Calendar in the Sky Articles

Bryan Mendez
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Easter and the Maya New Year

By Dr. Bryan Mendez

April 1st, 2018 is the Christian celebration of Easter in the Gregorian Calendar and it is also the start of a new year in the Maya Haab calendar. Next year they won’t occur on the same day. Both events are astronomical in origin. Let’s explore how these two calendars work.


The date of Christmas is December 25th, Groundhog Day is February 2nd, Dia de Muertos is November 2nd, but the date of Easter ranges between March 22nd and April 25th, why is it always different?

The reason is because the date of Easter is determined by more than one calendar cycle. The dates for Christmas, Groundhog Day, and Dia de Muertos only depend on the Gregorian Calendar, which is synchronized with the seasonal cycles of the Sun.

However, Easter is set as the Sunday after the full Moon that occurs on or after the March equinox. Its date is based on three cycles: the 7-day cycle of the week, the 29.53-day cycle of lunar phases, and the 365.2422-day cycle of the solar seasons.

The dates of the quarter and cross-quarter days shown with Earth's location in its orbit around the Sun.

The dates of the quarter and cross-quarter days shown with Earth's location in its orbit around the Sun.

Christian churches realized that there is a problem in using the astronomical occurrences for these events. The equinox and the full Moon are defined as moments when astronomical bodies cross through a set of celestial coordinates. This past equinox occured at 16:15 Universal Time on March 20th. Universal Time (UT) is the 24-hour mean solar time measured from 0-degrees longitude (in Greenwich, England). The equinox was 12:15 PM for the east coast of the United States and 9:15 AM for the west coast on March 20th. But, it was 4:15 AM on March 21st for New Zealand. The following full Moon occured March 31st at 12:37 UT. That’s Saturday at 8:37 AM Eastern Daylight Time and 5:37 AM Pacific Daylight Time. But in New Zealand the time was 12:37 AM on Sunday, April 1st. By the simplistic scheme for Easter this would require Easter Sunday to be April 8th in New Zealand but April 1st in the United States.

In 325 AD, the first council of Nicea determined the first rules for the date of Easter and decided to set the date of the equinox to be March 21st regardless of whether the astronomical event occurs on that day. This guarantees that Easter will occur on the same date for everyone. It was later determined that the full Moon on or after March 21st would be determined by a table that approximates lunar months as 29 or 30 days with the 14th day being the full Moon. This is again regardless of whether the astronomical full Moon falls on this day. The tables were set up to closely match the lunar phases so they are usually within a day of the actual full Moon. The rules governing the Gregorian lunar calendar are very complex. But a vestige of older calendars determined that the lunar month ending with a New Moon in April should always have 29 days (that’ll be important in a moment).

The Phases of the Moon shown next to their location around Earth relative to the Sun

The Phases of the Moon shown next to their locations around Earth relative to the Sun, which is to the right of the image.

When these rules are all considered together we can calculate that the earliest possible date for Easter would be when the full Moon occurs on a March 21st that is also a Saturday. Then, the next day, March 22nd, will be Easter Sunday. The last time that happened was 1818 AD and the next time won’t occur again until 2285 AD. For the latest possible date of Easter, a full Moon would occur on a Saturday dated March 20th. Then, a full lunar cycle (29 days) would have to go by until the following full Moon on Sunday, April 18th, making the latest Easter Sunday be on April 25th. The last time this happened was 1943 AD and the next time it happens again will be in 2038 AD.

The Gregorian Calendar, now used by much of the world for civic purposes, is a solar calendar named after Pope Gregory XIII, who enacted it in October 1582 AD. It was a reform of the much older Julian Calendar, named for Julius Caesar. When the Julian Calendar was enacted in 45 BC the date of the spring equinox was March 25th. But the Julian Calendar slips relative to the true solar year by 1 full day every 128 years. By the time of the first council of Nicea in 325 AD, the astronomical equinox was occuring on March 21st. By Pope Gregory’s time the date of the occurrence of the equinox had slipped to March 11th. This big difference from the church decreed date of March 21st, used to determine the date of Easter, was intolerable and the Gregorian Calendar was devised to fix the problem.  

The last day of the Julian calendar was Thursday, October 4th, 1582 and this was followed by the first day of the Gregorian calendar, Friday, October 15th, 1582 (the cycle of weekdays was not affected). The Gregorian reform was to skip 3 leap days within every 400-year period. This calendar only slips by one day relative to the true solar year every 3,223 years. Easter therefore keeps its astronomical connections much more closely since the Gregorian reform.  

