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What is a Blue Moon?
On New Year’s eve we will have a Blue Moon. The astronomical Blue Moon has been defined in various ways over time, but mostly (since 1946) it relates to the occurrence of a second full moon in a calendar month. The last time that we had a Blue Moon on December 31 was in 1990, and the next time will be 2028. Months with 31 days are more likely to have a Blue Moon, and poor February can never have a Blue Moon.
The time between full Moons is 29.53 days. The mean year (including the leap-year effect) is 364.75 days. Thus the number of full Moons per mean year is 12.352. So, each mean year we have 12 full Moons, and we gain an extra 0.352 full Moons left over per year. The inverse of 0.352 is 2.841 or roughly 3 years. Approximately every 3 years we will have 13 full Moons, so in one of the 12 months (except February) we will have two full Moons and the second one is called a Blue Moon.
A previous method of defining a Blue Moon employed a solar-based calendar. The first day of the year was winter solstice and there were four quarters: Winter solstice to spring equinox, spring equinox to summer solstice, summer solstice to fall equinox, and fall equinox to winter solstice. In a normal year each quarter had three full Moons, but on the approximate 3-year cycle one of the quarters would have four full Moons. The Blue Moon was designated as the third full Moon in the quarter having four full Moons. Why the third? The church, using the ecclesiastical calendar determines the date of Lent and Easter using full moons; the Lenten Moon is the last full Moon of winter, and the Easter Moon is the first full Moon of spring. Easter is then the Sunday following the Easter Moon, and Lent starts on Ash Wednesday 46 days before Easter. The Lenten Moon occurs during this 46 day period. Since the dates of Lent and Easter are determined by full Moons and the equinox, having a fourth full Moon in the winter quarter would have to be the Lenten Moon, hence the Blue Moon would need to be the third. If the extra full Moon was the third then the ecclesiastical calendar would remain in the designated bounds. It is unclear when or why the term blue became attache to the third full moon in a quarter. Thankfully this usage has been trashed in favor of the second full moon in a calendar month.
The accompanying photo on the front web page: As the Blue Moon was setting in the west, the Sun was rising in the east; in the photo the tops of the trees in the west are just being illuminated by the rising sun.
My thanks are extended to daughter Alice Few (web site) for providing Blue Moon websites (NASA, Sky and Telescope)
“why are some clouds dark?” – Karen
Hailstones, how are they made? – Arthur
Noctilucent clouds seen in Nebraska. – Ken
Explain the Moon’s Major and Minor Standstills. – Karen
Spectacular lightning photo from Gold Hill. – Gary Siemer
What makes tsunamis so different and destructive?
Observations of Equinox sunrises on the Gold Hill Town Meadow?
To see Arthur’s responses to these questions click on “More” below.