Julian Calendar. Also known sometimes as the Roman Imperial Calendar, the Julian Calendar takes its name from the Roman politician Julius Caesar under whose auspices it was devised and implemented. It was a synthesis of the Roman Republican Calendar and the Egyptian Calendar and it first went "live" on 1st January 45 BCE. Its principal designer was the astronomer Sosigenes of Alexandria (Pliny the Elder, Natural History 18.82), and although the basis of its operation had been known for at least two centuries, its introduction into the unwieldy Roman Calendar was novel. Inasmuch as it was a solar (rather than lunisolar) calendar, it represented a radical break with previous western attempts to regulate time, much more so than its revision into the Gregorian Calendar in 1582 CE.
By the middle of the C1st BCE the deficiencies of the existing Roman calendar had long been evident. Despite occasional intercalary months and days declared by priests, the republican calendar had become around 70 days behind the seasons. Caesar, who appears to have had a reasonable grasp of astronomical matters, realised that the solar calendar of the Egyptians offered a way out of the synchronisation problems that were intrinsic to calendars that relied on the Moon. The Roman year was 355 days long, whereas the Egyptian year was 365 days long, much closer to the true figure but still not right. However, Caesar would have known, via Sosigenes, of a failed attempt to introduce an extra day into the Egyptian Calendar in 238 BCE. We don't know how the idea evolved in detail but the upshot was that the traditional Roman months were reformed to allow 365 days for three consecutive years with every fourth year having 366 days, according to the scheme:
| Roman Month | Days before 45 BCE | Days after 45 BCE | English Month |
|---|---|---|---|
| Januarius | 29 | 31 | January |
| Februarius | 23 / 24 / 28 | 28 /29 | February |
| Martius | 31 | 31 | March |
| Aprilis | 29 | 30 | April |
| Marius | 31 | 31 | May |
| Junius | 29 | 30 | June |
| Quintilis | 31 | 31 | July |
| Sextilis | 29 | 31 | August |
| September | 29 | 30 | September |
| October | 31 | 31 | October |
| November | 29 | 30 | November |
| December | 29 | 31 | December |
| Totals | 350 / 351 / 355 | 365 / 366 |
The extra day was added to February to give 29 days for that month every fourth year, the so called leap year, and the whole calendar was reset to sychronise it with the seasons. Unfortunately, the new scheme did not get off to a good start. Julius Caesar died in 44 BCE and Rome descended into civil war. With Sosigenes presumably in Alexandria and not able to oversee the implementation of the new calendar, it seems the Roman priests misinterpreted the rule to make every fourth year a leap year and made every third year a leap year instead. Probably this was because they counted inclusively, starting with 1, as was a common in the ancient world. The result was that the calendar soon went wrong, though not by much as compared to previously. The error was not spotted for nearly five decades, and it was 8 CE before the leap year was implemented properly.
Unless specified otherwise, dates before 15th October 1582 CE (the start of the Gregorian Calendar) are normally given in the Julian Calendar (which goes up to 4th October 1582 CE). Dates before the stabilisation of the Julian Calendar in 8 CE are given in the proleptic Julian Calendar, that is, as if the Julian Calendar had always been in operation prior to that date. Note that dates between 45 BCE and 8 CE are difficult to determine precisely because of the flawed operation of the calendar between those dates.
Julian Day. Also known as the Julian Day Number (JDN) or somewhat ambiguously as the Julian Date (JD), this is the continuous count of days, each of 86400 seconds, from the start of the Julian Period, defined as 12:00 Universal Time on 1st January 4713 BCE (–4712) in the proleptic Julian Calendar. It is used in astronomy and commerce for calculating the number of elapsed days between two dates or times. Time of day is expressed as a decimal part of a day. Thus, 12:00 on 1st January 2000 has JDN 2451545.000000 and 13:00 on 2nd January 2000 has JDN 2451546.041667.
Julian Days were devised for astronomical use by Ludwig Ideler in 1825 and further developed by John Herschel in 1849. The adoption of midday as the reference point was already common in astronomy, because of the solar meridian transit at noon, and ultimately goes back to Ptolemy (c. 100 – c. 170 CE). Initially, Ptolemy's meridian of Alexandria was chosen, but the reference point was realigned with the Greenwich meridian when the latter was established internationally in 1884.
There are several variants of the Julian Date, the most common of which is the Modified Julian Date (MJD), which was introduced by the Smithsonian Astrophysical Observatory in 1957 in order to simplify the recording of the orbit of Sputnik using an IBM 704 computer. The start date is midnight on 17th November 1858 CE, obtained by removing the leading two digits, 24, from the JDN, and setting the day back by 12 hours to align it with the Gregorian calendar. It can be easily calculated from the formula MJD = JDN - 2400000.5. It is commonly used in astronomy and food production.
Julian Period. This is a chronological period of 7980 years formed by multiplying together the Indiction Cycle (15 years), the Solar Cycle (28 years) and the Metonic Cycle (19 years). The first Julian period started on at 12:00 UT on 1st January 4713 BCE and the next will start at 12:00 UT on 1st January 3268 CE.
The Julian Period was introduced by Joseph Scaliger in 1583 (Iulianam vocauimus: quia ad annum Iulianum duntaxat accomodata est 'we call it Julian simply because it fits in with the Julian Year') shortly after the introduction of the Gregorian Calendar in order to provide a means of continuously recording events which was arithmetically unencumbered by the recent calendar change or the BC/AD (BCE/CE) transition. By continuing all three cycles backwards until they were all zero together, the year 4713 BCE is reached, which had (and still has) the advantage of being well before any known historical date.
Julian Year.
