Watch Out! Here Comes May’s Super Duper Full Moon

This weekend's full moon will be the biggest of the year. If you catch it around rising time, the thickness of the atmosphere at low elevation will distort the moon's shape and color it orange. Photo: Bob King

Flower Moon. Super Moon. Call it what you like, May’s full moon is coming to a sky near you this Saturday night. I like Flower Moon, the traditional name, because of its obvious seasonal connection and my own love of wildflowers. On Monday this week the marsh marigolds flashed their yellow blossoms along the edges of our many creeks here in Duluth, Minn. While not the first flower, they’re one of spring’s showiest.

The moon's orbit around the Earth is a flattened circle or ellipse. As it revolves, its distance and speed change continuously over time. The near point, called perigee, happens this weekend. Illustration: Bob King

We get a full moon a month, rarely two, but this one’s special. Full moons vary in size because the moon’s orbit around Earth isn’t a circle but an ellipse, a sort of flattened circle with Earth a little off to one side. As the moon revolves around our planet, it’s distance varies from about 222,000 miles to 252,000 miles. When closest to Earth, the moon’s at perigee (PEAR-up-gee), and when farthest, apogee (AP-uh-gee).

Because the degree of flatness of the moon’s orbit varies slightly over the year, each month’s perigee and apogee distances vary too. The closest perigee of 2012 will be this weekend, the farthest perigee occurred on January 17.

As the moon’s distance varies, so does its size. A perigee moon is 14 percent larger than an apogee moon. Every month the passes through these points in its orbit, sometimes as a crescent or a half, but occasionally when it’s full. No one particularly cares if the moon’s at perigee when it’s a 5-day-old crescent. When it’s full, we pay attention because, well … full moons are big, bright and round, and we love ’em.

Seen side by side, the difference in perigee and apogee moons is obvious. Click image to find lunar apogees and perigees for any month. Credit: Tom Ruen

Saturday night and early Sunday morning the moon happens to be at perigee at the same time it’s full, an event that happens a little more than once a year.

Not only will the Flower Moon be the closest full moon of 2012 but it will shine 30 percent brighter than a typical full moon. The question is – will you notice the extra brightness and size of this “super moon”? Maybe if you have a photographic memory, but what you really need a side-by-side comparison with an apogee full moon. Unfortunately we have but one full moon.

The moon this Saturday evening around 10 o'clock local time. Saturn and Spica will lie "two fists" to its upper right. Created with Stellarium

For the adventurous, it’s still worth a try. You can compare Saturday’s moon with the size of something fixed in your environment, say the width of a power pole or the peak of your roof. Note exactly where you’re standing when you make the observation and then return to the same spot on November 27-28 this year when the full moon will be at apogee.

The tighter the fit the moon makes with your marker, the better the chance you’ll see the change in size with your very own eyes.

A caliper with a digital readout might be an ideal tool for measuring the size of the moon.

Ideally you’d want to erect some kind of post with a circular disk on top. That way you could sight the moon any time of year in any part of the sky.

Another idea would be to use a caliper carefully held at a precise arm’s length. A friend could adjust the jaws until the moon just fit between them. You could either leave the caliper locked or record the width and then compare it to the new width measured in November at apogee. I’d love to hear if anyone attempts either one of these methods or uses another of their own devising.

As the nearest celestial body to Earth, the moon has a significant gravitational effect on our planet, raising tides in both the planet’s crust and its oceans. Tides are strongest when the moon and sun line up with Earth at new moon and full moon . The two bodies work together to give us a double tug. Though the sun is fabulously larger and weightier than the moon, its tidal influence is only half as strong.

When the moon and sun line up with Earth, the planet has high tides called spring tides. When the moon is in first quarter phase, the gravity of sun and moon partially cancel each other out, creating low or neap tides. Credit: NOAA

As you might guess, Saturday’s extra-close moon means higher tides than normal, but don’t expect anything terribly dramatic. In most places it will amount to an extra inch with a maximum of six inches depending on local geography.

Can you see the faces in the moon? This weekend will be a perfect time to test your powers of perception.

I encourage you to go out for the fun of watching a big moon rise in the east. We’re all familiar with how much larger the moon appears when near the horizon, an optical illusion based on how we perceive objects near the horizon versus overhead. This moon will of course be even bigger. For the Duluth area, it rises at 8:19 p.m. Saturday just five minutes before sunset. Click HERE, select your state or country, key in your town to get your own personal moonrise time.

