The hyped story about the “Super Moon” is near the top of our on-line news feeds. I’ll try to untangle the hype from some facts. It’s true that this full Moon will be the closest since 1948-01-26.
The next closest super Moon will be in 2034, only four years before our UNIX calendars run out of steam.
But how big a deal is this? And what’s really going on?
If you don’t want to dive into the details below, here’s the long story short. Every month the Moon is closer and farther from the Earth as it follows its elliptical orbit. Due to the tidal force from the Sun, sometimes those distances are a little more extreme than others.
The so-called “Super Moon” occurs when the full Moon coincides very closely with the Moon’s closest approach, apogee, to the Earth for the year. But it’s really only around 3% larger than any other close approach during the year, and these happen every year. They just don’t always occur when the Moon is almost exactly full.
Kepler’s first law
Kepler’s first law is that orbits have the shape of an ellipse with the primary body at one of the two foci. In this case the Earth is at the focus. The ellipse means the Moon moves closer to and farther away from the Earth as it goes around. It’s closest point is called the perigee, and the farthest point is the apogee.
Sun’s tidal forces
Tidal forces stretch things. The tidal force due to the Sun’s and Moon’s gravity stretch the Earth’s oceans so they have high points and low points. The tidal force of the Sun also stretches the Moon’s orbit. When the Sun is in the direction of the long ellipse, the Sun stretches it in a longer ellipse. The extreme distances are even more extreme:
apogee = 406,700 km, perigee = 356,400 km.
When the Sun is at right angles to the major axis, the orbit is pulled into a less eccentric ellipse and the extremes are reduced.
apogee = 404,000 km, perigee = 370,400 km.
Every month the Moon passes apogee and perigee as it orbits the Earth. In the excellent diagram above from the Wikipedia article on the “Orbit of the Moon,” you can see the varying distance of the Moon from the Earth plotted. The minimum points are the perigee, closest, distance. Each month, when the Moon is at perigee, it will appear a bit larger, than when it’s farther away.
Notice how the minimum and maximum distances are not as extreme in April as they are in August. That’s the Sun’s tidal effect.
In this diagram, which was for 2014, it looks like the Moon was closest to the Earth in about the second week of August at perigee.
The other thing in this diagram is the phase of the Moon. The diagonal lines show the Moon going from new (phase = 0) to full (phase = 0.5) and back to new each month. The vertical lines show when the new Moons occur.
What we’re really interested in here is the full Moon so it would have been nice if there were vertical lines halfway in between the new Moons to mark the phase = 0.5 full Moons.
The big deal
So now we have all the pieces to understand what the big deal is. It’s this. How close does the full Moon come to the closest perigee of the year? Well, it almost never hits it perfectly, but it varies from year to year. In the year in the diagram, it looks like the halfway full-Moon point would fall nearly on the mid-August closest perigee.
Using the perigee-apogee calculator from below for 2014 we find the full Moon hit the closest perigee on 2014-08-10 almost exactly, zero hours difference! The perigee distance was 356,896 km.
In contrast, this year they missed by about 2 hours on 2016-11-14 at a distance of 356,511 km. So the full Moon was off a bit, but the closest perigee this year was a bit closer. Why a bit closer? It has to do with the angle of the Sun to the ellipse of the Moon’s orbit.
But it isn’t really
So that timing of full Moon and closest perigee of the year is what it’s all about. But notice that the Moon gets this close every year! It’s just not necessarily at, or exactly at, the time of the full Moon. Further, the Moon gets almost that close every month!
As we said above, the closet perigee distance is 356,400 km while the farthest perigee is 370,400. That’s 3.93% larger from the farthest perigee to closest. That’s how much the “close” Moon varies through the year.
So where did the big 14% come from that I kept seeing in all of these articles.
That’s the difference in size between when the Moon is at apogee vs perigee, and that happens every month, two weeks apart. Actually it’s about 13.36% when the ellipse is stretched, and about 9.8% when it’s not.
So again, this happens every month, but just not when the Moon is full.
Distance and Moon phases in 2014, calculated using algorithms from: Jean Meeus, Astronomical algorithms, Willmann-Bell, Richmond, Virginia, 1991 Jean Meeus, Astronomical formulae for calculators, Willmann-Bell, Richmond, Virginia, 1988. Moon phases: 0 (1) – new Moon 0.25 – first quarter 0.5 – full Moon 0.75 – last quarter
CC BY-SA 3.0