Time. Since the earliest days of mankind, humans have been obsessed with time. The time of the seasons, the times of night and day, time to plant, time to sow, the time in the heavens, and the time on earth. Cyclical time, linear time, eternal time, and time travel. All told, Time is a complex issue. Once the realm of priests and shamans, politicians and merchants, time is no less important today, and our quest to study, measure, and understand time still continues.
Calendars in one form or another, are literally, as old as time, or rather as old as the first human who realized the passage of time. Measuring this elusive concept, particularly over years or seasons, has been important to all facets of human activity. Historically, calendars have been used for a variety of purposes:
agricultural, organization of activities, control populous, religious, understanding natural cycles, and measurement.
Even today in our own society, we continue to use a number of different calendars simultaneously. We have a yearly Gregorian calendar, religious, fiscal, astrological, merchandising, moon phase, Julian dates used for programming, and so on. Some important dates are fixed, others are determined through calculations, some are dependent on astronomical events, and some dates are even moved for convenience, such as celebrating certain national holidays in the US on Mondays rather then midweek.
One of the most popular calendars in the US is the Farmers Almanac, which is a combination of astronomical events, agricultural planting cycles according to the moon, and religious holidays from both the Christian Church and the Old Religion of Europe. Although this is America's oldest published periodical, the roots of such almanac's date back to ancient times and the calendars of yore.
A calendar is a system of organizing units of time, and the word itself stems from the Latin word for calendar, which is kalend. The smallest unit is a day which is based on the revolution of the earth around the sun. In some cultures, the day starts at the sunset prior, while in others, the day begins immediately after midnight, or at sunrise. Next comes the week, which is a period of 7 days in our own calendar. These were originally based on the planets, and each attributed to a different deity. Weeks are grouped into months, which reveals the older lunar origins of our calendar, as month literally means "moonth", or a lunar cycle. Months are then grouped into a year, while some calendars such as fiscal and business calendars go a step further and divide a year into four quarters. In some cases, cycles of years are also noted, such as decade for 10 years, a century for 100 years and so on. The start of a new year may also differ according to the culture that devised it, for instance it may fall upon the spring equinox, winter solstice, or based be on the lunar phases.
In theory, calendars are based on the cycles of astronomical events in order to determine the accuracy of the length of the year, though the length itself is not steady, and has increased slightly over the ages. This is a result of the slowing of the earth's rotation, variations in orbit, and the effects of precession. [16] But to understand the basis of calendar making, we must first understand the key element of a calendar - that is, the relationship of the earth, moon, and sun.
The moon travels across our sky 13° a day, and orbits the earth 13 times in a year. Using the stars as a backdrop, the moon has a sidereal month of 27.322 days. The moon also experiences phases, which is known as the lunation, or the synodic month. This is the interval between conjunctions of the sun and moon, and corresponds to a lunar cycle of 29.53059 days to orbit the earth. The earth completes approximately 1/13th of its journey around the sun in a sidereal month, leaving the moon's phases lagging slightly behind in the cycle by 2.21 days. [13] This equates to 12.368 full months in a solar year, which is approximately 11 days short of the solar year. The fractional nature of the lunar cycles makes the moon difficult to base a calendar on, and cultures using purely lunar calendars often resorted to using months of either 29 or 30 days to try and match the calendar months to lunar "moonths", otherwise the result is that the lunar calendar will slowly slip put of sync with the solar year and seasons by 11 days each year.
The earth's orbital period, or tropical year around the sun is 365.242199 days, though it is commonly expressed as 365 or 365.25 days long by most people. This is the interval between vernal equinoxes, once considered the start of the new year. This corresponds to the cycle of the seasons, however the annual cycle may vary each year. Due to these variations, it is very difficult to keep a calendar in sync with the tropical year.
Used together, the lunar and solar years do not easily fall into sync, which makes calendars based on them rather complicated. Every 19 years however, the sun and moon return to the same position in the sky on the same days. This was first recorded by a Greek named Meton, a 4th century bc astronomer. The cycle, named the Metonic cycle after him, was used by some cultures to maintain their calendar systems over long periods of time, and to simultaneously track both the solar and lunar cycles.
