> Leap seconds deal with accumulating precession that is not dealt with effectively by the definition of the Gregorian calendar.
I don't *think* so. I think they deal with the rotation of the earth on its axis taking more than 24 hours, as opposed to a rotation around the sun taking more than 365 days (the February 29 and associated leap year adjustments). A clock problem as opposed to a calendar problem if you will. "A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time." -- http://en.wikipedia.org/wiki/Leap_seconds Neither of those is the *usual* astronomical meaning of precession, which refers to the "top-like wobble" of the north pole. "Axial precession is the movement of the rotational axis of an astronomical body, whereby the axis slowly traces out a cone." -- http://en.wikipedia.org/wiki/Precession#Astronomy Charles -----Original Message----- From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of John Gilmore Sent: Friday, June 07, 2013 4:25 PM To: [email protected] Subject: Re: Age of datasets in hours, not days? Julian-calendar leap years are defined very simply as those that have serial numbers divisible by 4, those in the doubly infinite sequence . . . -12, -8, -4, 0, +4, +8, +12 . . . A four-year Julian-calendar cycle thus contains a mean of (3 x 365 + 1 x 366)/4 = 365.25 days. This is unsatisfactory for many terrestrial uses; precession accumulates gradually; and the seasons of the tropical year move slowly toward the beginning of the calendar year. (The Julian calendar nevertheless remains in use, in its traditional, medieval form by the Orthodox Churches and with Joseph Scaliger's 'new' epoch origin, the Gregorian date of which is -4713 November 24, by observational astronomers.) To reduce this precession the Gregorian calendar adds a second-order correction. Years are of two sorts: o centurial years, e.g., ..., -100, 0, 100,....1500, 1600, 1700, 1800, 1900, 2000,... that have serial numbers divisible by 100, and o non-centurial years, all the others. Then 1) every fourth non-centurial year is a leap year and 2) every fourth centurial year is also a leap year, In other words, a non-centurial year is a leap year iff its serial number is divisible by 4; and a centurial year is a leap year iff its serial number is divisible by 400. In the upshot there are occasional intervals of seven successive non-leap years centered on a non-leap centurial year, e.g., 1897, 1898, 1899, 1900, 1901, 1902, 1903 2097, 2098, 2099, 2100, 2101, 2102, 2103, and the like. (2000 was of course a leap year.) .* A year in the 400-year Gregorian-calendar cycle thus contains a mean of (303 x 365 + 97 x 366)/400 = 365.2425 days. This is still very imperfect. It is better, yields less long-term precession than did/does the Julian calendar.. A mean tropical year, the time between successive vernal equinoxes, is shortening very slowly. Its current length is about 365.2421_9668 days. The year +3600, a centurial year having a serial number that is exactly divisible by 400, satisfies the definition of a leap year; but it will be a non-leap year by fiat in order to sop up some accumulated precession. Leap seconds deal with accumulating precession that is not dealt with effectively by the definition of the Gregorian calendar. There are many reasons for this precession, most of them classical and well understood. If you know German consult the relevant volumes of C F Gauß, Werke. If you know French, consult Laplace's Mécanique celeste. I am a little weary of correcting misapprehensions. John Gilmore, Ashland, MA 01721 - USA ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
