Super Moon Phases Blue Super Blue Phases!
Keyword Bonanza!
Ok, I lied. I was working on one of my favorite topics — how the 1,500-year-old idea of an Earth-centered Solar System was finally refuted with observational evidence, and intended for that to be the next article, but the big idea relies on an understanding of why objects exhibit phases in the first place. I figured maybe it’d be cool to step the F back and write about Moon phases, and with the media going nuts about the Blue Super thing last week, it was time to crack the ole knuckles and get to work. And, anyway, Moon phases just delight me every time I look up and see them, so it’s awesome for me any way you slice it.
SuperBlueMoon Stuff
Ok, friends… this might come across as a rant, but I don’t mean it that way. Or maybe I do? This is gonna be live therapy, I think: regarding the hoopla about the Blue Super Moon, I just… don’t get it. It hurts me to write that! Two general principles that mean a lot to me are
Let people enjoy things — it’s supremely annoying to read some thread where two people love a movie, say, and someone else replies “I thought it sucked”. I was raised in a reticent house where, generally, if someone were interested in my opinion, they’d ask for it. An exception to that is when you can participate in a little group to validate (“me too!”) each other’s joy.
Few enough people are the curious sort (but that does include all you ‘Stackers, amirite), fewer still are the lucky ones whose curiosity includes the night sky, so I really try to be excited along with everyone else when there’s some celestial event.
Now, really, the focus of my irritation is the all-too-credulous media, many of whom should know better. This phenomenon of SuperMoons just seemed to come out of nowhere maybe a few years ago — the idea is that the Moon’s orbit around the Earth isn’t a perfect circle, so it’s closer sometimes (360,000 km at the nearest) than others (405,000 km at farthest):
When that cycle syncs up with the full Moon, then you get the SuperMoon. On the little plot, that’ll be July 2022, August/Sept 2023, Oct 2024, and so on. Because it’s a little closer than average, it’ll look a little bigger than average (maybe 6% or so larger in diameter). Will it appear brighter? Kind of a complicated thing, really — all things being equal, bringing a light bulb closer to you means it’ll appear brighter, for sure, but sometimes we mean that the “surface brightness” of the thing increases, which is a little trickier since that extra brightness is spread out over a larger area. Anyway, roughly, yes, it should be brighter. But compared to… the previous Moon that might be in your memory? Looking at the plot, each Moon is only a little bit brighter or dimmer than the one before it, so if someone is going outside to look at it to expect that it’s way brighter than last month’s full Moon then they might be disappointed.
And this might be what is frustrating to me. I just don’t want people to get hyped up for something that isn’t reasonable to expect and then be disappointed. The regular full Moon is only about 0.5 degrees across on the sky (about half the width of your little finger held at arm’s length. No, really! This blows me away every time I compare the two). So then the Super Moon is about 0.53 degrees across. Probably noticeable if you looked at side-by-side images, but this isn’t the way we experience the Moon!
We see this in other intersections of science and media — take climate change: It’s so easy for the media’s reporting on the problem to either be super-inflated or easily misquoted that the general public is jaded by the hyperbolic headlines. I take great delight in looking at the Moon whenever I get the chance, but hearing people making special plans to travel to get the best view of the “last time we’ll see such an event until 2037” just seems weird and out of proportion. And, of course, the headlines keep throwing the word “Blue” in there, which just refers to two full Moons in the same month (nothing physical about the Moon), but you know there will be people expecting a literally blue LED Moon in the sky.
Anyway, I hate being the “well, actually” dude, especially when there are folks who are genuinely stoked after seeing the bright full Moon, but for some reason this is one of my new peeves. Do you all have some similar sorts of pet peeves you wish you didn’t have? Sometimes I feel indignantly right about things, so I’m sort of proud of my stubbornness, but other times I just feel annoyed and a bit shamed I feel that way. Group therapy, yo.
Moon Phases!
Now let's try to understand why the Moon exhibits different phases in the first place. The key is appreciating the geometry and alignment between the Earth, Sun, and Moon. That the Moon cycles through a repeating pattern of phases is a consequence of the following 3 simple ideas:
The Moon does not emit its own light. It does not glow --- it's essentially a giant rock in space. The only reason we see it at all is because sunlight bounces off of it and is reflected to us.