For even more fun, see if you can make out some of the interesting patterns in the moon’s face seen by generations of people before us. The dark spots are ancient, lava-filled impact basins called lunar seas or maria (MAH-ree-uh). The lighter regions are the crater-saturated lunar highlands. I’ve included the rabbit, the woman and the familiar man-in-the-moon for your eyes and imaginations. Enjoy the moonlight!

11 Responses

  1. Thanks for at least encouraging your readers to try to see the changing size of the full moon over its elliptical orbit, in contrast to ill-informed bloggers or even NASA who flatly state that you can’t do that, period. Nope, you can! I discovered that by chance last year in a true – if inadvertent – blind experiment, which I’ve repeated successfully a few times since. Make sure the Moon is full, high (no psychological ‘moon illusion’ must be at play) and far from distracting foreground objects, then try to ‘take it in’. You will notice that the next full moon will look equally large, but as the year progresses, the size will shrink quite noticeably until full moon and apogee coincide. A fun experiment is also to look at a full moon without looking up the distance and trying to guess where it is on its orbit; I got a surprisingly precise result a few months ago …

    1. astrobob

      Hi Daniel,
      Thanks, I figured it might be worth a try. I like that you tried and saw the changes in the moon’s apparent size, especially without knowing in advance where it was in its orbit. That’s what I call creative naked eye astronomy and a wonderful way to learn.

  2. caralex

    Bob, with regard to your experiment in comparing sizes, won’t the moon rise, reach zenith and set in a different part of the sky in May than it does in November? It might be difficult for an observer to find a fixed reference point when standing in the same position in both months. What I mean is, the moon might pass behind a pole in May, but soar right above it in November, from the same fixed point of view of the observer.

    Also, not all perigees are at the same distance – some are closer than others. Can you explain why this happens? Why isn’t the closest part of the ellipse not always at the same distance from us?

    1. astrobob

      Hi Carol,
      Excellent thoughts on the subject. I was thinking of picking something that might cover the moon no matter what path it takes in the sky, the only difference being the time you’d make your observation. It would be later for a May moon and earlier for a November one. But your question prompted me to think it through more thoroughly, and I realize now it would be more difficult because the May and November paths are radically different. What you’d need to do is stick a pole topped by a metal disk in the ground, walk far enough away for the pole for the disk to precisely cover the moon and note your distance from the pole. Come November, you’d sight the moon again with the disk while standing the same distance but in a different direction from the pole. If you wanted to do this for anytime of year, you’d draw a circle on the ground with the pole at center. Every observation you made would be on that circle equidistant from the center.

      There must be other ways as well – I’m open to your suggestions. As for the those varying perigees and apogees, that has to do with the moon’s changing eccentricity throughout the year. Here’s a good explanation:

      1. caralex

        Thanks, Bob. I hadn’t realised that the ellipse was ‘elastic’! I thought it was a fixed shape. Thanks for clearing that up.

        Regarding sighting the moon, how small would your disc have to be? The moon is actually tiny, if you were to hold a paper at arm’s length, and draw its outline. How big would it be? Half an inch? A quarter of an inch?

        I’ve been trying to think of ways to measure the difference between May and November, and came up with the idea of a small piece of transparent plastic or a piece of glass, such as a photographic filter. You wouldn’t have to be in a fixed location to do this. In May, you hold this piece of glass or plastic at arm’s length, and with a fine-tipped marker, mark the left and right side of where the moon shines through the glass. In November, hold it again at arm’s length, and see if the moon fills the space between your marks. It should be smaller. However, and this is why I suggested a fine-tipped marker, the difference would be tiny, and very difficult to measure with precision, given the already tiny size of the moon.

        What do you think?

        1. astrobob

          Hi Carol,
          Cool idea! As I was writing about a way to measure I also thought about using something held at arm’s length but rejected it because I figured fingers and wrists might be too flexible and too thick and introduce error. Then again, maybe there’s no more error there than sighting along a pole. A fine-tipped marker is a step in the right direction, but maybe there’s something even narrower and very straight. I’m thinking a razor blade coated in ink or thin wires to mark either side of the moon on the filter. Or how about a vernier caliper with fine adjustment held at arm’s length? You’d then have a precise number for comparison.

          1. caralex

            Well, we’ve been driving all evening, so I didn’t get a chance to do any ‘measurements’! However, I must say, the moon DOES look really bright tonight. The sky is absolutely cloudless here in Hamilton, and the moon is noticeably brilliant, though I really can’t say if it looks any bigger than any other full moon.

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