# Gregorian vs Julian calendar for dummies **URL:** \ **Category:** Factual Questions\ **Created:** [February 23, 2006, 9:14pm UTC](https://boards.straightdope.com/t/gregorian-vs-julian-calendar-for-dummies/345849 "2006-02-23T21:14:24Z")\ **Posts on this page:** 7\ **Page:** 3
**Author:** ![Kimstu](https://avatars.discourse-cdn.com/v4/letter/k/ecd19e/32.png) [@Kimstu](https://boards.straightdope.com/u/Kimstu)\ **Post date:** [February 25, 2006, 12:00am UTC](https://boards.straightdope.com/t/gregorian-vs-julian-calendar-for-dummies/345849/41 "2006-02-25T00:00:47Z")
> [@Gorilla Man](#): > > Nice - it’s like Douglas Adams’ description of a puddle, so self-satisfied that the dip in the pavement was created just for it to exist in ? I don’t want to belabor what might have been just a joke, but these aren’t actually analogous, right? There isn’t any physical reason that the moon’s size and distance from the earth had to work out the way they did, so that the disk of the moon is just about the same angular size as that of the sun. That really does count as a gen-u-wine coincidence (although not a perfect one, as the apparent size of the moon’s disk fluctuates somewhat). > [@Zeldar](#): > > I’d like to hear your reasoning on the 24-hour and 360-degree connection. Well, if that doesn’t bring back memories! I think this topic was the subject of my very first post here on the SDMB, many years ago. Fortunately, since it comes up quite a bit in my line of work, I kept the file and still have it. 🙂 Lemme go dig: > [@Kimstu in Dec. 1999](#): > > […] The Babylonian calendar used 12 months of either 29 or 30 days, depending on the first visibility of the crescent moon, with intercalary months […] thrown in on an ad hoc basis to keep the months aligned with the seasons. In the last half of the first millennium BCE, intercalation was standardized as the 19-year or “Metonic” cycle similar to that in the modern Jewish calendar, with seven intercalary months at assigned intervals in 19 years. (This system, by the way, produces an error of only about 1 day in about 7000 years […]) > > - Babylonian astronomers divided the day into 12 “beru”, each of which contained 30 “ush” or time-degrees, making 360 time-degrees in a day. The corresponding unit of distance was applied to the rotating sky, denoting the amount of its motion in one time-degree. (Yes, it was handy to have a unit that roughly corresponded to the average daily solar velocity, but don’t kid yourself that at this period anybody actually thought there were 360 days in a year!) This was used in measuring ecliptic longitudes (the twelve zodiacal signs, originally constellations of unequal lengths, were by this time standardized into equal twelfths), giving twelve signs of 30 degrees each. The sexagesimal system was borrowed (and never returned!) probably sometime in the Achaemenid period by the Greeks, who divided the equator (of earth or sky) into 60 parts. (The earliest surviving evidence for this sexagesimal division is in the work of Eratosthenes, around 250 B.C.E.) Over the subsequent years of development of arc and angle measurement among Greek scientists, this division was broken up into various smaller parts (sometimes 360, sometimes 720), and by about the middle of the second century B.C.E. the division into 360 had evolved into the accepted standard. And that’s why there are 360 degrees in a circle; it’s really not all that “ancient” a tradition, with its origins only about 3000 years ago and reaching the level of standard scientific convention nearly 1000 years later. > > (I get all this, by the way, from O. Neugebauer, _A History of Ancient Mathematical Astronomy_, 3 vols., Berlin: Springer-Verlag 1975, and Hermann Hunger and David Pingree, _Astral Sciences in Mesopotamia_, Brill: Leiden 1999.) IIRC, the 24-hour day division was first used by the Egyptians, possibly influenced by the Babylonian 12-beru day division. ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [February 25, 2006, 12:04am UTC](https://boards.straightdope.com/t/gregorian-vs-julian-calendar-for-dummies/345849/42 "2006-02-25T00:04:10Z")
> [@Kimstu](#): > > ? I don’t want to belabor what might have been just a joke, but these aren’t actually analogous, right? There isn’t any physical reason that the moon’s size and distance from the earth had to work out the way they did, so that the disk of the moon is just about the same angular size as that of the sun. That really does count as a gen-u-wine coincidence (although not a perfect one, as the apparent size of the moon’s disk fluctuates somewhat) It was a joke with a purpose, although on rereading I realise it wasn’t entirely appropriate…the moon is the correct size sometimes. Sometimes it’s larger than the sun, sometimes it’s smaller. One produces the same total eclipse, the other an annular eclipse. ---
