Add README for the Julian calendar

README.md

@@ -5,3 +5,4 @@ calendars from major cultures.
 
 Currently, the following calendars are supported:
 * [French Republican Calendar](frcal/README.md)
+* [Julian Calendar](jcal/README.md)

jcal/README.md

@@ -0,0 +1,32 @@
+# Julian Calendar
+
+![demo](demo.png)
+
+## What is this?
+
+The [Julian calendar][0] is a calendar introduced by Julius Caesar in 46 BC. A
+reformed version, the [Gregorian calendar][1], forms the basis of civil calendar
+in most countries.
+
+Julius Caesar (with aid of Sosigenes of Alexandria) introduced this calendar to
+replace the earlier Roman calendar, which required manual addition of leap
+months to keep it in synchrony with the seasons. Unfortunately, it had leap
+days every four years without question, resulting in the average year having
+365.25 days. However, the actual tropical year is roughly 365.2422 days,
+resulting in the Julian calendar gaining a day every 129 years. This means
+that seasons drift, starting earlier and earlier in the year.
+
+In 1582, Pope Gregory XIII decided to fix this drift by reducing the number
+of leap days, motivated by a desire to keep the March equinox on March 21st,
+since that value was hardcoded in the calculation for the date of Easter. To
+achieve this, he made years divisible by 100 but not by 400 non-leap years,
+resulting in 97 leap years every 400 years, resulting in the Gregorian calendar.
+To bring the equinox back in alignment, October 5th to October 14th in 1582
+were deleted. The result is the Gregorian calendar.
+
+This website extends the Julian calendar indefinitely into the future for
+reference reasons. Note that in the 20th and 21st centuries, the Julian
+calendar is 13 days behind the Gregorian calendar.
+
+[0]: https://en.wikipedia.org/wiki/Julian_calendar
+[1]: https://en.wikipedia.org/wiki/Gregorian_calendar