What is a Supermoon?

I wish I had created my blog before the Supermoon had risen last Saturday night to help dispel some of the myths around the event. But then again, mythology is fun and if the media attention got you looking up at the sky then I’m all for it! For those of you who are curious here are some of the details about the supermoon that the media didn’t cover. Hang on, this gets a little geeky but I’ll try to stick to the details that will help you enjoy your next supermoon event even more.

The first thing you should understand about the Moon is that it is in what is called a “Tidally Locked” orbit around the Earth. What this means is that the Moon rotates about its axis at a rate such that as the Moon orbits around the Earth the same side of the Moon is always facing the Earth. Most moons that orbit in a relatively close orbit to their primary bodies are in these types of orbits. The reason this happens is due to gravity. When two objects are in close proximity to each other they each exert a force on the other body. For the Earth and the Moon, the Moon exerts a force on the Earth and depending on where the Moon is above the Earth that extra gravitational tug causes our ocean tides. That same force that is behind our tides also causes a slight bulge in the Earth and that bulge changes the Earth’s gravitational field. The slight change in the Earth’s gravitational field changes the natural torque (a rotational force) on the Moon’s rotation. Eventually this small torque caused the Moon’s rotation about its axis to change ever so slightly. All of this happened early in the Earth-Moon history and eventually over millions of years an equilibrium was reached such that Moon’s rotational rate settled to a value that essentially “locked” one side of the Moon to always face in the direction of Earth. That is why we always see the same side of the Moon from Earth.

Before I get into what makes a super moon special, lets set the stage for a normal full moon and how a regular lunar cycle works. The phases of the moon are a result of how much of the Moon’s face (the side of the Moon facing us on Earth) is illuminated by the Sun. At any given time approximately 50% (it’s approximately 50% because the Moon is not a perfect sphere) of the Moon’s surface is illuminated by the Sun but at any given time that may or may not be the same 50% of the Moon’s surface that is facing us on Earth. When the side of the Moon that is facing Earth is the same 50% of the surface that is illuminated by the Sun that is when we experience a Full Moon. The reason the Moon rises right around sunset on a Full Moon is because the Sun has to be 180-degrees away (on the opposite side of the Earth) in order to fully illuminate the side of the Moon facing the Earth. This Full Moon phasing only happens once each time the Moon orbits around the Earth, which takes 27.3 days (or ~655 hours). Another interesting fact is that the “dark side” of the Moon isn’t always dark. If you were standing on the surface of the Moon you would experience “days” that were 327.5 hours long and nights that were 327.5 hours long, even on the “dark side.” It’s called the “dark side” of the Moon because it is not visible from the Earth, not because it is always dark.

Now, what was so “super” about the Full Moon this past May 5th? Well, the Moon is not in a perfectly circular orbit around the Earth. That means that approximately every 27.3 days the Moon cycles through varying distances from the Earth because the shape of the orbit around the Earth is slightly “squashed” or “flattened.” The figure below shows how the distance from the Earth to the Moon varies as the Moon orbits around the Earth as opposed to how the distance would always be EXACTLY the same if the Moon were in a perfectly circular orbit around the Earth. Since the orbit is not circular there is a point in the orbit where the Moon is closest to the Earth (called perigee) and the point in the orbit where the Moon is furthest from the Earth (called apogee).

So why did I say that the Moon cycles through it’s closest point and furthest point from the Earth approximately every 27.3 days? If you draw a line between the point in the orbit where the Moon is closest to the Earth (perigee) and the point in the orbit where the Moon is furthest from the Earth (apogee) that line is called the line of apsides. That line also rotates around the Earth, but at a different rate than the rate at which the Moon rotates around the Earth. So by the time the Moon has gone from perigee all the way around its orbit to perigee again, perigee has actually moved slightly! Why is that important and what does it have to do with the Supermoon? 


The term “Supermoon” was first coined by an astrologer named Richard Nolle in 1979 and he defined it as:


“...a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit perigee. In short, Earth, Moon and Sun are all in a line, with Moon in its nearest approach to Earth.”


By that definition there are approximately 4-6 Supermoons every year. Not so “super” huh? But the Supermoon this past May 5th was a little more “super” than normal because it was much closer to perigee than the 90% minimum threshold in Nolle’s definition. The timing of the Full Moon on May 5 2012 was such that the Moon was exactly “Full” 6-minutes after the Moon had passed through perigee. That’s less than 1% away from perigee (or within 99% of perigee)! So what does being at perigee do to the apparent size of the Moon? 2%. Yes, that’s right. 2%. The Moon’s diameter only appears 2% larger when compared to the April 2012 and June 2012 Full Moons. A 2% difference is the Moon’s diameter is essentially impossible to detect with the human eye, especially when the Moon is away from the horizon and there is nothing near the Moon to compare its size against. What is a little more apparent is the brightness. That number gets larger when you compare the apparent diameter of the Moon at perigee verses apogee. That is a 14% difference in apparent diameter. Larger, but still difficult to detect in an empty sky. The 14% difference in diameter equates to a 30% increase in brightness, and brightness differences are a lot little easier for our eyes to detect.

So what does all this mean to the casual observer? You really don’t need to know any of this to enjoy the breathtaking site of a Full Moon rising, but knowing why that moon may look just a bit brighter might make the view that much more enjoyable.


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