From 5cc54d020b144b92d90fabd6cb6febbf0165bb51 Mon Sep 17 00:00:00 2001
From: Quantum <quantum2048@gmail.com>
Date: Mon, 1 Jul 2024 01:48:36 -0400
Subject: [PATCH] gcal: explain the Julian calendar and Gregorian transition

---
 gcal/public/index.html | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/gcal/public/index.html b/gcal/public/index.html
index 27713f6..a1a2b3b 100644
--- a/gcal/public/index.html
+++ b/gcal/public/index.html
@@ -75,6 +75,35 @@
                 of making 1 BCE year 0 and extending this into the past was used.</p>
         </div>
     </div>
+    <div class="card">
+        <div class="card-body">
+            <h4 class="card-title">What is the J (Julian calendar) date?</h4>
+            <p class="lead">The <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian
+                calendar</a> is the predecessor to the Gregorian calendar, originally introduced by Julius Caesar in 46
+                BC.</p>
+            <p>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.</p>
+            <p>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.</p>
+        </div>
+    </div>
+    <div class="card">
+        <div class="card-body">
+            <h4 class="card-title">How are dates before the adoption of the Gregorian calendar handled?</h4>
+            <p class="lead">This version uses the proleptic Gregorian calendar for those dates.</p>
+            <p>Since the Gregorian calendar was adopted at different times in different countries, there's no nice way
+                of handling the transition. In the future, I might make a version that implements the transition with a
+                country selector, but this version will stay proleptic forever.</p>
+        </div>
+    </div>
     <div class="card">
         <div class="card-body">
             <h4 class="card-title">What is the JD (Julian day number) value?</h4>