Let’s talk about the Moon. It’s been particularly spectacular recently and you might have been hearing about “supermoons” and all that in the media. If nothing else, maybe it will help you answer your grandkids’ questions, one of these days.
The Far Side of the Moon. Credit NASA.
First of all, I’ve always wondered why it was just called “the Moon”. We don’t call Mars “the planet” and we don’t call the Sun “the star” (although “sun” has come to be pretty generic, as well, now that it is used to mean any star with planets orbiting it). As is usually the case, there’s actually a good reason why the Moon (which is its official, scientific name) is called the Moon. The word “moon” is actually derived from a very ancient Indo-European word for “month”, which makes sense, since the Moon orbits the Earth on a (more-or-less) monthly cycle. The Latin word associated with the Moon is “Luna”. Luna was the name given to the Roman goddess of the Moon. The Greeks called it “Selene”, after their own goddess. “The Moon”, however, is the oldest name for it, by far. The reason the name “Moon” seems generic now is that, like the name “Sun”, we have come to apply the term to any similar astronomical objects. Any satellite (“satellite”, by the way, just means any object, natural or otherwise, that is in orbit around another astronomical body) of another planet is now referred to as a moon (lower case “m”), and they are given other, cooler-sounding names to distinguish them from the Earth’s moon. So now you know why our “moon” (little “m”) is called “the Moon” (big “M”). “Monday”, by the way, is named after the Moon, just like Sunday is named after the Sun.
The Moon is, by far, the largest satellite in the Solar System, relative to the size of the planet it orbits. There are four other moons in the solar system larger than ours, but they orbit the gas giant planets, Jupiter and Saturn, so they are smaller relative to the size of the planets they orbit. The origin of our Moon is still not entirely clear. The best theory currently is that the Moon was formed very soon after the Earth started to form, about 4.5 billion years ago. In this theory, the still-mostly-molten Earth collided with another large body in the Solar System, roughly the size of Mars, called Theia. The result of this collision created a huge amount of debris in orbit around the Earth that eventually accreted (“accretion” is a gradual process of growth by the accumulation of new material to an object) into the Moon. This isn’t the whole story, because, although most of the known facts fit, there are some facts that are not fully explained by this theory. However, it’s the most complete idea currently out there. I remember, back in the days of the Apollo missions, that people used to fuss a lot over the cost of the moon missions, “just to bring back a bunch of rocks”. Well, that was obviously not the purpose or main benefit of the Apollo missions, but one of the questions those rocks did answer was that the chemical composition of the Moon and the composition of the Earth are the same, and that composition is different from everything else in the Solar System. That means that the Moon and the Earth are, fundamentally, made of the same thing, meaning that the Moon didn’t start out somewhere else and get captured by the Earth’s gravity as it was passing by. Whatever the details of its formation, the Moon and Earth share the same history, starting with the formation of the Moon about 4.5 billion years ago, very shortly (only 100 million years, or so) after the Earth started to form. There is no cheese, green or otherwise, involved.
The Moon actually orbits the Earth once every 27 days, but because the Earth is also moving around the Sun, the phases of the Moon change on a cycle of 29.5 days, which is the “lunar month”. The months that we use in our calendar are “solar months” which are defined as 1/12 of the solar year, which is the length of time it takes the Earth to complete one complete orbit of the Sun. You may or may not know this, but throughout the phases of the Moon as it orbits the Earth, we are always seeing the same side of the Moon. Even though the Moon rotates on its axis, just like the Earth does, it rotates at a speed that the same face of the Moon is toward the Earth all the time. We never see the back side of the Moon, and with the exception of probes that we and other countries have sent and the Apollo missions, that orbited the Moon, we have never seen the back side. It’s called the “dark side of the Moon”, but it’s actually no darker or dark more often than the side we see. It’s just lighted when we can’t see it.
Now, about this “supermoon” thing—it’s not a really scientific definition and there are a couple of ways to look at it. In general, a supermoon is called that because the Moon looks (slightly) bigger sometimes than it usually does. The actual reason it looks bigger is that the orbit of the Moon around the Earth, like the orbit of the Earth around the Sun, isn’t a perfect circle. It’s actually an ellipse, or oval-shaped path. The average distance from the Earth to the Moon is 238,855miles (384,400 km). In its oval-shaped orbit, when the Moon is closest to the Earth (called the “perigee”), it is 225,623 miles away. At its furthest (called the “apogee”) its 252,088 miles away. A “supermoon” happens when the Moon is in its full phase at the same time it is at perigee. A full Moon at perigee doesn’t happen very often, just a few times a year. Sometimes, the perigee is even closer than usual, which would make a ‘super-duper moon”, or maybe if the really rare ones were called supermoons, the others would be “great moons” or something. Anyway, we had a “super-duper moon” back in 2016. The last one prior to that was in 1948 and the next one won’t be until 2034.
