On September 17 Earth will be treated to a “supermoon.” If you believe the hype, the full moon will appear huge in the sky and will be so bright you will be able to drive at night without headlights.
I would advise against that. While it’s technically true that the moon will appear bigger and brighter than usual, in practical terms, you almost certainly won’t notice the difference.
There is, however, a bonus with this particular event: it comes with a (very) partial lunar eclipse that will be visible from anywhere in the U.S., so that’s cool.
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Let’s get back to the idea of a supermoon, though, which, I regret to inform you, isn’t a real scientific term. Astrologer Richard Nolle claims he coined the word in 1979, and while I have no reason to doubt this pedigree, the fact the term originated from an astrologer may indicate how I feel about its etymology. Nolle defined it as a new or full moon that occurs when the moon is within about 90 percent of its closest approach to Earth in every orbit. Call it a supermoon a day or two before and after those phases, and that’s about right; Nolle’s supermoon is meant as a rough concept more than a hard-and-fast definition.
Two key facts underpin the supermoon moniker. The first is that the moon’s orbit around Earth is not a circle but instead an ellipse or oval, so at some times it’s closer to our planet than at others. The closest point in the orbit is called the perigee (literally “near to Earth”) and the farthest point is called apogee (“away from Earth”). The moon takes about 27 days to orbit Earth once, so it reaches perigee about two weeks after apogee.
The second fact is that the moon also goes through phases. It goes from new (when it’s near the sun in the sky and essentially invisible because we are seeing an unilluminated lunar hemisphere) to crescent, waxing gibbous and then full. After that, the process reverses to waning gibbous, crescent and then old (when it is once again near the sun). This is caused by geometry and the angle we see between the sun and moon. It takes the moon 29.5 days to go through its phases; if you’re wondering, that’s where the word “month” comes from.
You may notice that that’s about two days longer than the moon takes to physically orbit Earth. The reason for that is once again geometry. It takes 27 days for the moon to complete an orbit, but during that time, our planet is also orbiting the sun. That changes the direction to the sun in the sky slowly over the course of a month, and it takes a couple of extra days for the moon to “catch up” to the sun as seen from Earth.
What this means is that the moon may be at perigee when it’s at any given phase. It might be new, full or anything in between. Although Nolle’s original use of the word had a supermoon when a full or new moon is near perigee, in the more modern usage, it’s only for when the moon is within a day or two of being full at perigee. (Conversely, if this happens at apogee, it’s a “micromoon,” a term I have never heard used in real life).
The moon is brightest when it’s full, of course, and if it’s closer to Earth at that point, it’s true that it’ll look bigger and brighter—hence a supermoon! But will you actually be able to tell the difference?
On average, the moon is about 384,000 kilometers from Earth (measured from the centers of the two bodies, which is how astronomers prefer to do things). On September 18, the morning after the moon is full, the moon will be at perigee at roughly 9:30 A.M. EDT, when it will be at a distance of 357,300 km. That’s 27,000 km closer than usual, or about 7 percent! Certainly that should be noticeable, right?
Right? Well, technically, yes. The size of the moon in the sky depends on its physical size and its distance. That means it should be 7 percent wider in the sky than average, which is enough for your eye to discern. But you have nothing to compare it with—the night before, the moon was only a little bit farther away (about 1,000 km), so the change from night to night is small and hard to distinguish. Worse, you’d be comparing it with your memory of how big the moon was, and memory is notoriously unreliable under such circumstances. If you try to think back to the last time you saw a full moon, that would be at least around a month, if not several months, in the past, making the memory even fuzzier.
I’ll note that the full moon is significantly brighter when it’s at perigee, by about 15 percent, than a full moon at its average distance from Earth. But that’s hardly noticeable either. Our eyes don’t respond to light linearly (meaning we don’t see an object emitting twice as much light as being twice as bright), so that 15 percent difference is perceptually smaller than you’d think. Again, you’re also comparing it against your memory, so it’s hard to be objective.
Either way, the “super” moon won’t look all that huge or be bright enough to allow you to drive without headlights. Caveat observator.
So having thrown all this kryptonite at the supermoon, then, is it still worth going out to look at it?
Yes. This is especially true because on September 17 the moon will slip a bit into Earth’s shadow, creating a partial lunar eclipse. This starts at about 10:15 P.M. EDT (check your local listings), reaches its maximum at 10:45 P.M. and ends around 11:15 P.M. At its maximum, about 3.5 percent of the moon will be darkened; look to the northern portion of the moon (the top left of its disk as seen from the U.S.) to see what looks like a slice taken out of it. While it’s not as dramatic as a total eclipse, it should still be fun to see.
Mind you, it’s always a good idea to go look at the moon, be it super or full or apogean or eclipsed or not. It’s gorgeous, and the changes in its phase from night to night mean there’s always something new to see. On any given night that it’s full, it casts a magical silvery light everywhere. Binoculars reveal more detail, and even a small telescope can show you amazing wonders, such as mountains, craters and even more bizarre features.
And to be honest, this is why I can’t entirely dismiss the idea of the overhyped supermoon; if it gets people to go outside and look up, then that’s a good thing. Just be aware of what you can and cannot see and appreciate the moon for what it is: an immense ball of rock that’s the closest cosmic neighbor to us in the entire universe; a world we’ve studied for millennia; a place where people have stood and explored and where they may do so again soon; and an object of beauty that inspires art and awe—and science.
(PS: the next two supermoons are on October 17, when the moon will be full at 7:30 A.M. EDT and at a perigee distance of approximately 357,200 km 10 hours earlier, and on November 15, when it will be full at 4:30 P.M. EST and at a perigee distance of 360,100 km the day before at 7 A.M. Plan accordingly.)
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