In ancient times, calendar development followed a typical pattern, which developed as the society who conceived it perfected their own sciences. Some of the earliest calendars were based on the lunar cycles, as the moon's phases are easy to see. An example of this can be seen in cave art. Considered one of the earliest known calendars, paintings dating from 15,000 years ago where found at the Lascaux caves in France. These beautiful images depict counts and cycles of the moon. Another well known example is the carving known as the Venus of Laussel, dating to 12,000 years ago and also found in France. This relief depicts a goddess holding up a horn with 13 markings. The horn has long been a symbol of the moon in many cultures, and it is believed that this relief depicts lunar measurements. Some modern cultures continue to use a lunar calendar today, such as the Jews and Muslims.
Recognizing the problems of the lunar calendar's variable months, many cultures then turned to the sun. At this point, calendars turned to solar cycles, or a combination of solar and lunar known as lunisolar. Historically, we can find many examples of such attempts to track the lunisolar cycle in the construction of megalith alignments and stone circles such as those found in Europe. [15] One of the most famous of these is Stonehenge in Britain. Lunisolar calendars were also found to have inherit problems, as the months were based on lunar phases, and as a result, every few years intercalary days or months would be required in order to bring the calendar back into sync with the tropical year.
Following this, some cultures turned to stellar calendars. Constellations are also easy to see and follow in the night sky, and they have annual cycles that appeared to match up to the seasons. These types of calendars often focused on planets such as Venus, important stars like Sirius, and recognizable asterisms such as the Pleiades. The problem here of course, is that stellar patterns change throughout the world, and recognizable alignments could not be maintained over vast distances.
Finally, after millennia of calendar attempts, many cultures ended up with a complex system comprising several varieties of calendars, often using them simultaneously, and methodically attempting to keep them in harmony. But keeping the calendar in sync with astronomical cycles has always been a challenge to timekeepers.
There are approximately 40 calendars in use throughout the world today, [16] the majority of which had their beginnings in religious or ceremonial practices, and were since adopted by governing bodies. This is a review of some of the most notable calendars, many of which are still in use, or have contributed in some way to modern calendar keeping.
Mayan - The Mayans were obsessed with time, and developed the most accurate calendar system to date. Originally using a lunar calendar, they eventually developed a complex system that used three calendars simultaneously. Among their many achievements were accurately tracking the cycles of the moon, sun, Venus, and Pleiades. The tzolkin was a 260 day calendar based on a lunar cycle and associated to the 9 month period of human gestation. It appears to have been the oldest of the three systems, and was generally used for purposes akin to modern day astrology. The second calendar was the haab, a 365.2422 day calendar based on the solar year. This comprised a year of 18 months of 20 days each with an additional 5 intercalary days added at the end as needed. The haab was used for civil and agricultural purposes. [7] Together, the tzolkin-haab were used in a 52 year cycle known as the Calendar-Round, which also included timings to the Pleiades. [35] Lastly, the Long Count calendar was used to track long periods of time as the name suggests. This consisted of 13 baktun cycles, where each tun consisted of 360 days, and a baktun equaling 144,000. The Toltecs and Aztecs would later adopt these calendars, and some of the ancient calendar practices and religious traditions associated with them are still used by their modern day descendents. [38]
To the Mayans, the calendar was both sacred and political, and many Mayan rulers are depicted with symbols of astronomical time keeping. In addition, many monuments such as the Pyramid of Kulkulcan reflect the significance of the calendar. In this example, the pyramid itself represented the calendrical year. There are four sides to the structure representing the four quarters, each composed of 91 steps, and having a platform at the top, which adds up to 365, the approximate number of days in the year. In addition, the building was constructed so that each year on the equinox, a shadow play was incorporated into the visual imagery. A serpent representing the Pleiades, one of the primary units of tracking astronomical time in the Mayan world would form along the steps marking the date. [35] To this day, many tourists and local residents still flock to the area to witness the event.