The Moon orbits the Earth. It takes about 27 days for the Moon to make one trip around the Earth (which is the origin of the word month).
The Sun is farther away than the Moon. This means that the Moon *always* comes between the Earth and Sun, and that we're often seeing the unlit "dark side" from behind during crescent phases.
The result is the exact same repetition of the above phases every 4 weeks — over 50 billion cycles in a row!
Here’s the idea: the image below does not represent the true sizes and distances of the objects, but is meant to show what happens when you look at a lit sphere (the Moon) that is lit by a distant source (the Sun)
So this would be the geometry that would result in a waxing crescent as seen from the Earth — exactly half of the Moon is always lit by the Sun, but we’re only seeing a sliver of that lit part from behind. A few days later the Moon would progress along its orbit (counterclockwise, as seen looking down on the North Pole) and we’d see a 1st quarter phase, and so on. Here is the whole thing animated, both from a “God’s eye” view looking down, and also from the Earth’s point of view looking at the Moon the whole time (I've suppressed the motion of the Earth around the Sun as well as our 24-hour spin as that can get a little confusing)
You might have to play the movie a few times to get the hang of it, but you can start to see what's happening: from above, you can see that half of the Moon's surface is always lit by the Sun as it goes around the Earth. The different phases happen when we look at the half-illuminated Moon from different points of view. For the new and crescent phases, we're looking at the Moon from behind, and we're between the Moon and Sun during the gibbous and full phases. That's really about all there is to it.
The next question people usually ask is why we don’t see eclipses every month, since it sure looks like the Earth/Moon/Sun line up perfectly. Well, they actually don’t. The plane of the Moon’s orbit is actually inclined about 5 degrees from the Earth-Sun line, so it spends most of its time either above or below the Earth-Sun line.
Just to make the point, because so many times there’s the “not to scale” disclaimer on illustrations, here's an one showing the relative sizes and distances between the Earth and Moon (below). The Moon is only about a quarter of the diameter of the Earth and about 30 Earths away. Also you can see the inclined orbit relative to the direction of the Sun. This means that only very rarely are we in the Moon's shadow (a solar eclipse). When the Moon is behind the Earth, it is possible for it to be in the Earth's shadow, which is a lunar eclipse. The lunar eclipse is somewhat more likely, mostly because since the Earth is bigger than the Moon, it casts a bigger shadow.
How to Be Badass
Here’s the whole enchilada in one image. And the cool bonus is that if you teach yourself how to read it, you can generate rise/high-in-the-sky/set times for each phase. Just imagine, your partner says something like “ooh, beautiful full moon tonight”, and you can’t resist piping up with “yeah, and in 3 weeks it’ll be a first-quarter Moon, high in the sky at sunset!” and then 3 weeks later you’re all like
Guaranteed hero time.
The diagram shows the animation in static form --- the inner set of circles shows the half-lit surface of the Moon as it orbits the Earth, and the outer set shows what the Moon at that time would look like from the Earth. Notice the time of day for someone standing on Earth's surface (noon if the Sun would appear directly overhead, midnight if the Sun is on the other side of the Earth). Now you can predict when a certain phase would rise and set!
Draw an imaginary line from the phase back to the Earth — that tells you when that phase is high in the sky (so the 1st quarter phase should be high in the sky at sunset). In that case, the moon would then rise at noon (6 hours earlier!) and set at midnight (6 hours later!). The full moon should always rise at sunset, be high in the sky at midnight, and set at sunrise.
What you actually see from the Earth should be something like the image above. At sunrise, the third-quarter moon should be high in the sky. The Sun is about 400 times further away than the Moon so the position of the Sun should be taken as just the direction of the sunlight. The waxing crescent moon (below) at noon should be visible as shown, although it might be tough to see if the atmosphere is bright.
On Deck:
For next time (probably!): I’ve been taking some pictures of Venus over the last few months. When I teach beginning astronomy, I love it when we get to the topic of competing models of the Universe at the time of Galileo, and how his observations of the changing phases of Venus forever fractured the then-1,500-year accepted model. I’ll revisit those old ideas with these new images, and try to use them to show the actual scale of the Solar System!
If you’re a student/teacher and want to see lots of worked examples/derivations that I like to include in my classes when I teach the “standard” University Physics 1 and 2 courses, feel free to browse the (growing) collection of 150+ videos at
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