The 7-day week, used to determine the date of Easter, also has astronomical origins. The 7-day week has been used in Christian Europe in an unbroken way for nearly 2,000 years, given to the tracking of Easter Sunday as far back to at least 311 AD. It probably began in ancient Babylonian or Jewish traditions. The 7 days are related to the creation story of Genesis: Earth is created in 6 days and the creator rests on the 7th. Also, there were 7 celestial objects observed by the ancients that wandered against the backdrop of the fixed stars (the original meaning of ‘planets’): Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. The Latin names for the weekdays illustrates those associations. During Roman occupation of northern Europe, germanic/Norse gods were associated with the Roman gods for whom each weekday was named.

Table of the days of the week and their associated planets and Norse gods

The days of the week and their associated planets and Norse gods.

While many celebrations and traditional festivals have close ties to astronomical events, Easter takes the cake by using 3 astronomically connected cycles all together.


The Maya Long Count Calendar had several different cycles within it. The first part was a linear count of days since the legendary creation date. Each day of the Long Count was also recorded with corresponding days of the 365-day Haab and 260-day Tzolk’in calendars. While the Long Count was an invention of the Maya, the Haab and the Tzolk’in predated them and were used by the Olmec civilization in about 500 BC. The 365 and 260 day calendars were eventually used throughout mesoamerica and are still in use by several communities. The Haab and the 260 Tzolk’in cycle together on a 52 Haab/73 Tzolk’in period called a Calendar Round.

0 Pop written in Maya glyphs

The first day of the Haab.

The Long Count had not been used by Maya people for centuries by the time of the Spanish conquest, but the Calendar Round was used in some places. So to correlate the Long Count with European calendars modern scholars have used Julian Calendar dates recorded by the Spanish and Calendar Round dates that the Aztec and Maya people were using during the conquest. It’s not so easy since many communities were not using the same Calendar Round dates. The correlation accepted by most scholars right now gives the Long Count creation date ( 4 Ajaw 8 Kumku') as August 11th, 3114 BC on the Gregorian Calendar. That makes April 1st, 2018 AD the start of a new Haab (0 Pop).  

The 365-day count is known as the Haab in Yucatec Mayan and Jaab in Quiche Mayan. It contains 18 named “months,” called a Winal (or Uinal), each with 20 numbered days, plus one final month of 5 days named the Wayeb. The Wayeb is a month of abstinence and reflection, not unlike Lent for Christians. The Maya say that during the Wayeb people are most susceptible to evil and should stay home and avoid many foods and activities.

Maya numbers from zero (seating) to 19

Maya numbers from zero (seating) to 19.

The glyphs for the Winals of the Haab

The glyphs for the Winals of the Haab.

The Haab is nominally a solar calendar, but the cycle of solar seasons is 365.2422 days. Therefore, the Haab slips 1 day every 4 years relative to the seasons. For the most part, the ancient Maya prefered to express numbers as integers and ratios of integers. They also preferred regular rhythms of time and chose not to keep their calendars synchronized with the fractional-day rhythms of nature. European, North African, and Asian cultures frequently used leap days or leap months to adjust their calendars to the astronomical cycles. The Maya prefered to keep the same number of days between dates rather than insisting that solar events always occur on that same date.

It appears that the Maya simply tracked the slip. If the spring equinox occurs on 13 Kumk'u this year, then in 4 years it will occur on 14 Kumk'u. There are a few stone inscriptions at several archaeological sites and a passage in one of the surviving Maya books that indicate that the ancient Maya had accurate knowledge of the length of the solar year, possibly to better precision than their European counterparts. They expressed this knowledge by using the numbers of days between dates. For example, some inscriptions reference a period of 6,940 days between two dates. This is a period equal to 235 cycles of the Moon’s phases and 19 solar years. The precise value is 6,939.6 days, but the Maya never expressed numbers as a fraction. Another example gives an interval of just over 754 years. That is half the time it takes for the Haab to cycle around and come back into alignment with the solar year: 1,508 Haab = 1,507 solar years. Several other inscriptions appear to reference periods that are one-fourth, one-half, and three-fourths of this relationship. If true--and there is some debate on the interpretation of these inscriptions--it means that the ancient Maya knew the value of the solar year as 365.2422 days (a difference of only 1 second from the modern observed value). Such precision is accomplished with many observations over a long time to achieve a long-term average.

As we celebrate special days in our calendars, remember that we do so on these days because of the motions of the Sun, Moon, and planets carefully watched by our ancestors long ago. So, take a moment, look to the sky and know that you can still watch those motions and feel a connection to those who came before us.

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