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Conference hydra::dejavu

Title:	Psychic Phenomena
Notice:	Please read note 1.0-1.* before writing
Moderator:	JARETH::PAINTER

Created:	Wed Jan 22 1986
Last Modified:	Tue May 27 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	2143
Total number of notes:	41773

310.0. "How often Friday, the 13th full moons?" by PBSVAX::COOPER (Topher Cooper) Thu Feb 19 1987 19:15

    My daughter was born last Friday night.  This was both a "Friday
    the 13th" and the day of the full moon.  I wondered how often these
    occur together, and did a quick calculation to get an estimate.
    I thought readers might be interested.
    
    Step 1. The probability that a given day will be a Friday is 1/7.
    
    Step 2. In a four year period, the number of days is 365+365+365+366
    which equals 1461 days.  In that same period there will be 12*4
    or 48 thirteenths.  Thus the probability of a randomly selected
    day being a thirteenth is 48/1461 = 16/487.
    
    Step 3. (This parts a little bit sophisticated) Since the length
    of time between Friday's and that between thirteenths (on the average)
    don't divide each other, the probability that an arbitrary day is
    a Friday is "independent" of the probability that it is a 13th.
    This means that the probability that any particular day (such as
    a full moon day) is 1/7 * 16/487 = 16/3409.  This means that, on
    the average, out of 3409 full moons, 16 of them will occur on Friday
    the thirteenth.
    
    Step 4. The average time between full moons (the length of the
    "synodic" month) is 29.5 days.  If it were *one* full moon out of
    3409 then the average times between them would be 29.5*3409 days.
    But it is 16 of those 3409 full moons, so the average time is
    
    			29.5*3409
    			--------- days
    			   16
    
    between full moon Friday the thirteenths on the average.  This comes
    to roughly 6285 days or about 17.2 years between full moon Friday
    the thirteenth's on the average.
    
    NOTE: This caculates an average.  Since we are dealing with a periodic
    (in the case of the 13ths, a complicated periodic) phenomena, we
    might actually tend to get clustering, i.e., a bunch of such events
    all together, and then none for a very long time.  I don't know.
     But this should be an accurate estimate of the *average* time between
    them.
    
    			Topher

T.R	Title	User	Personal Name	Date	Lines
310.1	Three firday the 13's this year.	NEXUS::MORGAN	Organized Religion? Just say NO!	`Fri Feb 20 1987 01:31`	6
	I was watching STARHUSTLER on PBS and the old guy said that any year that's not a leap year that starts on thursday will have 3 friday the 13's in that year. Now I know that is not what you are asking about but I had to chime in anyway. Mikie?
310.2	How wonderful for you.	MANTIS::PARE		`Fri Feb 20 1987 10:32`	4
	Congratulations on the birth of your daughter!!! How wonderful for you. I have a feeling that this child, born on that day under a full moon, will have a very special role to play in life. Take good care of her.
310.3	Some _good_ luck for Fri the 13th, too	NATASH::BUTCHART		`Fri Feb 20 1987 16:33`	19
	Friday the 13th on a full moon does seem especially auspicious for the birth of a daughter. Reasons being, in ancient (_really_ ancient) mythology, the deity worshipped was the Triple Goddess, who was considered to be manifested in the waxing and waning of the moon. There are 13 (not 12) lunar months in a single year; therefore the number 13 was considered somewhat sacred, especially to the triple goddess. And Friday is a day that has, in some goddess religions, been associated with her. So a Friday the 13th was actually considered a day of great spiritual significance and good luck. To have a full moon occur on this day would probably have been considered extremely special, since the full moon phase represents the goddess in her mother role. (Interesting note: the really ancient ideas of the moon gave it 3 phases, not 4; the Maiden phase was the new to gibbous waxing moon, the Mother phase was the gibbous to full to gibbous moon, and the Crone phase was the waning to new moon.) A very good day to become a mother . . . Marcia
310.4	TGIF	PBSVAX::COOPER	Topher Cooper	`Fri Feb 20 1987 20:18`	16
	RE: .3 One doesn't have to look very far to find that Friday is sacred to (or at least "ruled" by) the Goddess. The word "Friday" comes from the Old English "Frigedaeg" (with a bar over the i). Frige is a form of the Old English goddess Freo (with a bar over the e). For Norse mythology fans she corresponds to Freya. She is associated with Venus. Venus/Freya/Freo are versions of the triune Goddess (there are others, e.g., Juno) as absorbed into the broader pantheons of the religions they are associated with. They are all multi-aspected though, probably because of their association with the Morning/Evening Star, the Crone aspect is played down. Note that in French, the word for Friday is "vendredi" or "Venus's day" (and similarly for other Latin based languages). Topher
310.5	When Friday the 13'th occurs.	PBSVAX::COOPER	Topher Cooper	`Wed Feb 25 1987 18:22`	45
	RE: .1 Knowing the day of the week on which the year starts and whether or not it is a leap year is always sufficient to determine what day of the week any date of that year is on, and therefore how many (and which) Friday the 13'ths there are. Roughly speaking, assign numbers to the days of the week as follows: Sunday = 0, Monday =1, ... Saturday=6. To find out the day of the week for any date, calculate the day of the year for that date (using "30 days hath ..."). For example Feb. 1 will be day 32 of the year. Add the number for the starting day of the week for Jan. 1, divide by 7 and take the remainder. That remainder will be the day of the week for that date (there are some shortcuts). I used this technique to build the following tables -- Non-leap years J F M A M J J A S O N D # NY ------------------------------------ Su X X 2 Mo Mo X X 2 Tu Tu X X 2 We We X 1 Th Th X X X 3 Fr Fr X 1 Sa Sa X 1 Su Leap years J F M A M J J A S O N D # NY ------------------------------------ Su X X X 3 Tu Mo X X 2 We Tu X 1 Th We X X 2 Fr Th X X 2 Sa Fr X 1 Su Sa X 1 Mo The rows in these two tables represent the days of the week on which the year starts. The first twelve columns represent the twelve months of the year. An "X" indicates that the thirteenth of that month on a year starting on the given day of the week will occur on a Friday. The column headed "#" is the number of Friday the thirteenths which will occur in a year starting on that day of the week. The column headed "NY" gives the day of the week which the Next Year will start on. Topher