posted on Oct, 20 2011 @ 11:25 PM
Calendars are pretty weird because they try to do all sorts of different things. Here we have 2 very different calendars. One is the calendar we use.
Now that is actually interesting since only some people use the Gregorian calendar. But that is the one I think most people are aware of. Understand
that there are many calendars in use today such as the Hebrew calendar and the Chinese calendar and the Islamic calendar and the Julian calendar and
probably lots of other calendars. We just get used to one calendar and forget about the others.
The Gregorian calendar is a calendar that is supposed to track the movement of the Earth around the Sun. We don't think of it that way, but that is
what it does. The problem is that the way the Earth turns is not easy to relate to the time it takes for the Earth to go around the Sun.
A year is a little longer than 365 days. How do we know that? After a bit we notice that the longest and shortest days of the year are not on
particular days of the calendar. The Sun is highest at noon on the longest day of the year and lowest in the sky at noon on the shortest day of the
year.
So if the shortest day is supposed to be on December 21 and we think a year is 365 days, then we notice that the longest day is on December 22 in 4
years. In 8 years the longest day is on December 23. The goal is to have a calendar that has the longest day always on the same day.
To make that happen leap days are added. The problem is that the fraction of a day is not .25, it is .24. In 100 years we need to add 24 days, not 25
days. So every 100 years there is no leap year. There are further refinements as well.
So this is all about the Gregorian calendar. It is well worked out will be spot on for a long time.
The other calendar in question is the long count calendar used by the Mayans. It is different in that it is a day counter. There is no goal to match
other astronomical issues. So the count is simple. It goes up and up without repetition.
The question with all calendars is how to relate a date in one with a date in another calendar. Calendars in use today are easy to relate. You can
find out when Chinese New Year happens. You can find out when dates in the Hebrew calendar or the Islamic calendar occur. Eid al-fitr and Yom Kippur
are determined by other calendars, but the Gregorian date is known.
The Mayan long count calendar is a bit harder to figure out since it was not in use when Europeans arrived. There is also evidence today that
different Mayans cities did not agree on the count. Despite that efforts have been made to relate the long count calendar to the Gregorian calendar.
A method to match up different calendars is known as a correlation. The correlation most often used to match up the Gregorian calendar and the Long
count is known as the GMT correlation. Notice that this correlation matches up the Gregorian calendar and the long count. Such a correlation addresses
the fact that the Gregorian calendar and the long count are different types of calendars.