Re: Friday 13 and Friday 13
On 06/01/2015 23:07, John Pickard wrote: Good morning everyone, Many thanks for the replies to my (slightly trivial) question re Fri 13. I now have a better understanding of how to predict which days to stay in bed, avoid walking under ladders, and not kicking black cats! If nothing else, the replies show that dates are rather tricky things with some interesting mathematics behind them. Wouldn't it be simpler to move to decimal time and a decimal calendar? (NOT a serious question!) Cheers, John John Pickard john.pick...@bigpond.com Napoleon tried decimal time, and he was defeated and exiled twice (this year is the 200th. anniversary of Waterloo) As for the calendar, the orbits of Earth and Moon would have to be shifted to give us different length years and months; and, as for the week, I quite like the idea of Sunday, Monday, Tuesday, Wednesday, Thursday, Friday and Saturday being associated with the Sun, Moon, Mars, Mercury, Jupiter, Venus and Saturn. If we were to have a ten day week, we would have to add Uranus, Neptune and Pluto; and some people say that Pluto is only a 'minor planet' so that would start new arguments :-) -- -- Richard Mallett Eaton Bray, Dunstable South Beds. UK --- https://lists.uni-koeln.de/mailman/listinfo/sundial
RE: Friday 13 and Friday 13
Richard Mallett wrote: >Just to clarify : The answer to the original question is that this >situation recurs every 28 years, so will have occurred in 1987, 1959, >1931, 1903 and 1874 (29 years because 1900 was not a leap year) and will >occur again in 2043, 2071, 2099 and 2128 (29 years because 2100 will not >be a leap year) Although the situation does usually recur every 28 years, I do not think this is the complete answer, nor that it will occur in 2128, as that is a leap year - when March 13 will fall on Saturday. We would guess that February 13th must be a Friday about one year in seven, or there would not be enough days of the week to go round; and that about one-quarter of these will be a leap year and March 13th will be Saturday. So: Three times in a 28 year cycle (an ordinary one, not including a century non-leap year) February 13 falls on Friday and the year is not leap, so March 13 is also Friday. The intervals are 6, 11, and 11 years, according to whether the start year is 1, 2, or 3 after a leap year, so that they include 1, 3, or 3 leap years. This works because 6+1 and 11+3 are both multiples of 7. The Friday 13th pair in February and March occurred in 1953, 1959, 1970, 1981, 1987, 1998 then 2009 (as 2000 was leap it did not disturb the 6, 11, 11 sequence). Then add 6 to give the present year, 2015, next add 11 and 11 to give 2026 and 2037, then 2043, 54, 65; 2071, 82, 93, 2099. However 2100 not being leap changes the sequence in this case to make three six-year intervals in a row, 2093, 2099, 2105, 2111, before settling back to 2122 and 2133 followed by 2139, 50, 61 ... The six year interval spanning the century includes just one leap year. The same happens around 2200 with 2189, 2195, 2201, 2207. But at 2300 (as happened at 1900) there is a twelve-year gap, including two leap years, in place of an eleven-year one including three: 2285, 2291, 2303, 2314, 2325, then 2331, 2342, 2353. Similarly 400 years earlier the sequence ran 1874, 1885, 1891, 1903, 1914. 2400 being leap means the 6, 11, 11 year interval sequence runs through it undisturbed, as happened at 2000. Regards, Andrew James Secure Meters (UK) Ltd. is a registered company in England: 2199653 Secure House, Moorside Road, Winchester, SO23 7RX This correspondence is confidential and solely for the intended recipient(s). If you are not the intended recipient, you must not use, disclose, copy, distribute or retain this message or any part of it. If you are not the intended recipient please delete this correspondence from your system and notify the sender immediately. --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Friday 13 and Friday 13
Merry 358 and Happy 1, or is it Merry 357 and Happy 0 :) On 2015-01-06 18:07, John Pickard wrote: If nothing else, the replies show that dates are rather tricky things with some interesting mathematics behind them. Wouldn't it be simpler to move to decimal time and a decimal calendar? (NOT a serious question!) -- --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Friday 13 and Friday 13
