Saros and the Blue Moon

If you multiply the days in a year (365.25) by 19 years you will get 6939.75 days. If you multiply the days from in a synodic month [New Moon to New Moon] (29.53) by 235 synodic months you will get 6939.55 days. The two periods almost match. This odd fact has been known for at least 2500 years. The ancient Egyptians called this Saros. The Saros cycle means that any alignment of the Sun, Earth and Moon that occurs at some date will occur again 19 years later to the day. The only difference is the time delay causes the event to occur later in the day. So if a solar eclipse occurs here on some day, it will occur 19 years later but somewhat to the west. It is worth noting that this cycle isn't perfect. Eventually the little mismatches accumulate to the point where the cycle fails entirely, starting a new set of Saros cycles.

If you multiply 19 years by 12 months you will get 228 months. This falls 7 synodic months short of our Saros cycle. This means that there will be 7 more New Moons every 19 years than months. Today this is just an odd fact, but when the phase of the Moon was used to keep track of the calendar, this was a serious problem. The extra months had to be tucked in somewhere or important holy days would be observed on the wrong date. For example, Easter must fall in the same week as the first full Moon in Spring according to most Christian churches. If an extra synodic month was inserted in the Winter, Easter follows the fifth Full Moon, not the Fourth full Moon. These extra months, tucked into various seasons, were given a variety of names, but the most famous in a Blue Moon. For some reason, the Blue Moon was always tucked in between the second and last months of a season. [The full details of this rule was explained in the Maine Farmer's Almanac {MFA}].

Blue Moons occur on average every 2.71 years. However with the unequal number of days in the months [courtesy of the inflated egos of Julius (July) and Augustus (August) Caesar], the actual date of a Blue Moon is not easy to calculate. This irregularity in the period gave birth to the phrase once in a Blue Moon.

Back in 1946, an article in Sky and Telescope {S&T} magazine written by a contributor established a new [incorrect] rule for Blue Moons. This writer decided the rule was equivalent to being the second Full Moon within a calendar month. By the S&T rule both January and March of this year were Blue Moons, but by the MFA rule this year has no Blue Moons. Next year has a Blue Moon in February by the MFA rule but has no Blue Moons according to the S&T rule. The S&T March 1999 and May 1999 issues explain this confusion in great detail. I'm not sure that Sky and Telescope's solution will solve anything. They now suggest [I suspect facetiously] combining both rules and letting astronomers explain to interested questioners. Well, I've done my bit, but I doubt that anyone will remember the rules by next Thursday.

Early July has a rather major spectacle for those are willing to stay up to the wee hours of the morning. All the planets will be visible during the night. Usually, some planets are in the daytime sky, but not right now. Unfortunately, the opportunity will vanish rather rapidly. As the Moon waxes during the month, it will begin to obscure Pluto. By the time the Moon is new again, Mercury will have moved back behind the Sun.

By Early August all the planets [except Mercury] will be in the night time sky. For those willing to stay up until 1 AM, the last of the planets [Saturn] will rise. Frosty Drew Observatory will be open to the public each Friday night where you can view everything in our 7" refractor. By the time we replace this with our new 16" Schmidt Cassegrain Catadioptric computerized telescope in the middle of September, only Venus and Mercury will be unavailable.

Leslie Coleman
Leslie Coleman
Entry Date:
Jul 1, 1999
Published Under:
Leslie Coleman's Columns
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