May 31, 2020

Event of the Week: A “Blue Moon” Eclipse!

This New Year’s Eve rounds off the calendar with an especially rare treat, although its one that not everyone will get to witness; a partial eclipse of the full “blue” moon. The second full moon of December, the first occurred on December 2nd of this month. This eclipse, however, is extremely shallow; at maximum, the moon will only be 7.63% immersed in the dark umbra of the earth’s shadow.

Viewing favors Asia, Europe, and Africa; Australia will see a setting eclipse at sunrise, and most of the Americas will miss out. Only the Canadian Maritimes and extreme northeastern New England can expect to see a shallow, rising eclipse at dusk on the 31st. Umbral contact begins at 18:52 Universal Time (UT), greatest eclipse occurs at 19:23 UT, and last umbral contact is at 19:52 UT. This particular eclipse it a part of saros cycle 115. For those who keep track, the last blue moon under the “two fulls in one month” convention (see below) was June 30th, 2007, and the next is August 31st, 2012. And the last blue moon eclipse? That was on Jan 31st, 1999, and the next won’t be until January 31st, 2018!

This week’s astro-term is Blue Moon. This term has crept into the popular vernacular to mean the second full moon of a calendar month, or something extremely rare. The moon takes 29 days 12 hours 44 minutes and 2.9 seconds to transit from two identical phases (remember our friend, the synodic month?). Hence, a second full moon can only occur on the 30th or 31st, and never in February, leap years not with standing. Generally, this man-made idiosyncrasy of our modern day Gregorian based calendar happens every 2-3 years, although some years such as 2018 will have two! Much ado was made in a May 1999 Sky & Telescope article as to the 1946 mis-interpretation of the definition in the term; as taken from the 1937 Maine Farmer’s Almanac, a strict definition would be a season (as measured from equinox to solstice or vice versa) with four full moons… but I challenge anyone to come up with a meaningful deference! Under the seasonal convention, the third full moon in a season with four is “blue”…thus, the full moon on the 31st is the 1st full moon of the winter season… but I think the two full moons in one month convention is here to stay. It certainly easier to explain…under the esoteric “3rd full moon of the season with 4” convention, the next blue moon isn’t until November 21st of next year. Cool fun extra credit meaningless fact; under this convention, you can have four full moons (three complete lunations) in one season…but…the first full moon in the cycle must occur directly after the solstice or equinox! Of course, these constructs are merely fanciful names; the moon isn’t scheduled to appear blue on either date! Where the discoloration factor came into play is a true mystery. The moon can appear bluish under certain rare conditions, such as following a volcanic eruption or forest fire; this may appear more visible during any given full moon, as there is simply more illumination to be had. Interestingly, a volcanic eruption may be brewing as we write this, as Mount Mayon in the Philippine Islands may be preparing to blow its top. The convention of an extra full moon may have gotten its odd reputation from way back in the medieval day, when the lenten moon would arrive too early on the calendar and would be termed the “betrayer moon” to differentiate it from the true lent moon and the “egg” or Easter moon. We here at Astroguyz think its wonderful that we now commemorate an obscure and misinterpreted construct that has little to do with reality, but is sure fun to write about! And a blue moon eclipse, now that’s truly rare! If nothing else, its a good excuse to crack open a Blue Moon brew or three to commemorate the event, and serve as meaningless astro-banter at that New Year’s Eve party… now, will mother nature throw some volcanic ash into the mix as well?


  1. [...] the original post here:  Event of the Week: A “Blue Moon” Eclipse! : By admin | category: Object | tags: although-its, blue, calendar, full-moon, [...]

  2. [...] Event of the Week: A “Blue Moon” Eclipse! : [...]

  3. [...] “Blue Moon Lunar Eclipse” occurs on January 31st, 2018. The last one occurred during a partial in 2009) A Blue Moon can never occur in February, not even on a leap year, because the shortest synodic [...]

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