There’s a lot of fuss on the Internet right now about the supermoon, so you might be wondering: what is it? And what’s the big deal?
The easy answer: a “supermoon” is the biggest full moon of the year.
The long answer: the Moon doesn’t always cover exactly the same amount of sky — it can appear slightly larger or smaller depending on where it is in its orbit (although truthfully, the difference isn’t really perceptible). This is because the Moon isn’t always the same distance from us. Like most orbits, its orbit is elliptical, not circular, and the Earth (or, more accurately, the common center of mass) sits at one focus of the ellipse. The closest point is called periapsis (or perigee, when talking specifically about objects orbiting Earth), and the farthest point is called apoapsis (or apogee). The supermoon is, therefore, the full moon that occurs closest to perigee.
A full lunar cycle, or lunation (which is not exactly the same as a lunar orbit, but close), takes about 29 and a half days. This is also precisely the length of a lunar day, but about two days longer than the lunar orbit. This is because the Earth-Moon system is moving around the Sun, which affects the angle of sunlight. (This is also why there’s a difference between a solar day and a sidereal day. The former is how long it takes to go from midnight to midnight, and is what we set our clocks by, but the latter is how long it takes the Earth to complete one rotation with respect to the “fixed” stars.) So all this basically means the Moon will be at different points in its orbit when it hits different points in its cycle, but it may take a long time for the cycle to repeat. Tomorrow, the Moon will be the closest to perigee when it hits full moon than it has been since 1948. So that’s why people are calling it “the closest supermoon” or the “biggest supermoon”.
Now, what it isn’t is the closest pass the Moon has made sine 1948. In fact, the last time the Moon was thi close was . . .
. . . a whopping 27 days ago. And it will be this close in another 27 days. It just won’t be full at the time.
Still, it’s kind of a cool thing. And the Lunar Reconnaissance Orbiter team has put together this amazing visualization of all the lunations for 2016 — including the current one. They didn’t do it just for the supermoon; in fact they did this last year too. It’s a beautiful visualization of the process, showing just how much bigger or smaller the moon may appear, and showing at top left where the Moon is in its orbit (not to scale, but clearly indicating perigee and apogee). Skip to 4:24 to see the supermoon. Also notice how the Moon moves more quickly near perigee than apogee; the slider to the right, showing the Moon’s distance from Earth (to scale) is a good place to watch for this. They rendered this for both northern and southern hemisphere viewers; I’ve inserted both below:
By the way, this may also give you some insight into why not all solar eclipses can be total. If there can be a perigee full moon, there can be perigee new moon as well, and all eclipses are at new moon. A perigee solar eclipse will produce a total eclipse. An apogee solar eclipse will produce only an annular eclipse; the Moon at perigee appears too small to cover the Sun completely.