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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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3 Answers

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
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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 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 at 20:19
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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 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 at 21:23
    
Nice, great! Thanks a lot for the additional info. I've edited the answer accordingly. –  Juan A. Navarro Feb 6 at 8:55
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Sol Invictus was said to have died and gone to hell on the Noctis, 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". It's my own understanding from research and reading the material that this is why Christmas is celebrated on that day, although, as you can read in the link below this position isn't held by everyone.

Wikipedia

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I can't find any reference to it being celebrated three days after Noctis, nor in fact any information about what "Noctis" would be. Can you point me somewhere? –  Lennart Regebro Feb 6 at 5:37
    
@LennartRegebro The "winter solstice", or the longest night of the year. –  Nathan C. Tresch Feb 7 at 2:20
    
References, please. All information I can find claims that Sol Invictus was the 25th. No one says it was three days after the solstice, which is not the same thing. –  Lennart Regebro Feb 7 at 4:16
    
@LennartRegebro From the link in my answer: The Philocalian calendar of AD 354 gives a festival of "Natalis Invicti" on 25 December ... Maybe reading the entirety of the link I gave might be a good start, and then following up with the references at the bottom. –  Nathan C. Tresch Feb 7 at 23:42
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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 at 22:02
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