger, it would shift closer to the surface.

Pluto and its largest moon Charon are much closer in size than the Earth and moon. Can you guess where their barycenter is? It’s located at a point in space between the two but closer to Pluto because it’s the more massive of the pair. Think of a sledgehammer: the center of mass is much closer to the heavy hammer end than the wooden handle. Now imagine two orbiting objects with identical masses. They would orbit around a point in space exactly in the middle between the two.

Barycenters are a big deal when hunting for extrasolar planets. Larger, closer planets tug harder on their host suns, causing them to wobble a tiny bit back and forth, first closer to us, then farther in a repeating pattern. When the star moves toward us, the color of its light changes slightly (becomes bluer). When it’s moving away, it’s slightly redder. Astronomers measure the shift in color and uncover planets (and their masses) that would otherwise be lost in the stellar glare.

After reading this far, I hope you’re feeling a little more centered than when you started 🙂