Special Sub-Topic: The Julian Period Click Here To Play: The Julian Period

The original Roman Calendar consisted of ten months and had a total of 304 days. Which two months from our current calender were NOT included on the original Roman Calendar?

January and February.. It was the second Roman King (Numa Pompilius, 715 to 673BC) who established the months of February and January between the months of December and March - (interestingly) in that order. This increased the number of days in the year to 354 or 355 days. In 450BC February was promoted to its current position on the calender.

In 46BC the Roman Calendar was replaced by the Julian Calendar. Who was it that persuaded Julius Caesar to reform the Roman Calendar in order to produce a much more manageable format?

Sosigenes of Alexandria. Very little is known of Sosigenes. He was known as 'Pliny the Elder' and was noted as an astronomer when consulted by Caesar. There are sources that indicate that the format of the calendar was designed by Aristarchus of Samos, a Greek mathematician and astronomer, some 200 years prior. Ptolemy III was also cited to have proposed similar reform in 238BC but it was never adopted. Aristaeus was a Greek mathematician who was a contemporary of Euclid. Bryson, also Greek, contributed toward the calculation of pi, and Hippocrates attempted to explain the phenomena of comets and the Milky Way.

The Julian Calendar was decreed by Caesar to be made up of three 365 day years and one 366 (leap) year. In 9BC, however, it was discovered that the priests assigned to the computing of the calendars had been adding the leap year incorrectly. How many years apart had they added the leap years?

3 years. To rectify this error it was decided that no further leap years be added until 8AD.

Now we come to the Gregorian Calendar, which is the one we use today. It is based on a cycle of 400 years, made up of 146,097 days. By equally dividing the days by years, each year is an average of how many days?

365.2425 days. The Gregorian Calendar omits leap years in years that are divisible by 100 but NOT divisible by 400. In this way, the year 1900, which is divisible by 100 but not by 400, is therefore, not a leap year. The year 200, on the other hand, is.

Now we get to look at the Julian Period. Please do not confuse this with the Julian Calendar. The first Julian Period commenced at noon on January 1st. In in which year?

4713BC. Did you remember that in the introduction we mentioned that the second Julian Period was due to commence in 3268AD?

The start of a Julian Period is determined by the convergence of three calendrical cycles. The first is called the 'solar cycle' and is represented by the number of years it takes the days of week, as they apply to each particular day of a particular year, to be repeated in the Julian Calender. How many years is this cycle?

28 years. To clarify this: take any given year as an example. The day of the week that applies to January 1st. Follow the pattern of days through to December 31st. It will take 28 years for this exact pattern to repeat itself under the Julian Calendar. Now don't go rushing out and buying yourself 28 consecutive years of calendars and think you're set for life. Remember, we no longer use the Julian Calendar, we use the Gregorian Calendar and its cycle is 400 years long.

The second converging cycle used to determine the Julian Period is called the 'Metonic cycle'. This is represented by the period (in years) it will take the phases of the moon to recur on (almost) the same dates of the year. How many years make up this cycle?

19 years. This cycle is also referred to as the 'Golden' number.

The third converging cycle that is used in the determination of the Julian Period is 15 years in length. It is called the 'Indiction'. What is used to determine this factor?

The Roman tax cycle of Emperor Constantine. The indication was an important numbering system for documents, records and tax forms. It also provided the Romans with a census cycle. Halley's Comet re-appear approximately every 75 years, rather than 15 years. Tables of the Motions of the Moon was a 1919 work by Ernest W. Brown and the Astronomical year numbering process is (literally) the process of numbering years i.e. 1BC is year zero, 2BC is year minus one and 4713BC is year minus 4712.

How long, in years, is a Julian Period?

7,980 years. Solar Cycle (28 years) x Metonic Cycle (19 years) x Indiction Cycle (15 years) = 7,980 years.

A derivative of this is the Julian Date, which essentially assigns a number to every day since the commencement of the first Julian Period. In which of the following fields is the use of this Julian Date most likely to be used?

Astronomy. The concept of the Julian Date and the Julian Period was created in 1583 by a French religious leader and scholar, Joseph Scaliger. There are arguments that the concept was named after his father, (coincidently named) Julius Caesar Scaliger. However, in the translated introduction to Joseph's Book V of the Opus de Emenatione, he indicates that it was named as an accommodation to the Julian Calendar, which, in turn refers to Julius Caesar.