Middle East - Five thousand years ago, the ancient Sumerians had developed a sophisticated lunar calendar that divided the year into 12 months of 30 days. Many of their methods later found their way into the civilizations that followed. However due to the number of different cultures in the area during this period, the calendar often become quite confusing. Many months had different names, and fiscal, civil, and agricultural calendars were in use simultaneously.
This area also gave rise to one of the most unique calendar systems ever developed. In Egypt, the lifecycle of the community thrived around the annual flooding of the Nile. Early attempts to base a calendar on the river's annual cycle were unsuccessful, so a change was made to incorporate a lunar calendar in order to time the flooding and agricultural cycle. This too was found to be problematic, so a solar calendar was developed. Yet the problems appeared resolved when the priesthood responsible for timekeeping discovered the use of Sirius. They found that the star Sirius ("Sothis" or "Isis") in the constellation of the dog Canis Major had a cycle that closely matched the true solar year, being only 12 minutes shorter. Much of the priesthood at the sacred city of Heliopolis was dedicated to observing Sirius and developing an extensive calendar system based on it. [18] They concluded that the Sirius calendar would remain in sync with the seasons, while the civil calendar was a fourth shorter then the solar year. This would continue throughout the duration of the Sirius cycle, yet the two would come into sync again every 1420 years. Today, we still refer to the Egyptian calendar when the hot days of late summer roll around, and we mention the "dog days of summer" are upon us. The dog of course, refers to the annual rising of Sirius in Canis Major, the Great Dog!
The Hebrew calendar is also of middle eastern origin, and has it's beginnings in the year 359. This remains the official calendar of Israel, and is used by Jews around the world for religious purposes. The calendar is based on the lunar cycle, and a year is comprised of 12 months that alternate between 29 and 30 days each in order to start each month and year with a new moon. Leap years have 13 months, and can have a variable number of days such as 383, 384, or 385. [33] These are inserted over a 19 year Metonic cycle of 235 lunations. An unusual aspect of this calendar is also that it has 4 different New Year days. Rosh Hashanah is the most widely celebrated, and represents the creation of the world. Tu B'shevat is the new year for fruit trees, 15 Nisan the new year for kings, and 1 Elul the new year for animals. Hebrew years are counted starting with the Era Mundi, or Era of Creation, which equates to -3760 bce in the Gregorian calendar. This makes the year 2001 the equivalent of AM 5762 in the Hebrew system. Due to the lunar nature of the calendar, days start in the evening rather then at the dawn, although today the calendar is set according to astronomical calculations rather then observational sighting of the moon, as is the case with the Muslim calendar.
The Islamic calendar is also a lunar calendar, and is still in use by Muslims around the world. This calendar dates to the year 622, beginning with the journey of the prophet Mohammed from Mecca to Medina. [35] The calendar uses a 12 month year, with each month beginning on a new moon, and alternating between 29 and 30 days each. Once again, the problematic nature of the fractional lunar cycle had raised the need for adjustments, so the 12th month is given a variable length. Even with this adjustment however, the calendar drifts, as the lunar and solar cycles move out of sync. The result is that months in the Islamic calendar do not align to the seasons. In an effort to adjust this, an 11 year leap year cycle is used, by which a year can be either 354 or 355 days in length. [35] Unlike the Hebrew calendar which is calculated in modern times, the Muslim calendar remains largely observational. This means that months begin at the first actual visual sighting of the lunar crescent. [34] Historically, this has been problematic for Muslims, as visibility may be poor, and the sighting of the moon varies by location.
Europe - Many of the early calendars in Europe were based on the annual agricultural or hunting cycles of the native people. The development of the calendar followed the usual pattern, originating with lunar calendars and then shifting to solar during the Bronze Age. [13] Many standing stones and astronomical markers in Europe are based on alignments to the solstices, equinoxes, quarters, or cross-quarter days, and in some cases the annual cycles of particular stars or constellations. Extensive studies have demonstrated that such alignments do exist, and played an important role in the religious practices as well. An often overlooked example of an old stellar calendar of Europe still exists in modern timekeeping. In ancient times, the cycle of the Pleiades was used to mark important calendar dates, such as the beginning and end of summer. Referred to as the cuckoo bird in Germany, this timekeeper eventually found its way into the famous cuckoo clocks we are all familiar with.
