They need not actually be spectacular

Our astronomer, somewhat reluctantly, talks about the Supermoon

Our astronomer writes:

We’ve just had another “supermoon.”  I’m not sure what people in general understand by that term; I suppose those who are really paying attention realize that it means a Full Moon closer to the Earth than normal.  This month, it’s closer than any over a period of thirty years (1990-2020).  I should explain a bit.

The Moon orbits the Earth in an ellipse, so it’s closer sometimes and farther other times.  If there were only these two bodies to worry about, that would be the end of it, because the ellipse would stay exactly the same.  But the Sun also tugs on each one, so the ellipse changes shape and direction slightly.  Also, the point where the Moon is closest in its ellipse (perigee) doesn’t stay at the same phase (New, Full, Quarter), but migrates over the course of a year.  Sometimes things line up almost exactly, and we get a nearer than normal perigee about Full Moon.  It was only a few years back that someone started calling it a “supermoon.”

The difference between a perigee Moon and an apogee Moon is about 14% in size, which translates to 30% in brightness.  It’s obvious if you take pictures of each and set them side by side.  If you could arrange them both in the sky, say one setting while the other rose, one would be perceptably brighter than the other, but not by much.  Most people can distinguish stars that differ in brightness by 20%; a few are good to 10%; so the maximum difference in brightness is just enough to be clear to most people.  But that’s if you had them both in view at the same time, while they actually occur about six months apart.

November’s Full Moon is 356,509 kilometers away, while October’s was 357,861.  That’s not quite half a percent difference, giving not quite a one-percent brighter November than October Moon.  You certainly wouldn’t notice that, even if you had them both in the sky to compare.  So this “supermoon,” while measurably nearer and brighter if you have the right instruments, wouldn’t appear to your eye to be any different from many another Full Moon.  The good news is that, if you missed this month’s display, next month’s should be very similar.

I used to worry that people, excited by talk of a “supermoon” or comet or meteor shower or other astronomical event, would be disappointed at the real thing.  Most such events are spectacular only if you’re familiar with the sky already, and can appreciate how unusual they are.  [A total solar eclipse is an exception! See the one next August if you can!]  A total lunar eclipse is probably the most arresting of nighttime events; the rest are less spectactuar.

But I’ve come to think that almost anything to get people out and looking up at the sky is worthwhile.  Any Full Moon is worth observing, whether you set out with a telescope and sophisticated camera, telescope and sketch pad, or just look.  And if it gets you into the habit of looking up, I’m all for it.