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I understand that the Julian Calendar was introduced to align the length of the calendar to the tropical year, i.e. make the average year 365.25 days long. However, what I'm still trying to figure out is that were they trying to align the start of the calendar with? Or, put it another way, why 1 January is not aligned to a solstice, equinox or some other astronomical event?

Wikipedia currently states (emphasis mine):

The first step of the [Julian] reform was to realign the start of the calendar year (1 January) to the tropical year by making 46 BC (708 AUC) 445 days long, compensating for the intercalations which had been missed during Caesar's pontificate. This year had already been extended from 355 to 378 days by the insertion of a regular intercalary month in February. When Caesar decreed the reform, probably shortly after his return from the African campaign in late Quintilis (July), he added 67 more days by inserting two extraordinary intercalary months between November and December.

Realign the start of the year to what of the tropical year? A footnote further explains:

It is not known why he decided that 67 was the correct number of days to add. Ideler suggested (Handbuch der mathematischen und technischen Chronologie II 123-125) that he intended to align the winter solstice to a traditional date of 25 December. The number may compensate for three omitted intercalary months (67 = 22+23+22). It also made the distance from 1 March 46 BC, the original New Years Day in the Roman calendar, to 1 January 45 BC 365 days.

But why was the date 25 December special or traditional at that point in time?

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Possibly they wanted to match it to Brumalia. The Roman winter solstice festival. wikipedia: "The Brumalia was also celebrated during the space of thirty days, commencing on 24 November and ending with the "Waxing of the Light", December 25" citation

Much the same can be said about Saturnalia, they're very similar. The "Birthday of the Unconquered Sun" is another Decemeber 25th shortest day thing, but it's usage seems to be late roman empire.

The tropical year is the solar year. It's a measurement of the position of the sun. It was probably based on Hipparchus's work on equinoxes.

Also the choice of this date is to do with Pliny: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0137%3Abook%3D18%3Achapter%3D59

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Yes, but the question is why they aligned these astronomical events to the (seemingly) arbitrary date of December 25th? – Juan A. Navarro Jan 3 '13 at 12:28
I see how that's a bit different. – Nathan Cooper Jan 3 '13 at 12:36

After some reading about the early Roman Calendar, it is relevant to note that originally the calendar had only ten months and began on March, with an uncounted “winter” period after December. The number of days on each month were more or less flexible, and they usually tried to align the 15th of March, the mid of the month, with Ides, a full moon.

At the time of the ruler Numa Pompilius the months of January and February were introduced at the end of the year, but apparently the calendar became a mess while they tried to keep it both in sync with the moon and the seasons. Julius Caesar then fixed the lengths of the months and the year in order to more closely match it to the length of the tropical year, thus keeping it synchronised with the seasons, but at the expense of loosing synchronisation with the phases of the moon. He also wanted to move the “start” of the year to January, but this didn't last for long and the calendar was still considered as beginning on March.

It seems that, when introducing the new Julian Calendar, the first 15th of March was aligned to fall on a full moon (such alignment would, of course, be lost on the following years) and so that March would contain the Spring Equinox (which, due to the alignment with the tropical year, would remain true thereafter). As a side effect, all of this caused the “1st of January” to fall at some arbitrary point along Earth's orbit around the Sun.

Furthermore, even after the Julian Calendar was set and fixed, the “start of the year” has been celebrated at many different dates including: the 1st of May, 15th of March, 1st of January, 25th of December (which by then was significant because on the fixed calendar it aligned with the winter solstice and the christian Nativity), 25th of March (Annunciation), Easter, 1st of September, 1st of March, and others. It's only from the 16th and 17th century that most countries settled on celebrating (and legally establishing) the start of the year on the 1st of January.

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"My guess is that, even if they tried to keep the first few 15th's of March still on sync with the full moon" Nope, they were fully aware of that going to a solar calendar they would loose the sync to the full moon. Perhaps they were syncing the first year to a full moon, though. All of the rest is good. – Lennart Regebro Feb 5 '14 at 19:10
Sure I agree about the phases of the moon, and they knew that sync with the moon was going to be lost. My “guess” is about the fact that maybe they tried to start the new calendar with the 15th of March on a full moon and, thus, 1st of January was as a consequence fixed to some arbitrary point. / This is probably the crux of my question, to which I haven't yet found a satisfactory answer: how was the “correct” start of the year determined? – Juan A. Navarro Feb 5 '14 at 20:19
Yes, that's what I thought to, and so it is. 46 BC was the long year, so 45 BC (or year -44) was the first year of the new calendar, and indeed, the full moon is on the Ides of March. eclipse.gsfc.nasa.gov/phase/phases-0099.html – Lennart Regebro Feb 5 '14 at 20:45
And the Spring Equinox the 23rd of March, it seems. So it seems they aligned the first year to the first day of the moon in which the Spring Equinox would happen. I'm actually quite surprised that it's the 23rd. I would have expected it to be the 25th, so the Winter solstice would have been the 25th as well. – Lennart Regebro Feb 5 '14 at 21:23
Nice, great! Thanks a lot for the additional info. I've edited the answer accordingly. – Juan A. Navarro Feb 6 '14 at 8:55

According to Feeny, "Caesar's Calendar", p.196 (preview here), the concept of aligning with the celestial objects was not even present to Roman minds in Cicero's time.