Good morning everyone, Many thanks for the replies to my (slightly trivial) question re Fri 13. I now have a better understanding of how to predict which days to stay in bed, avoid walking under ladders, and not kicking black cats! If nothing else, the replies show that dates are rather tricky things with some interesting mathematics behind them. Wouldn't it be simpler to move to decimal time and a decimal calendar? (NOT a serious question!) Cheers, John John Pickard john.pick...@bigpond.com -Original Message- From: John Pickard Sent: Tuesday, January 06, 2015 8:35 AM To: Sundial List Subject: Friday 13 and Friday 13 Good morning all, and a happy 2015 to everyone. Looking at my diary this morning, I noticed that 2015 is a bit unusual in having Friday 13 in consecutive months: February and March. Does anyone know how often this occurs, when was the last time, and when is the next time it will happen? (And no, I don't place any particular significance in the day!) Cheers, John John Pickard john.pick...@bigpond.com --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Friday 13 and Friday 13
On 06/01/2015 11:53, Andrew James wrote: As there are 24 leap years in three out of four centuries and 25 in the fourth, in a 400 year period there are 303*365+97*366 = 146097 = 20871*7 days. So the weekday pattern repeats every 400 years, as this is exactly 20871 weeks. The relationship of weekdays to dates is such that the 13th of the month falls more often on Friday in the 400 year period than on any other day of the week - not a large bias, but one which doesn't go away! The 13th falls on Monday 685, Tuesday 685, Wednesday 687, Thursday 684, Friday 688, Saturday 684, and Sunday 687 times. Regards, Andrew James Secure Meters (UK) Ltd. is a registered company in England: 2199653 Secure House, Moorside Road, Winchester, SO23 7RX This correspondence is confidential and solely for the intended recipient(s). If you are not the intended recipient, you must not use, disclose, copy, distribute or retain this message or any part of it. If you are not the intended recipient please delete this correspondence from your system and notify the sender immediately. --- https://lists.uni-koeln.de/mailman/listinfo/sundial Just to clarify : The answer to the original question is that this situation recurs every 28 years, so will have occurred in 1987, 1959, 1931, 1903 and 1874 (29 years because 1900 was not a leap year) and will occur again in 2043, 2071, 2099 and 2128 (29 years because 2100 will not be a leap year) -- -- Richard Mallett Eaton Bray, Dunstable South Beds. UK --- https://lists.uni-koeln.de/mailman/listinfo/sundial
RE: Friday 13 and Friday 13
As there are 24 leap years in three out of four centuries and 25 in the fourth, in a 400 year period there are 303*365+97*366 = 146097 = 20871*7 days. So the weekday pattern repeats every 400 years, as this is exactly 20871 weeks. The relationship of weekdays to dates is such that the 13th of the month falls more often on Friday in the 400 year period than on any other day of the week - not a large bias, but one which doesn't go away! The 13th falls on Monday 685, Tuesday 685, Wednesday 687, Thursday 684, Friday 688, Saturday 684, and Sunday 687 times. Regards, Andrew James Secure Meters (UK) Ltd. is a registered company in England: 2199653 Secure House, Moorside Road, Winchester, SO23 7RX This correspondence is confidential and solely for the intended recipient(s). If you are not the intended recipient, you must not use, disclose, copy, distribute or retain this message or any part of it. If you are not the intended recipient please delete this correspondence from your system and notify the sender immediately. --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Friday 13 and Friday 13
Two sequential Friday 13ths can only happen on Feb & March, and only in non-leap years. So, i would guess it happens 3/28 years, or about 11% of the time. On 2015-01-05 16:35, John Pickard wrote: Good morning all, and a happy 2015 to everyone. Looking at my diary this morning, I noticed that 2015 is a bit unusual in having Friday 13 in consecutive months: February and March. Does anyone know how often this occurs, when was the last time, and when is the next time it will happen? (And no, I don't place any particular significance in the day!) Cheers, John John Pickard john.pick...@bigpond.com --- https://lists.uni-koeln.de/mailman/listinfo/sundial -- --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Friday 13 and Friday 13
Good morning all, and a happy 2015 to everyone. Looking at my diary this morning, I noticed that 2015 is a bit unusual in having Friday 13 in consecutive months: February and March. Does anyone know how often this occurs, when was the last time, and when is the next time it will happen? (And no, I don't place any particular significance in the day!) Cheers, John John Pickard john.pick...@bigpond.com --- https://lists.uni-koeln.de/mailman/listinfo/sundial
