We start the year with an astronomical bang — a big, bright supermoon on New Year’s Night. January’s full moon, called the Wolf Moon, also happens to be the new year’s closest full moon of the year. Exceptionally close full moons have come to be called supermoons because they appear brighter and bigger than the average.
The moon’s distance from Earth varies because its orbit is an ellipse (something like a squashed circle) with Earth off-center to one side. All the planets orbit the sun in ellipses, too. A circular orbit, which requires that all forces acting on a body be in perfect balance, is rare. This lovely lack of perfection means that the moon’s distance constantly varies along its orbit.
When the moon arrives at the point in its orbit closest to Earth, it’s at perigee. When it swings around to the most distant end of its loop, it’s at apogee. The moon moves through perigee and apogee once every 27 days, the time it takes to complete one orbit. Sometimes the moon’s new, half or gibbous at perigee. A supermoon occurs when a full moon occurs within hours of perigee. Easy, huh?
On average, the moon’s about 239,000 miles (384,633 km) from the Earth, a distance the Apollo astronauts covered in about 3 days. NASA’s New Horizons probe, which made a spectacular flyby of Pluto in July 2015, was launched at a much higher velocity and reached the moon in just 8 hours and 35 minutes. Perigee occurs on January 1 at 3:56 p.m. Central time, only 4 hours before the moment of full moon. That means we’ll get to see an unusually close moon on New Year’s Night, just 221,559 miles (356,565 km) away or 17,441 miles closer than average. That amount is more than two Earth-diameters.
Closer means the moon looks larger or as we like to say, it has a larger apparent size. Of course it’s physical size remains the same throughout its orbit. Normally the full moon spans 30 arc minutes (written 30′), equal to ½° or twice the width of your pinkie held up at arm’s length against the sky. During the New Year supermoon, it will be almost 34 arc minutes across or about 13% larger than average.
If you have have good recollection of full moons past, you might be able to see that the moon really does look bigger than usual. It’ll be brighter, too. In early January, the full moon is located in the constellation Gemini in almost the same place the sun occupied in early summer. Remember how high the sun was then? Well, now it’s the moon’s turn! From S. Florida and S. Arizona, the moon will stand almost directly overhead around local midnight.
I’m kind of a freak, so one of my favorite activities is seeing how much of the world I can see in color during high full moons especially when there’s snow. Normally, we can’t see color at night because there’s not enough light stimulate the cone cells in our eyes responsible for color vision. Only the rod cells are active and they provide a 1950s black-and-white view of the world. If I manage to brave the cold, I’ll be out there tomorrow night in living color.
At full moon, take a minute to notice the two main types of lunar landscapes: the broad, white areas called the lunar highlands and the dark patches called seas or maria (MAH-ree-uh). The former is the original lunar crust that cooled and crystallized from the time the moon was nothing but a ball of molten magma more than 4 billion years ago, while the seas are younger (3.1-3.8 billion years) and formed when wayward asteroids punched into the lunar crust, setting off lava flows that topped over the impact scars. Nowadays, we have fun seeing these light and dark spots as faces or animals.
I wish you the best of the new year and clear skies so you can enjoy a gorgeous moonrise. To find the time when the moon comes up tomorrow evening (it will be close to sunset), click here. January is a special month because we’ll have a second full moon on the 31st. It will also be in total eclipse!