It's easy to go about your days and nights here on Earth and ignore that giant companion that orbits us. However, this week a lot is going on for the moon, so let's give it some attention.
First off, it's a full moon tonight! Specifically, a Harvest moon. In some parts of the world it's also a lunar eclipse tonight, although not for North and South America. Bummer for us.
Second, this week there was a cool bit of news concerning a new theory on the formation of the Earth's moon.
Let's tackle the Harvest moon/ lunar eclipse tidbit first. A Harvest moon is the name of the full moon that occurs closest to the autumnal equinox, which is September 22 this year. The equinox is an important date because it marks the day of the year when the northern hemisphere transitions from summer to fall.
Equinox actually means "equal night" in Latin which implies that day and night should be 12 hours each. This doesn't actually end up happening due to the fact that latitude also determines the length of one's day/night. However, this is mostly true in general due to the fact that the Earth's polar axis has a tilt that is pointing neither at the sun nor away from it during the equinoxes.
As you can probably guess, the full moon nearest the fall equinox would be named after the harvest due to the fact that farmers in the northern-hemisphere-centric world are generally harvesting their crops during this time of year.
Okay, moving on to lunar eclipses. A lunar eclipse happens during a full moon because the moon is on the opposite side of Earth from the sun (so we can see it fully illuminated at night). However, recall that the Earth is rotating over the course of the night AND that the moon is orbiting Earth about every month. Let's do some quick math. Over the course of one 24 hour period, or ~1/30th of a month, the moon will orbit through ~1/30th of its full orbit or 1/30th of 360 degrees, which is 12 degrees. So the lunar eclipse is not visible for everyone, because the moon will eventually leave the shadow of the Earth. Additionally, the orbit of the moon is on a slight tilt relative to the plane of the Earth and the sun, meaning that the moon is rarely perfect aligned with the shadow of the Earth. So while most of the Earth can see the eclipse today/tonight, the moon has rotated out of the shadow of the Earth by the time night rolls around for the North and South America.
Let's talk about the big moon news from this week. Astronomers have known that the moon is weird for quite some time. It's highly unusual for a planet as puny as the Earth to have such a large companion. It's hard for Earth to gravitationally capture an object this large. Just look at the other planets and moons in our solar system. Saturn and Jupiter have a plethora of large moons that compare in size to the moon, but planets similar to the Earth in size have tiny moons (such as Mars' Phobos and Deimos).
So what gives?
The most widely accepted scientific theory for the formation of moon is the giant impact hypothesis. It is exactly what it sounds like. A giant impactor, named Theia, smashed into the Earth in a relatively low energy collision. The debris from this collision coalesced into the moon. However, computer simulations of this low energy giant impact predicted that most of the moon should consist of material from Theia.
The problem is that the composition of the Earth and moon match almost exactly. This seems unlikely if the moon were built mostly from another unknown composition. Statistically, Theia's composition should not exactly match that of Earth. Scientists thought more precise measurements of composition might resolve this problem but a study in 2016 only confirmed the existing problem.
So what are the new theories? How can the moon's composition so closely match that of Earth? The leading theory so far is a higher energy collision that vaporized Theia and the early Earth down to the entire mantle! This would explain how the surface compositions of the moon is slightly richer in potassium-41, which a heavier isotope of potassium. This heavier isotope would have a chance to condense in a high pressure cloud that the moon formed from in the higher energy impact event.
So as you take the time tonight to gaze up at the Harvest moon, spend a second to think about the fact that we're still in the process of learning about our closest companion.