**Author:** ![Zeldar](https://avatars.discourse-cdn.com/v4/letter/z/b77776/32.png) [@Zeldar](https://boards.straightdope.com/u/Zeldar)\ **Post date:** [February 25, 2006, 12:13am UTC](https://boards.straightdope.com/t/gregorian-vs-julian-calendar-for-dummies/345849/43 "2006-02-25T00:13:23Z")
Thanks for the additional comments on the 24/360 issue, **Kimstu**. It squares with what I’ve read from various sources. I was expecting a new wrinkle on the “24” issue, as opposed to just doubling “12” which I realize was the choice for having more divisors (2, 3, 4, 6) than 10 (2 and 5) for developing fractions that made common sense. On a tangential issue, have you consider how Roman Numerals are partial to 1, 5, and 10 and multiples thereof. In fact, is there any basic Roman Numeral that isn’t a 1, 5 or 10? Examples: 10, 50, 100; 100, 500, 1000. They were pretty tied to fingers and toes, I guess. ---
**Author:** ![Odesio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/odesio/32/20003_2.png) [@Odesio](https://boards.straightdope.com/u/Odesio)\ **Post date:** [February 25, 2006, 12:22am UTC](https://boards.straightdope.com/t/gregorian-vs-julian-calendar-for-dummies/345849/44 "2006-02-25T00:22:05Z")
> [@Askance](#): > > You call him up? On what? Think, person, think! You’re dealing with people by letter carried by boat or horse. Everything took place at a slower pace then and the exact time of day or nomenclature for the current month or year was totally irrelevant. The people of Ulm might disagree. When Napoleon laid seige to the city the people of Ulm expected the Russians to arrive in short order at a date that the Austrians and Russians had previously agreed upon. Oops, the Russians were on the Julian calendar and were two weeks behind the Austrians and therefore didn’t arrive in time for the battle. Marc ---
**Author:** ![GuanoLad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/guanolad/32/42_2.png) [@GuanoLad](https://boards.straightdope.com/u/GuanoLad)\ **Post date:** [February 25, 2006, 1:02am UTC](https://boards.straightdope.com/t/gregorian-vs-julian-calendar-for-dummies/345849/45 "2006-02-25T01:02:23Z")
> [@Zeldar](#): > > I’d like to hear your reasoning on the 24-hour and 360-degree connection. Other than 24 X 15 = 360 and 15 X 2 = 30, what other relationship(s) do you know of? Isn’t that enough of a reason? I was just thinking in terms of circles and divisions, and that the cycle of the day is divided into 24 because it fits into 360, and can also be further broken down into convenient parts that also fit the 360 degree pattern. Apparently I’m wrong, so never mind. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [February 25, 2006, 1:11am UTC](https://boards.straightdope.com/t/gregorian-vs-julian-calendar-for-dummies/345849/46 "2006-02-25T01:11:05Z")
BTW, for lots of background on calendars in general, you can start at this site, maintained by known calendar freak Peter Meyer: [http://www.hermetic.ch/cal\_stud.htm](http://www.hermetic.ch/cal_stud.htm) Re to the pope hiring astronomers: Popes prior to Gregory XIII had the problem investigated. Paul III, a few popes befory Gregory XIII, for instance. In 1563, the Council of Trent had approved a calendar reform to restore the equinox to the dates recorded in 325. This was a few years before Gregory XIII’s papacy. It’s got Gregory’s name on it because a proposal was finally implemented during his tenure. In fact, Roger Bacon, who lived about 3 centuries earlier, had sought to get the calendar reformed. People probably began noticing the slippage of the equinox long before then - by Bacon’s time the 325 AD observations would have been off by about 7 days. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 25, 2006, 5:00am UTC](https://boards.straightdope.com/t/gregorian-vs-julian-calendar-for-dummies/345849/47 "2006-02-25T05:00:37Z")
> [@](#): > > Trouble is, the rate at which the earth spins on its axis and the rate at which it orbits the sun don’t coincide neatly (it would be spooky if they did!). Not necessarily all **that** spooky, since there are planets where the rates do coincide nicely. Mercury’s rotation, for instance, is exactly 2/3 as long as its year. And the Moon rotates once in exactly the same time it takes to orbit the Earth. It would be a bit surprising if a planet as far away from its star as the Earth showed such a pattern, and it would also be a bit surprising to see such a pattern with a ratio as high as hundreds to one. But even there, I think that it falls rather short of “spooky”. Spooky I would reserve for things like constants of cosmological significance being 10[sup]120[/sup] times smaller than expected, but still not quite zero. And yes, we are indeed spending a lot of effort trying to figure out _that_ one. [Previous page](https://boards.straightdope.com/t/gregorian-vs-julian-calendar-for-dummies/345849.md?page=2)