Supermoon returns twice in January
The moon’s elliptical orbit takes it about 30,000 miles closer to Earth (perigee) some months than others (apogee). A supermoon is a full moon that is at or near (90 percent of) its closest point in its orbit around Earth (perigee).
If you caught moonrise Sunday night, or moonset Monday morning before the fog rolled in, you saw the only supermoon of 2017 and the first of three that will straddle the new year. We’ll see supermoons on Dec. 3, Jan. 1 and Jan. 31.
So why doesn’t this happen more often?
The time between perigees (an anomalistic month, about 27.555 days) does not align very well with the time between full moons (a synodic month, about 29.531 days). So most full moons occur at a point on the moon’s orbit anywhere but perigee.
An interesting pattern appears when you map out the motion of the moon relative to the Earth (apogee/perigee) and relative to the Sun (lunation or phases like full, new, etc.). Just how close that closest point to Earth is goes through a regular cycle. Those supermoons tend to occur when the moon is at its closest, often two or three months in a row. The same is true of mini-moons which are when the full moon occurs at or near lunar apogee.
The next supermoon will occur on Jan 1, 2018, and it will be much like December’s though just a bit dimmer.
The final supermoon of 2018 will occur just a few weeks later, on Jan. 31, with extra superlatives. The second full moon to occur in a calendar month is known as a blue moon.
Blue moons aren’t that rare. We will see two in 2018, and they generally occur every two to three years. A “super blue moon eclipse” might seem like the unicorn of astronomical events but we last experienced one in September of 2015.
This lunar eclipse is going to be a challenge to observe. Unfortunately, the moon will set just as the best part lunar eclipse is beginning.