Do you suffer from Friggatriskadekaphobia? If so, you might want to stay in bed a week from Friday.
Friggatriskadekaphobia is fear of Friday the 13th. In many cultures, 13 is the unluckiest number, although it is 17 in Italy. Friday is usually considered the least lucky day, although Tuesday is the unluckiest day in South America.
This is the only Friday the 13th, this year. Every year has at least one. About as many years have two as have one, both of which occur about 43 times each century. Every non leap year that begins on a Thursday — like 2015 — will contain three occurrences of Friday the 13th.This happens 14 or 15 times per century. Leap years that begin on Sunday — like 2018 — will also have three of them.
Friday falls on the 13th slightly more often than any other day of the week. The Gregorian calendar started in 1582. It made the calendar year actually match the astronomical year. It is easy to calculate the day of the week for every day of the year. In every span of 400 years, the complete Leap Year cycle, Sunday falls on the 13th 687 times, Monday 685 times, Tuesday 685 times, Wednesday 687 times, Thursday 684 times, Friday 688 times and Saturday 684 times. Occasionally, as was the case in 2007 and last year, there were two Fridays that fell on the 13th, which were 13 weeks apart. The year 2012 contained three Friggatriskadek’s separated by 13-week intervals.
Friday the 13th isn’t an uncommon occurrence. Another regular, commonly referred to interval is a month of Sundays. Thirty days make up a month, so a month of Sundays equals 30 weeks — about two-thirds of a year.
What happens “once in a Blue Moon?” A Blue Moon. A Blue Moon occurs July 31, 2015. The next one after that won’t be until Jan. 31, 2018. That year will have two Blue Moons, the second on March 31.
So Friday the 13th occurs one to three times per year, on average about six months apart, but sometimes in pairs. A month of Sundays occurs every 30 weeks, roughly seven months. Once in a Blue Moon occurs, on average, once or in a pair each two-and-a-half year period.
There are even longer time intervals marked by recurring events, like the span between the times a teenager appreciates his parents’ advice.
•At 5:51:17 a.m. June 21, our local star — the sun — will be as far north as it ever gets, marking the beginning of summer in the northern hemisphere. From that day until Dec. 21, the days get shorter and the nights get longer. As an astronomer, I appreciate that.
On Friday the 13th, and all other days of the year, we’ll keep you informed of astronomical happenings in our daily planetarium program “Tonight’s Sky.” Call 602-3761 or log on to www.sciencemuseumok.org/content/planetarium for information and show times.
• The Oklahoma City Astronomy Club meets on June 13 — Friday the 13th. Check out its website at www.okcastroclub.com for more information.
• Planet Visibility Report: June starts out with four planets visible during evening twilight. Mercury, deep in the twilight glow and therefore the most difficult to locate; Jupiter, one-third of the way up from the western horizon and much brighter than Mercury; Mars, nearly due south about halfway up; and Saturn, low in the southwester sky. Venus dominates the morning twilight as the brilliant “Morning Star.” Full moon occurs on June 13, with new moon following on the June 27.
Wayne Harris-Wyrick is director of the Kirkpatrick Planetarium at Science Museum Oklahoma. Questions or comments may be emailed to email@example.com.