What was the Julian Calendar aligned to?

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.

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.

i am unable to give references to support my mention above that the possible source to which the roman calendar was being aligned .... was the Canopic calendar .... except to say that I have sat down and attempted to reconstruct how the Canopic calendar would have looked ... starting from back in the time of the Ptolemies.... and projecting it down to roman times.

I have noted the historical names of important people who have looked into the starting date of the reformed calendar who have indicated this first year of the reforms should be a leap year.

I have when looking at the Canopic calendar ... looked at the use of the 7 day planetary week used by the Greeks in association with this new 365.25 day calendar ... which results in a 28 year calendar cycle ...

It is to be noted that in 45 BC .... the start of the Canopic calendar ... in the latter part of 45 BC .... is the first year of such a 28 year cycle ... meaning that the prior year ... which would be the last year of the 28 year cycle ... would be a leap year ... and as such would overlap with the first part of the reformed roman calendar year of 45 BC ....

This would put the leap day of the Canopic calendar year 28 ... to be in the same year as the projected leap year of the first year of the roman reformed year ....

This would then indeed give historical grounds for Ideler 1825 Mommsen 1859 to conclude that the first year of the roman calendar reforms of Julius Caesar to be a leap year.

Another interesting confirmation of this... is that if you take the corrections of Augustus ... which put the Roman reformed calendar back on to its intended cycle ... and project the corrected calendar back wards to 45 BC ... you again find that 45 BC ... should indeed be a leap year ....

There are several things that have not been mentioned in the other answers. The Roman calendar in 45 BC can shown to be aligned with the start of the Roman nundinal cycle.

There is also good reason to believe that the Roman calendar alignment was closely connected with the canopic calendar. The leap year cycle of the newly aligned Roman calendar, for example was aligned with the leap year cycle of the canopic cycle.

A wrong understanding of the 4 year leap year cycle, however, resulted in the leap year cycle being 3 yearly until the reign of Augustus. He did some more refinements that put the Roman calendar back onto its originally intended alignment.

Caesar's goal with the Julian calendar may have been to correct the errors which had occurred in recent decades and restore the important holidays and other important days of the year to their rightful times - their rightful times as he and other older Romans remembered them, and not to when they might have happened in the time of Romulus centuries earlier.

And also to change the calendar enough to minimize the actions necessary to keep it correct and so make sure there would be no more calendar drift in the future. Recent events had shown that the administration could not be trusted to make the necessary actions at the necessary times to keep the calendar in line with the seasons, so minimizing those necessary actions and making them strictly periodic and automatic seemed like the proper course of action.

This question asks about the day that the consuls began their term of office:

When did the Roman consular year begin during the Republic and Empire?1

My answer lists the various dates for the start of Consuls' terms, and thus for the star of the Roman consular year. Obviously the data for the early Roman Republic is rather suspect, but it seems clear that the 153 BC date for starting the Roman consular year on January 1 should be solid.

So by 45 BC starting the Roman consular year on January 1 was a tradition over a century old and Caesar would have needed a strong reason to change it. And Caesar obviously had to keep January 1 in his new calendar reasonably close to January 1 in the old Republican calendar, thus somewhat restricting the possible date range for January 1 in the Julian Calendar.

This factor, combined with the factors mentioned in other answers, might be enough to account for the Julian calendar placing January 1 when it did.

Scaliger 1583 indicates that 45bc is not to be treated as a leap year. By doing so, the start of the year aligns with the dark moon conjunction.

Ideler 1825 Mommsen 1859 both indicate that 45bc should be treated as a leap year. this would mean that the start of the year falls one day before the dark moon conjunction.

However, it should be of note that the Canopic calendar year that straddles the start of 45bc is a leap year. The new canopic year that starts on Oct 22 is aligned to a dark moon conjunction, and is also the start of new 28 year solar cycle based on the use of the 7 day planetary cycle in which saturday is the first day of the 7 day cycle.

This Canopic calendar is the only 365.25 day calendar that is known to have been in operation at the time, which makes it a strong contender as being the basis for the calendar reforms of the roman calendar. Sosigenes was based in Alexandria of Egypt, in which there was at the time, one of the biggest libraries available. It was also in Alexandria, that the Canopic calendar originated.

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.

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

The epoch of the Julian calendar (i.e., January 1^st, 45 BC) was indeed synchronized with the first new moon following the previous year's winter solstice :

It was probably the original intention of Caesar to commence the year with the shortest day. The winter solstice at Rome, in the year 46 B.C., occurred on the 24^th of December of the Julian calendar. His motive for delaying the commencement for seven days longer, instead of taking the following day, was probably the desire to gratify the superstition of the Romans, by causing the first year of the reformed calendar to fall on the day of the new moon. Accordingly, it is found that the mean new moon occurred at Rome on the 1^st of January, 45 B.C., at 6h. 16′ P. M. In this way alone can be explained the phrase used by Macrobius: Annum civilem Caesar, habitis ad lunam dimensionibus constitutum, edicto palam proposito publicavit. This edict is also mentioned by Plutarch where he gives the anecdote of Cicero, who, on being told by some one that the constellation Lyra would rise the next morning, observed, Yes, no doubt, in obedience to the edict.

— William Smith, A Dictionary of Greek and Roman Antiquities, page 231, John Murray, London, 1875.

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.