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.
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.
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.
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.
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.
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.
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!