Caption: Bust/statue of Julius Caesar (100--44 BCE)---Old Gaius.

Features:

1. There is almost NO information about this bust/statue online. Yours truly assumes it's a Roman sculpture from classical antiquity, but maybe it's a pastiche. The bust/statue bears a reasonable likeness to the Tusculum portrait of Julius Caesar which is dated to 50 BCE--40 BCE and may come from Julius Caesar's lifetime and may be a good likeness. Yours truly prefers this sculpture to the Tusculum portrait itself which is just a head bust---and those always look just awful to yours truly.

   Yours truly always liked Old Gaius' hairstyle---with it, he wandered the world---and with it, he wooed Cleopatra VII (69--30 BCE).

The Julian Calendar and the Gregorian Calendar:

1. In yours truly opinion, Julius Caesar's best achievement was his calendrical reform of 46--45 BCE which established the Julian calendar. He was advised by ancient Greek astronomer Sosigenes of Alexandria (fl. 50 BCE)---who was portrayed in Cleopatra (1963 film) by Hume Cronyn (1911--2003)---whose father is the eponym of the Hume Cronyn Memorial Observatory.

2. Of course, the Julian calendar was only known in western Eurasia and NOT all of that initially. It did spread a little to the Americas and other outposts of European contact in the Age of Exploration (c.1400--c.1800). The successor to the Julian calendar, the Gregorian calendar (initiated 1582) was originally only used in Roman Catholic countries, but gradually spread throughout most of Europe (after a lot of kicking and screaming) and European colonies and in modern times (c.1800--c.1945 or so) became the modern de facto international civil calendar, of course.

3. The Julian calendar broke the connection between the Moon and timekeeping by dispensing with lunar months altogether and replacing them with the 12 semi-arbitrary modern calendar months (which we just call months) that divide up the year. Julius Caesar cut the Gordian Knot of lunisolar calendars.

4. But there is another calendrical difficulty, NOT caused by lunar months. The solar year = 365.2421897 days (J2000) is NOT an integer number of days and this creates a heck of problem for the solar calendar used for civil purposes.

5. The Julian calendar reform (46--45 BCE) solved the solar calendar problem by dividing years into common years = 365 days and leap years = 366 days. There was then leap year cycle of 4 years consisting of 3 common years = 365 days and 1 leap years = 366 days.

   The resulting average year is Julian year = 365.25 days exactly by definition which, indeed, approximates to good accuracy/precision the solar year = 365.2421897 days (J2000).

6. When Dionysius Exiguus (c.470--c.544 CE) in 525 CE established the zero-point (his estimate of the birth year of Jesus (c.1--c.30 CE)) for the Anno Domini (AD or Year of Our Lord) Era---which was renamed Common Era (CE) by Johannes Kepler (1571--1630) in 1615 which name has become somewhat widely used since especially in archaeology: see Wikipedia: Common Era---he chose to start the new year numbering in 532 CE which was a leap year by the former dating system, the Diocletian Era (DE), and which he kept as a leap year, thereby very conveniently making all leap year numbers evenly divisible by 4 which may be why he identified the year of Jesus' (c.1--c.30 CE) birth when he did since that year is NOT precisely known. The Anno Domini Era gradually came into use after being accepted by Charlemagne (747--814) probably advised by the scholar Alcuin (c.735--804) probably following the usage by Bede (672/673--735) (see Wikipedia: Anno Domini: History; Wikipedia: Anno Domini: Popularization; Wikipedia: Bede: Use of Anno Domini).

7. The Julian calendar worked pretty well for awhile, but as the centuries rolled by the completion of each year of the solar-year count became progressively earlier than the completion of the corresponding year of the Julian-year count because of the difference between the mean solar year = 365.2421897 days (J2000) and the Julian year = 365.25 days.

   By the 16th century, the discrepancy amounted to ∼ 11 days from the time of year 1 CE or 10 days from the Council of Nicaea (325 CE) (see Wikipedia: Gregorian calendar: Adoption). This meant, for example, that the vernal equinox was occuring circa Mar10 rather than ∼ Mar21 where it occurred circa year 1 CE. If this disacrepancy had been allowed to continue for millennia, eventually the vernal equinox would happen on Dec25.

   Something had to be done.

8. The aforementioned Gregorian calendar which followed from Gregorian calendar reform (1582) ordered by Gregory XIII (1502--1585, pope 1572--1585)---making use of an idea suggested astronomer Aloysius Lilius (1510?--1576) and supported by astronomer Christopher Clavius (1538--1612) (who is the eponym of a nifty lunar crater: Crater Clavius)---fixed the problem with the Julian calendar by the simple expedient of saying every centurial year of the Gregorian calendar NOT evenly divisible by 400 would be a common year, NOT a leap year as on the Julian calendar, and by jumping from the date of end of the day 1582 Oct04 by 10 days to the start of 1582 Oct15 in order make Mar21 again the approximate date of the vernal equinox as it was at the time of the Council of Nicaea (325 CE).

9. Note in the Gregorian calendar, 1700, 1800, 1900, 2100 are common years, but 2000 is a leap year.

10. The average year of the Gregorian calendar is the Gregorian year = 365.2425 days (exact by definition) which is a very close approximation to, but still slightly longer than, the solar year = 365.2421897 days (J2000).

11. It is estimated that Gregorian calendar will be 1 day longer than the count of solar years in approximately 7700 years (see Wikipedia: Gregorian calendar: Accuracy). The people in year 9700 CE (i.e., in the 10th millennium) can worry about that discrepancy---if they worry about anything at all.
    A simple calculation---that actually is NOT right---can be done of when the Gregorian calendar and count solar years becomes desynchronized by 1 day. Just using of the difference between the solar year = 365.2421897 days (J2000) and the Gregorian year = 365.2425 days (exact by definition), a count n of completed solar years will only arrive ∼ 1 day before the count n of completed Gregorian years when n = 3223. If this happened, the solar "Mar21" would for example arrive on Gregorian calendar Mar20. So starting a count from 1582, there would need to be jump of 1 day (e.g., a jump from Mar20 to Mar22) in the calendar year circa year 4800 to bring the Gregorian calendar back into synchronization with the count of solar years. But this calculation is WRONG and the reasons are discussed by Wikipedia: Gregorian calendar: Accuracy.

12. Note the Julian year = 365.25 days also approximates to good accuracy/precision the sidereal year = 365.256363004 days (J2000), but this is NOT directly relevant to the calendar.

Back to Julius Caesar (100--44 BCE):

1. Julius Caesar actually undertook the calendar reform in his role as Pontifex Maximus (or chief priest of Rome): as such, he was the legitimate official responsible for the calendar. A fictional portrayal of the some of the events of the reform is given in The Triumph of Caesar (2008 book) by Steven Saylor (1956--).

2. After Julius Caesar's death in 44 BCE, a month was name for him: Iulius which is the modern July. This change was proposed by Mark Antony (83--30 BCE) (see Wordinfo: Calendar, Julius). For July, see the figure July: Les Tres Riches Heures du Duc de Berry below (local link / general link: tres_riche_heures_07_july.html).

   ---

3. Also about Julius Caesar (100--44 BCE): He was a significant Classical Latin author and ancient Roman historian (see Wikipedia: Julius Caesar: Literary works). He exhibited mercy to Romans, but NOT to others. For the rest, he was a brutal conqueror---which phrase has a certain irony.