In 2020, there will be two Full Moons in October. One of which occurs on Halloween. A Full Moon will occur on Halloween about once every 20 years but a second Full Moon in October will occur about once every 44 years.
The year 2018 brought another relatively rare occurrence: four Full Moons before April 1, none of which occur in the month of February.
The cycle of lunar phases is determined by the particular alignment of the Moon and the Sun with Earth and this cycle has a period of approximately 29.5 days. This is the Moon’s synodic period, from the Greek συνοδος or sunodos, meaning a gathering. In this case the gathering is an alignment and the synodic period is the time between two identical alignments of three celestial objects. Thus the time between two Full Moons—when the Sun and Moon are exactly opposite each other as seen from Earth, is one synodic period or 29.5 days.
The first Full Moon of 2018 occurs on January 1. Although known by many different names across many cultures, this Full Moon is commonly referred to as the Full Moon After Yule or the Wolf Moon. The United States Naval Observatory provides a table of lunar phases for 2018. The times are given in Universal Time so corrects must be made based on your specific time zone. For example, while observing Standard Time, observers in the Midwestern United States must subtract six hours to get Central Standard Time and subtract five hours to get Central Daylight Time.
From the table we find that the first four Full Moons of 2018 occur on January 2 at 2hr 24m UT (8:24 p.m. on January 1 Central Standard Time), January 31 at 7:27 a.m. CST), March 1 at 6:51 p.m. CST, and March 31 at 6:37 a.m. CST. Since February is the only month that has less than 29 days, it is possible for no Full Moons to occur in February. This will occur when a Full Moon occurs on January 30 or 31.
With two Full Moons occurring in October of 2020 and both January and March of 2018, there is a lot of talk about Blue Moons. According to modern folklore, the second Full Moon that occurs within a single month is a Blue Moon. The phrase “once in a Blue Moon” supposedly signifies a rare event, but having two Full Moons in a single moth isn’t terribly rare and even having Blue Moons within three months isn’t especially rare. In fact, according to this definition, which can be traced back to a misinterpretation of an earlier definition of Blue Moon (see my earlier post for details), two Full Moons can occur in one month about once every two years. That doesn’t exactly make this event rare. The fact that February has no Full Moon this year is more rare.
The alignment of our Gregorian calendar and the Moon’s synodic period synchronize to cause such an occurrence once every 19 years. Thus, the last year February had no Full Moon was 1999 and the next time this will happen is 2037. Even more rare, however, is a year in which a 29-day February has no Full Moon. In that case, not only must a Full Moon occur at the end of January, but the year must also be a leap year. The last leap year that had no Full Moon in February was 1608 (the year before Galileo acquire his first telescope!) The next leap year in which February has no Full Moon will be 2572. Now that is a rarity, indeed!
By the way, according to the more traditional definition of a Blue Moon described in the Maine Farmer’s Almanac (and described in a Sky & Telescope article), there are no Blue Moons in 2018. According to this definition, a season must have four Full Moons rather than the more typical three Full Moons. This year, the January 1 Full Moon is the Moon After Yule, the Full Moon on January 31 is the Snow Full Moon, and the Full Moon on March 1 is the Lenten Moon. The next Full Moon on March 31 occurs in spring and is the Egg Moon. The Full Moon occurring before the vernal equinox must be the Lenten Moon and if it is the fourth Full Moon of winter, the Full Moon falling between the Snow Moon and Lenten Moon is the Belewe, or Betrayer, Moon. That Full Moon must always fall between February 20 and 23 in order for another Full Moon to occur before the vernal equinox.
Much of the details surrounding the naming of Full Moons has to do with the determination of Easter since that is the most important moveable holiday on the Christian calendar and seasonal changes and the phases of the Moon had to be precisely determined. As with most things, once you dive deeply into the subject, you find that the rules are intricate and never are as simple as they seem initially.