From the Farmers’ Almanac:
13 full moons in 2020
• The first full moon of 2020 was the “Wolf Moon” on Jan. 10.
• Usually, we have one full moon for each month, making the total 12 for a year. But on occasion, some months will have two full moons. This is the case for October 2020: We’ll have a full moon on Thursday, Oct. 1, and again on Saturday, Oct 31 — a Halloween full moon — which is not only rare in and of itself, but it will also be considered a “blue moon” because it’s the second full moon of the same month.
Rare Halloween blue moon in 2020
• For more than half a century, whenever two full moons appeared in a single month (which happens on average every 2 1/2 to 3 years), the second has been christened a “blue moon.” In our lexicon, we describe an unusual event as happening “once in a blue moon.” This expression was first noted back in 1821 and refers to occurrences that are uncommon, though not truly rare.
• When you look at the full moon on Halloween night, it won’t appear blue in color but you’ll be looking at something pretty uncommon. A full moon on Halloween only occurs roughly once every 19 years (known as a Metonic cycle). If the full moons are calculated using Greenwich Mean Time, that translates to approximately three to four times per century. So maybe the phrase “once in a blue moon” really belongs to a full moon on Halloween.
A blue moon by another definition
• There is an alternate definition of a “blue moon,” which is based on a seasonal rule rather than a monthly rule: Normally, there are three full moons each season of the year. But when a particular season ends up containing four full moons, the third then is referred to as a ‘blue moon.”
Two supermoons in 2020
• For a supermoon to occur, we can thank the shape of the moon’s orbit, which is not a perfect circle, but an oval shape. That means when the moon orbits the Earth each month, it reaches a point that’s farthest from the Earth, called apogee, and a point where it swings closest to Earth, called perigee.
• According to how most people define a supermoon, it occurs when the moon is at least 90% of the way to its perigee position at the same time it is in its “full” phase.
• Based on this definition, we will experience two supermoons this year:
• March’s Worm Moon turns 100% full on March 9 — with the moon at perigee on March 10.
• April’s Pink Moon turns 100% full on April 7 — with the moon at perigee on April 7.
• When we have a supermoon, the moon can appear as much as 14% larger and 30% brighter than a normal full moon.
Source: The Farmers’ Almanac