Saturday's Super Moon

Saturday night we will have a full moon on the same night that the moon is at its nearest point in its orbit to Earth -- known as perigee.

This event has been recently labelled the "Super Moon." I only started hearing that term in the last couple of years. So just how "super" is this Super Moon?

First, the coincidence of the full moon and perigee occurs about once every 14 months, so to say that this is an unusual occurrence is a bit of a stretch.

The Super Moon, however, will be 25 percent brighter than the full moon at apogee (farthest from the Earth) and that just might be noticeable.

Now let's deal with a couple myths. There is a widespread rumor that natural disasters, earthquakes in particular, are more common near a Super Moon.

These rumors became more widely known after last year's Japanese earthquake and a tsunami (March 11) occurred in the same month as the Super Moon (March 19).

But notice that even in that case, the two events were separated by over a week -- the moon was not at all close to perigee or full when the earthquake occurred.  A more serious study of major earthquake and Super Moon dates shows no correlation between these events.

A much more common observation is that when a full moon is seen rising above the horizon (which must always occur at sunset), it looks enormous compared to when it is seen overhead.

And so Saturday's rising Super Moon should look colossal. This is a real effect; however, the actual apparent size of the rising moon is identical to the apparent size when the moon is overhead.

What we experience is an optical illusion. You can test this one yourself. Next time you see a full moon rising, hold up your thumb at arm's length and put it over the moon, looking at it with one eye. Your thumb should just cover the moon.

Then, when you look at the moon later that evening when it is farther up in the sky (and appearing to be smaller), try the same experiment. Again, your thumb will just cover the moon. Unless your thumb has shrunk that evening, you've proven that the massive rising full moon is just an illusion.

Lastly, I personally learned something very interesting that I had never known about Super Moons while writing this article and reading through a site listing perigee and apogee distances to the Moon.

When we have a Super Moon, this means that the orbit of the Moon is aligned to "point" at the Sun -- the Moon's orbit is elliptical (which is why the distance to the moon varies), and the long axis of the ellipse is pointed toward the sun on the date of the Super Moon.

The sun's gravity pulling on the moon during its orbit around the Earth then forces the orbit to become more elliptical than usual when this alignment occurs -- which means that the perigee distance is less than usual, and the apogee distance is greater than usual at these times.

So the Super Moon is even closer to us than it would be at its normal perigee.  The difference I would have thought to be small, but it turns out to be 8,000 miles (out of an average distance of 230,000 miles, another 3.5 percent).

Editor's note about the author: Aaron Turner is an amateur astronomer and aerospace engineer from Southbury.