If you happen to be outside this evening, check out the Moon when it’s close to the horizon — it’s going to appear slightly larger than usual, thanks to a (somewhat) rare alignment of the Moon’s position in its orbit around Earth relative to the Sun.

The media are referring to it as a “Super Moon.” So, what exactly makes it “super”? The Moon has a slightly elliptical orbit around the Earth, which means sometimes it’s a little closer to the Earth (this position is called “perigee”) and sometimes a little further away (“apogee”). The difference is about 42,000 km, which sounds like a lot, but it’s only about 10% of the Moon’s average distance from the Earth. Today’s perigee is a smidgen¹ closer than usual, but not enough to have any significant impact on ocean tides, earthquakes, or volcanic activity. Anyway, when the Moon’s perigee is especially close and coincides with a full moon, that seems to be a “Super Moon.” But it really amounts to a not-so-super 14% increase in the Moon’s apparent size in the sky compared with its apogee size. (I think I just broke the record for how many times the word “moon” can be crammed into a blog post.)

This NASA vid explains it all with nifty images:

DIY Science: If you’re an empirical kind of person, you can try the following: hold up a ruler or some other reference object at arm’s length (steady your arm on a stable surface) and carefully measure the apparent size of the Moon tonight, then do the same on October 12th when there is another full moon, but this time its close to apogee. See if you can detect a slight difference. If you have a telescope equipped with an eyepiece with a linear-scale reticle, this should be a piece of cake.

[1] “Smidgen” is a loose scientific term that, for the purposes of this article, means less than 2% difference in distance.

Recommended reading:

• NASA Science: Super Full Moon
• NASA: Earth’s Moon