The modern names for the days of the week also have their origin in old pagan times, and revel the names of Teutonic and Roman deities:
| Sunday | Sun day |
| Monday | Moon day |
| Tuesday | Tir's day |
| Wednesday | Woden's day |
| Thursday | Thor's day |
| Friday | Frigga's day |
| Saturday | Saturn day |
Today, modern followers of the Old Religion continue to follow the ancient astronomical calendar which includes the 8 holidays found most often in ancient European alignments. Although four of the dates have since become fixed, the equinoxes and solstices remain based on astronomical events. This is known as the Wheel of the Year, and the names comprise a mixture of Celtic and Teutonic designations.
2/2 Imbolc - "in the belly"
Vernal Equinox Ostara - named after the goddess Easter
5/1* Beltaine - "Bel fire"
Summer Solstice Litha - "light"
8/1 Lammas - "loaf"
Autumnal Equinox Mabon - named for the harvest lord
10/31* Samhain - "summers end"
Winter Solstice Yule - "wheel"
*Used now mainly for religious purposes, the calendar has begun to lose it's astronomical foundations, as two of the holidays, once timed by the Pleiades, now fall on fixed dates.
Greek - The calendar systems in ancient Greece are some of the most confusing for researchers. This is due to the fact that the Greek city states tended to keep individual calendar systems of their own, and many of these are not well documented. [5] Generally speaking, these calendars were lunisolar, comprising a year of 12 months with an intercalary month as needed. The most well known of the Greek systems is the Athenian calendar, and it is among the records of Athens that the majority of our knowledge is gleaned. It is known that at least two calendars were in use in Athens, a lunisolar version and a second civil calendar. Evidence also points to an annual tracking of the Pleiades [28] as the equivalent of farmers almanacs have been recovered which describe the yearly agricultural cycle in terms of a stellar calendar. It was also in Athens that the astronomer Meton recorded that every 19 tropical years the moon phases realigned with the solar cycle.
Roman - We have inherited much of our modern day calendar keeping from the Romans, who based their methods on a traditional local calendar created by the legendary founder of Rome named Romulus, and later by incorporating Greek and Egyptian techniques. Their early calendar was lunar, comprising 10 months of 304 days, falling far short of the solar year. Many of these early month names are still with us.
Martius named after the god Mars Aprilus named after the goddess Aphrodite Maius named after the goddess Maia Junius named after the goddess Juno Quintilis fifth month, later renamed to Julius Sextilis sixth month, later renamed to Augustus September seventh month October eighth month November ninth month December tenth month
The Emperor Pompilius added the months of January named for the god Janus, and Februaris named after a ritual goatskin called the februa, used in the annual celebration of the Lupercalia. Having failed however at bringing the calendar into sync with the solar cycle, he then inserted a smaller intercalary month called Mercedinus after the god Mercury following February. At the time, February was the last and shortest month in the year, so this arrangement was considered the least disruptive.
Unfortunately this effort fell short at establishing a stable calendar. To complicate matters even further, over the years, Emperors often rearranged the calendar to suit their own needs and to extend or shorten the terms of their fellow politicians. In time, the Romans found that their calendar was drastically out of sync with the seasons. To resolve this problem, the Emperor Julius Caesar changed the calendar on January 1, 45 bce, based on advice from a Greek astronomer named Sosigenes who was studying in the Egyptian city of Alexandria. He instituted a new solar calendar of 12 fixed months with a leap day added every fourth year at the end of February to keep it in sync with the tropical year. He also wanted to establish the vernal equinox or winter solstice as the start of the year, but was overruled by the Senate who chose January 1, as that was the day they traditionally began their terms. Julius did succeed however in fixing some holidays such as assigning the vernal equinox to fall on March 21.[2] This drastic change in the calendar was ultimately beneficial, although harsh on the populous as it resulted in the addition of 80 days, extending the adjustment year to 445 days in length, and earning it the nickname "the Year of Confusion". [2]
In time the Romans converted to Christianity, and in the year 527, an abbot named Dionysis Exiguus began renumbering the years in accordance with the new beliefs. He estimated that Jesus had been born on March 25, 754, and so renumbered that as year 1. The following years were designated as A.D., standing for "Anno Domini", "Year of our Lord". [3] This calendar was soon spread across Europe as a result of the influence of the Roman Empire and Catholic Church, and became the basis for the Gregorian calendar which was to follow.
