# 儒略日

## 儒略日起点

28年为一太阳周期（solar cycle），经过一太阳周期，则星期的日序与的日序会重复。

19年为一太阴周期，或称默冬章（Metonic cycle），因235朔望月=19回归年，经过一太阴周期则阴历月年的日序重复。

15年为一小纪（indiction cycle），此为罗马皇帝君士坦丁一世所颁，每15年评定财产价值以供课税，成为古罗马用的一个纪元单位，

## 換算

### 公曆轉儒略日

${\displaystyle {\begin{matrix}a&=&\left\lfloor {\frac {14-month}{12}}\right\rfloor \\\\y&=&year+4800-a\\\\m&=&month+12a-3\\\end{matrix}}}$

${\displaystyle {\begin{matrix}JDN&=&day+\left\lfloor {\frac {153m+2}{5}}\right\rfloor +365y+\left\lfloor {\frac {y}{4}}\right\rfloor -\left\lfloor {\frac {y}{100}}\right\rfloor +\left\lfloor {\frac {y}{400}}\right\rfloor -32045\end{matrix}}}$

${\displaystyle {\begin{matrix}JDN&=&day+\left\lfloor {\frac {153m+2}{5}}\right\rfloor +365y+\left\lfloor {\frac {y}{4}}\right\rfloor -32083\end{matrix}}}$

## 參考資料

1. ^ Dershowitz & Reingold 2008, 15.
2. ^ Both of these dates are years of the Christian or Common Era (which has no year 0 between 1 BC and 1 AD). Astronomical calculations generally include a year 0, so these dates should be adjusted accordingly (i.e. the year 4713 Before Christ becomes astronomical year number −4712, etc.). In this article, dates before October 15, 1582 are in the (possibly proleptic) Julian calendar and dates on or after October 15, 1582 are in the Gregorian calendar, unless otherwise labelled.
3. ^ McCarthy & Guinot 2013, 91–2
4. ^ "Resolution B1" 1997.
5. ^ US Naval Observatory 2005