That explanation of a supermoon might not be what you think of when you think of the term “supermoon”, however. When the Moon is physically closest to Earth, it does appear slightly bigger. In the case of the supermoon, the full Moon appears about 7% larger than average and about 14% larger than the full Moon looks when it is the furthest away, at apogee. It’s also about 20 to 30% brighter, because it’s closer, looks bigger, and reflects more sunlight. However, sometimes the Moon looks absolutely ENORMOUS, right? That’s what I think about when the term “supermoon” comes up. That really has nothing to do with distance, but rather is just an optical illusion. When the Moon rises or sets, it looks much bigger when it is low in the sky than it does when it’s high in the sky. This is particularly apparent when the Moon is full as it rises in the evening. A Harvest Moon, for example, is the first full moon in the autumn. In reality, the Moon is, of course, always the same size. If you were to take a ruler and measure the diameter of the Moon near the horizon and then measure it again when it’s high in the sky, you would see that the diameter is the same. That it looks so much bigger near the horizon is a trick our brains play on us. It looks really big because you are seeing it against the horizon and all the trees and everything and your brain interprets the visual clues differently. The perspective of your brain is fooled because it is comparing the Moon to its surroundings. It fills up much more of your visual field. When it’s high in the sky, you have no reference points around it for comparision, and it’s just there in the middle of the sky, so it seems much smaller.
The Moon Rising. Credit NASA, Bill Dunsford
You’ve probably also heard the phrase, “once in a Blue Moon” to describe something that doesn’t happen very often. “Blue Moon” is another odd term, because color doesn’t have anything to do with it. Blue Moons are the same color as any other Moon. What the term actually refers to is when there are two full moons in a month or 5 full Moons in a given season. They occur every 2 ½ years, due to the difference in the solar month on our calendar and the actual period of the lunar orbit.
“Blood Moon” has a couple of meanings. One is the red appearance of the Moon during a total lunar eclipse, when the Earth is between the Moon and the Sun. The Moon appears red because the only light it is reflecting during the eclipse is light that passes through the edges of the Earth’s atmosphere, which scatters most of the blue light (remember the article on the electromagnetic spectrum?) and only the red wavelengths dimly reflect from the Moon.
Speaking of optical tricks, the Moon really isn’t all that bright, either. It is basically just sort of a dingy gray color and it’s not particularly reflective. It only looks bright because you usually see it when the sky is dark. When you see the Moon in the daytime, it looks much dimmer by comparison, although it’s just as brightly lit by the sun as it is when you see it at night. Which brings us to eclipses. A solar eclipse happens when the Sun, the Moon and the Earth line up so that the Moon blocks all or part of the Sun. Because the Moon is so much closer to Earth than the Sun, even though the Sun is so much larger, the Moon and the Sun look to be about the same size, and in fact, during a solar eclipse, the Moon almost exactly covers the Sun. Total solar eclipses aren’t all that rare, occurring somewhere on Earth about every 18 months, but because most of the Earth is ocean, total eclipses over land don’t happen all that often. Total solar eclipses average about once every 400 years for any particular spot. A lunar eclipse is when the Earth lines up between the Sun and the Moon and casts a shadow on the Moon. Sometimes, the Earth’s shadow covers the whole Moon, and that is a total eclipse. More often, the shadow only covers part of the Moon (a partial eclipse) or only the “outer ring”, or penumbra, of the shadow covers the Moon, causing a hard-to-see “penumbral eclipse”. We generally have from zero to four lunar eclipses a year and they can range from penumbral to total.
In addition to all these different ways we see and speak of the Moon, it also has other effects. The ocean tides are due (mostly) to the pull of the Moon’s gravity as it orbits the Earth. Whatever side of the Earth the Moon is on, it exerts a gravitational attraction on the Earth, and on the water in the oceans, and pulls the water toward it, causing a high tide on the side where the Moon is and a low tide on the side of the Earth away from the Moon. Since the Moon is closer than usual during a supermoon (or actually at any perigee), the tidal forces are greater, causing higher than normal high tides and lower low tides. There is also a tiny possibility that the greater gravitational pull might raise the risk of earthquakes, because the Moon’s gravity also pulls on the Earth’s crust, pulling slightly harder during a supermoon, although there is no data to suggest an increase of natural disasters during lunar perigee.
So, now maybe you know a little bit more about the Moon than you did before. The next time you’re looking at the Moon and thinking whatever looking at the Moon causes you to think, I hope you’ll be able to appreciate your view just a little bit more.
Oh, one more thing. Full moons don’t make people crazy, despite the word “lunatic” being derived from Luna, the Roman goddess. There is no actual evidence that human behavior is affected at all by full moons, despite what teachers, ER staff and cops might think. Believe it or not.