Stuffing itself on sunlight, the moon has fattened from a crescent to a portly gibbous over the past week and a half. Come Saturday night (Feb. 8) it will lie directly opposite the sun in the sky and rise around the same time the sun sets. Because it faces the sun directly the moon’s entire face will be illuminated, hence the term “full moon.” The moment of maximum fullness occurs at 1:33 a.m. CST Sunday (Feb. 9).
Full moons have lovely cultural and seasonal associations with February’s called the Snow Moon. That seems appropriate at least for mid-northern latitudes. Folks living in the Caribbean might scratch their heads and think otherwise. Full moons appear extremely bright, and the light they cast makes walking at night a pleasure. Venus, the brightest planet, bedazzles the eye, but a full moon is more than 3,800 times brighter!
Not all full moons are equally bright (or close) because the moon orbits the Earth in an ellipse with Earth offset slightly to one side. At one end of the ellipse, called perigee, the moon passes closest to the planet. At the other end, called apogee, it reaches its farthest point. It swings through the perigee and apogee points once every 27.5 days, the time it takes the moon to orbit the Earth.
When the moon is full and within 90 percent of its closest approach to Earth in a given orbit, we call it a supermoon. By this definition there are four supermoons in 2020 — the current one and those on March 9, April 8 and May 7. April’s will be the closest.
Close means bigger and brighter. A perigee or supermoon can be up to 12–14 percent larger than a full moon at apogee and 30 percent brighter. One degree (1°), twice the size of a typical full moon, equals 60 arc minutes. The average apparent size of the moon is around 31 arc minutes, but that varies from as small as 29.74 arc minutes at apogee to 34.06 at perigee.
If you could see a supermoon and a normal moon side by side the difference would be obvious. But in real life there’s only one moon, so you have to either have a very good memory or build a Supermoon Sighter I wrote about a while back to help you compare.
Whether or not you give a hoot about supermoons try to catch a moonrise either Saturday night (Feb. 8) or Sunday (Feb. 9). Click here to find your moonrise time. Watching the moon transform from a delicate, orange oval at moonrise into a hard, white disk when it’s high in the sky is one of life’s simple, cost-free pleasures.