Definition:Calendar/Julian
Definition
The Julian calendar is the calendar which was introduced by Julius Caesar in $45 \, \text{BCE}$.
It divides the year into:
and:
The years themselves are given an index number, and are known by that number.
A standard numbering system was introduced by Dionysus Exiguus.
He identified a particular year as being $525 \, \text{A.D.}$, where $\text{A.D.}$ is an abbreviation for Anno Domini, Latin for in the year of the Lord.
The year $1$ was conventionally supposed to identify the year of the birth of Jesus of Nazareth, although the accuracy (and indeed, the actuality) of this has since been questioned.
Years before $1 \, \text{A.D.}$ are counted backwards and assigned the label $\text{B.C.}$.
However, note that the year immediately prior to $1 \, \text{A.D.}$ is $1 \, \text{B.C.}$, not the intuitive year $0$, a discrepancy that can cause confusion.
Using this system of numbering, a leap year is identified by this number being divisible by $4$.
The $365$ or $366$ days in the year are divided into $12$ approximately equal sections called months, which are assigned both names and index numbers:
| \(\ds 1:\) | \(\) | \(\ds \) | January | \(\quad\) $31$ days | ||||||||||
| \(\ds 2:\) | \(\) | \(\ds \) | February | \(\quad\) $28$ days, or $29$ days in a leap year | ||||||||||
| \(\ds 3:\) | \(\) | \(\ds \) | March | \(\quad\) $31$ days | ||||||||||
| \(\ds 4:\) | \(\) | \(\ds \) | April | \(\quad\) $30$ days | ||||||||||
| \(\ds 5:\) | \(\) | \(\ds \) | May | \(\quad\) $31$ days | ||||||||||
| \(\ds 6:\) | \(\) | \(\ds \) | June | \(\quad\) $30$ days | ||||||||||
| \(\ds 7:\) | \(\) | \(\ds \) | July | \(\quad\) $31$ days | ||||||||||
| \(\ds 8:\) | \(\) | \(\ds \) | August | \(\quad\) $31$ days | ||||||||||
| \(\ds 9:\) | \(\) | \(\ds \) | September | \(\quad\) $30$ days | ||||||||||
| \(\ds 10:\) | \(\) | \(\ds \) | October | \(\quad\) $31$ days | ||||||||||
| \(\ds 11:\) | \(\) | \(\ds \) | November | \(\quad\) $30$ days | ||||||||||
| \(\ds 12:\) | \(\) | \(\ds \) | December | \(\quad\) $31$ days |
Thus, for example, the day following the $31$st of January is the $1$st of February, and the $30$th of June is followed by the $1$st of July.
Julian Year
A Julian year is the length of a year as defined using the Julian calendar.
- $1$ Julian year $= \begin {cases} 366 \, \text {days} & : 4 \divides y \\ 365 \, \text {days} & : 4 \nmid y \end {cases}$
where:
- $y$ denotes the number of the year
- $4 \divides y$ denotes that $y$ is divisible by $4$
- $4 \nmid y$ denotes that $y$ is not divisible by $4$.
Also see
Source of Name
The Julian calendar is named after Julius Caesar, under whose reign it was introduced.
Sources
- 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $365 \cdotp 2422$
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $365 \cdotp 24219 \, 878$