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mcal: explain what the Long Count is

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the <em>Wayebʼ</em>;
</li>
<li>The <a href="https://en.wikipedia.org/wiki/Mesoamerican_Long_Count_calendar"><strong>Long
Count</strong></a>, a modified base-20 system used to represent the number of days since the
mythical date of creation, which is August 11, 3114 BCE in the proleptic Gregorian calendar or
September 6, 3114 BCE in the proleptic Julian calendar. It is typically represented as five numbers
separated by dots, with each number counting from 0 up to 19, except for the second last number,
which counts from 0 to 17.
Count</strong></a>, represents the absolute number of days since the Mayan date of creation.
</li>
</ul>
<p>Since the <em>Haabʼ</em> is the closest equivalent to months in the Mayan system, this calendar
@ -123,64 +119,19 @@
</div>
<div class="card">
<div class="card-body">
<h4 class="card-title">What's so special about this version?</h4>
<p class="lead">Most versions of the calendar floating around doesn't use the original definition above.</p>
<p>Most versions uses the so-called <em>Romme</em> method for leap years, using the same leap year rules as
the Gregorian calendar, i.e. every year divisible by four, except century years not divisible by 400.
This method might make sense, except years 3, 7, and 11 were leap years under the original rules and
were observed as such in real life, but the <em>Romme</em> method instead makes years 4, 8, 12 leap
years instead.</p>
<p>This version uses the original rules. The <a href="https://ssd.jpl.nasa.gov/planets/eph_export.html">JPL's
DE440 and DE441 ephemerides</a> were used to calculate the exact timings of the autumnal equinoxes
between the Gregorian years 13201 BCE and 17191 CE (corresponding to the French Republican years -14991
to 15399). The times were then converted to UT1+00:09:21, the exact local time at the Paris Observatory.
UT1 was chosen to keep track of the Earth's rotation without having to worry about the issues posed by
leap seconds in UTC. Note that due to the uncertainty over
<a href="https://en.wikipedia.org/wiki/%CE%94T_(timekeeping)">ΔT</a> — the difference between UT1 and
Terrestrial Time (TT) used in the ephemerides — it is theoretically possible for there to be
inaccuracies when the equinox occurs very close to midnight.</p>
<p>For more details about how I calculated this calendar, please see
<a href="https://quantum5.ca/2022/03/09/art-of-time-keeping-part-4-french-republican-calendar/">my blog
post on the topic</a>. This is the fourth part of a series on time-keeping, and you are highly
encouraged to read the
<a href="https://quantum5.ca/2022/02/16/art-of-time-keeping-part-1-years-dates/">first</a>
<a href="https://quantum5.ca/2022/02/23/art-of-time-keeping-part-2-time/">three</a>
<a href="https://quantum5.ca/2022/03/02/art-of-time-keeping-part-3-astronomy-equinoxes/">parts</a>
for a more complete understanding.</p>
</div>
</div>
<div class="card">
<div class="card-body">
<h4 class="card-title">What are those names above the Gregorian date?</h4>
<p>Those are the names of the days in the
<a href="https://en.wikipedia.org/wiki/French_Republican_calendar#Rural_calendar">rural version of the
calendar</a>. This was intended to replace the Catholic Church's calendar of saints, as the French
Revolution wanted to reduce the influence of the church. Every day of the year has a unique name
associated with the rural economy and these names are supposed to correspond with the season.</p>
<p>Every <em>quintidi</em> is named after an animal, every <em>décadi</em> is named after an agricultural
tool, and the remaining days are named after various plants or produce. The only exception is the winter
month of Nivôse, which has the remaining days named after minerals.</p>
</div>
</div>
<div class="card">
<div class="card-body">
<h4 class="card-title">What are those numbers below the Gregorian date?</h4>
<p>The five (or more) numbers separated by dots is the corresponding
<a href="https://en.wikipedia.org/wiki/Mesoamerican_Long_Count_calendar">Mesoamerican Long Count
calendar</a> date. This is commonly known as the &ldquo;Mayan calendar.&rdquo; This calendar is not
available for dates before August 11, 3114 BCE (25 Thermidor -4905).</p>
</div>
</div>
<div class="card">
<div class="card-body">
<h4 class="card-title">What is decimal time?</h4>
<p class="lead">Decimal time is a time system used during the French Revolution that divided the day into 10
hours, each with 100 minutes, which contained 100 seconds each.</p>
<p>The result is 100,000 seconds in one day, compared to the 86,400 seconds with the normal 24-hour
system. This makes it very easy to denote time as a decimal fraction of a day. For example, decimal time
5:67:72 (around 13:37:31) on January 1, 2000 can be represented as <code>2000-01-01.56772</code>.</p>
<p>Also note that each decimal hour is 2.4 normal hours, each decimal minute is 1.44 normal minutes, and
each decimal second is 0.864 normal seconds.</p>
<h4 class="card-title">How does the Long Count work?</h4>
<p class="lead">The Long Count calendar is a count of days since the Mayan date of creation, which is August
11, 3114 BCE in the proleptic Gregorian calendar or September 6, 3114 BCE in the proleptic Julian
calendar.</p>
<p>It uses a modified base-20 system to represent this number, with each &ldquo;digit&rdquo; counting from 0
up to 19, except for the second last &ldquo;digit,&rdquo; which counts from 0 to 17. Typically, five
numbers are shown, separated by dots, but in theory, more numbers could be used for dates in the distant
future.</p>
<p>For example, the date 13.0.0.0.0 represents exactly 13&times;20&times;20&times;18&times;20 =
1&thinsp;872&thinsp;000 days since the date of creation, which is December 21, 2012 CE.</p>
<p>Each &ldquo;digit,&rdquo; of the Long Count have a name, from the rightmost: kʼin, winal, tun, kʼatun,
bʼakʼtun, piktun, kalabtun, kʼinchiltun, alautun, hablatun. Terms from bʼakʼtun and beyond were invented
by modern scholars and were not used by the classical Maya.</p>
</div>
</div>
</div>