I love when astronomy stuff happens on weekends. Untethered from the daily work schedule, many of us can make time for something like a moonrise. That’s my subtle way of suggesting you should see Sunday night’s moonrise. Not only will the moon rise full and bright, it will be at perigee, the point in its 27-day orbit when it’s closest to Earth. When this happens close to the time of full moon, our satellite appears larger and brighter than usual and we call it a supermoon.
The moon orbits in an ellipse with one side closer to the Earth than the other, so its distance varies from near to far during every orbital cycle. The moment of full moon, when the moon lies directly opposite the sun in the sky, happens about 10 a.m. (Central time) Sunday, Dec. 3. That’s when its “fullest,” making both Saturday and Sunday nights ideal for watching moonrise. Perigee takes place just 17 hours later in the wee hours on Monday morning. That’s a pretty close matchup, the reason why December’s “Cold Moon” will be the second closest full moon of 2017.
How close? Normally, the moon’s about 239,000 miles (384,630 km) away (a 3-day journey for the Apollo astronauts), but Sunday night it will just 222,137 miles (357,495 km) or 7% closer than usual. Closer also means brighter and larger. Some people can see the difference, others can’t, but we can all enjoy — weather permitting — a spectacular moonrise right around sunset that night.
Find your local moonrise time and then key your attention to the northeastern sky. Watch for curious distortions of the lunar disk as the light from different parts of it pass through air layers of different temperature and density. Binoculars will show a greenish-blue upper edge and purple-red lower edge. What you’re seeing is called dispersion: the air is so thick (dense) when we look along the horizon that it acts like a prism and spreads the light of the moon into the colors of the rainbow. Most of those colors overlap to make white, but the far ends of the rainbow — blue and red — leave a trace of color along the top and bottom edges.
We’ve discussed before how when the sun is low, as it is now in December, the full moon, being opposite the sun, is high. It will rise in Taurus Sunday night near the place the sun occupied back in mid-June. Call it the “hot seat.” As time goes by and atmospheric effects lessen, the full moon will whiten and climb highest above the horizon around midnight. If you’re on a late walk, look down at your shadow. See how short it is? That’s just how it looked around local noon in early June.
The perigee point swivels around the moon’s orbit because of the gravitational effects of the sun and Earth, the reason all full moons aren’t supermoons. Over time, full moons drift from perigee to apogee, the farthest point from Earth, and back again. The next apogee full moon occurs on July 27 next year, when the moon will be considerably farther than its normal distance. People have come to calling such remote full moons minimoons.
If you’d like to compare Sunday’s close moon with next July’s distant one, make one of my Supermoon sighters.