It would be interesting to know precisely where in Cicero and Ovid there are references to alignment.

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In your quote mentioning Ideler you come close to answering your own question. The only mistake is that Ideler's guess that the year was aligned to the Winter solstice. In fact, Caesar aligned the calendar to the Spring equinox and we know this from offhand contemporaneous mentions by Cicero and Ovid. In other words, the date VIII Kalends Aprilis (25th of March) was made to fall on the Spring equinox.

The beginning of the civil year in Rome was always the Kalends of January. You may hear of March as the beginning of the year, but that was the sacred year. Caesar only published new dates for use in the civil calendar.

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Thanks for your answer! It makes a lot of sense. Just a few questions: on this source 25th of March is stated as a.d.VIII.Kal.Apr. but you have it as VI, which one is correct? And, again the big question, why this date in particular? What was special about it at the time? Also would be glad if you could provide some sources or references! – Juan A. Navarro May 1 '14 at 7:30
It is 8, I miscounted. I have updated the answer. – Tyler Durden May 1 '14 at 14:16
@JuanA.Navarro The Romans celebrated the vernal equinox always on the same day, the festival of Hilaria on March 25th (our word "hilarious" comes from this joyful holiday). Why this date happened to be associated with the equinox is probably accidental. In other words, when the calendar was first used the equinox happened to fall on this day and thereafter it became traditional. It was a very important civil holiday and was the day of marriages so it had a high social and economic importance in Rome. Everybody, including slaves, got the day off. – Tyler Durden May 1 '14 at 14:34
Thanks again. This seems to point in the right direction, and is becoming my preferred theory thus far. However, as Roger Pearse notes in his answer, could you point to a source where Cicero or Ovid make references to this alignment (either to Hilaria or possibly to Brumalia). – Juan A. Navarro May 2 '14 at 8:23
I usually don't provide references, and according to the site guidelines, questioners are not supposed to ask for references. To fully document the link here would require a long essay because it is not as simple as "references", you have to analyze them as well. I am not going to start writing a 5,000 word essay for you or start writing up citations. I have given you the correct answer (unlike the other respondents who have given incorrect answers). Take your pick: wrong answers with references or a correct answer without. – Tyler Durden May 2 '14 at 14:16

Sol Invictus was said to have died and gone to hell, and three days later rose from the grave on that day, celebrated in the Roman festival of Dies Natalis Solis Invicti, or "Birthday of the Unconquered Sun". Both the nox maximus, or Winter Solstice, and Dies Natalis Solis Invicti were said to have occured on December 25th in the Julian calender.

For more information than you asked for about later changes to the celebration, this page presents a comprehensive study of how Christmas relates to Dies Natalis Solis Invicti.

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Deleted a series of near snark from multiple parties. I will posit that we like to avoid being a primary reference here. For that reason, a lot of our users will look askance at answers without a lot of links backing up their assertions. – T.E.D. Feb 10 '14 at 22:02
@LennartRegebro I changed my answer to reflect your contention and my inability to provide good sources. – Nathan C. Tresch Apr 29 '14 at 15:07
Yeah, I suspect you have things backwards here. It's not that the calendar is fixed to this festival, it's this festival that is fixed to the winter solstice. It probably happens the 25th because the calendar is fixed to the spring equinox. But why the spring equinox was fixed to the 25th of March is unclear. – Lennart Regebro Apr 30 '14 at 7:38
@LennartRegebro Maybe, but, since the asker is asking WHY the solstice was affixed to Dec 25th in the original question, "he intended to align the winter solstice to a traditional date of 25 December.", and the solstice actually falls on the 21st, your contending that the day is important because the solstice falls on that day doesn't hold up. The celebration pre-dates the time when the solstice was affixed to that day, and it's one of the earliest I can find which does. – Nathan C. Tresch Apr 30 '14 at 17:41
So to recap: I'm sure that the Julian Calendar was intended to be aligned with the solstices and equinoxes in some way. But I don't know why they are aligned so they end up the 25th of the months. It would make more sense to put the 1st of April on the spring equinox. Apparently the 25th of March is the traditional date, but the question then only becomes "Why the 25th of March?" – Lennart Regebro May 2 '14 at 13:34

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