Gregorian/Modern - It took hundreds of years for the flaws in the Julian calendar to become so problematic that calendar reform was once again an issue. By the 1530's, Pope Paul III had become determined to resolve the matter, and hired several astronomers to propose a solution. When Pope Gregory XIII later took office, he found the time was ripe for action, and in 1582 he established the Inter- Gravissimas, which has since become known as the Gregorian calendar. One of the principle objectives of the reform was to realign the Christian holiday of Easter with the vernal equinox and the spring season, as the two had slipped greatly apart. The new plan proposed that Easter should fall on the Sunday following the full moon that falls on or after March 21, while other Church holidays such as Christmas remained on fixed dates. In the new system, leap years were still retained, but restyled for the new times, and new years day was officially fixed to January 1, as many countries had continued to celebrate it on their own traditional dates. To enact this new calendar, 10 days were removed from the year, which was not well received by the people. Many refused to move their annual holidays and festivals to the revised dates. This is still noticeable throughout Europe, as many traditional holidays that were once celebrated on the same dates, now range 10 days apart depending on the locale.
Originally a calendar proposed by the Christian Church, the Gregorian calendar has since been adopted by all western nations. Catholic countries adopted it following the Pope's decree, while Germany did not accept it until 1700. An Act of Parliament in the United Kingdom enacted the calendar in 1751, which is the origin of it's use in the US today. Sweden followed in 1753, although east Europe would not convert until 1919, and China in 1949. On the other hand, the Eastern Orthodox Church voted to reject the Gregorian calendar in 1971 and has still retained the older Julian version. In these many years, the only recent change to befall the Gregorian calendar has been the movement to drop the Christian designations of A.D. and B.C. in favor of c.e. for "common era", and b.c.e. for "before common era". [3]
But does this longevity mean permanent stability for our Gregorian calendar? Not at all. Around the world, other countries and cultures have continued to utilize their own calendars, or have attempted to revise or replace the Gregorian calendar all together. Many are eager to point out the flaws within the Gregorian system:
- the lengths of the months and quarters are not equal
- the year does not comprise all full weeks, meaning dates can fall on any day of the week from year to year
- the number of working days is not consistent from month to month
- the Gregorian system is also off by a light fraction from the true solar year, though it will be many centuries until it starts to slip out of sync with the seasons
As a result there has been a number of proposals to institute calendar reform.
13 Month - This calendar was developed in 1849 by August Comte. His proposal was to use a 364 day calendar composed of 13 months with 28 days each in order to have a true perpetual calendar. The charm of this proposal was it's consistency, in that the first day of the month would always be a Monday, and the last day a Sunday. [20] Known as the George Eastman calendar in the US after one of its major supporters, this idea met with mixed reviews. Because the adoption of this plan would have changed the fixed dates of certain holidays, it was not considered a favorable approach and eventually lost its supporters. [24]
In 1922, the League of Nations appointed a Committee to research the possibilities of calendar reform, and studied over 140 proposals. [31] Their conclusion was that the time was not ripe for change, and the matter was again dropped.
World Calendar - This calendar was developed by Elisabeth Achelis [30] and has been promoted by the World Calendar Association since the 1930's. In this plan, the year is divided into four quarters that begin in January, April, July and October, and always start on a Sunday to form a 12 month perennial calendar. The first month of each quarter would have 31 days, and the second and third months would have 30 days. In total, there would be 13 weeks in each quarter. An intercalary day called "Worldsday" would be placed at the end of the year, and in leap years an extra day called "Leapday" would be inserted between June and July. Worldsday was also seen as an attractive benefit, as it was to be an international holiday not assigned to a week day, and solely devoted to the pursuit of world peace. The creators of this calendar believe it is such an excellent plan, that they have placed copyright protection on it. [22]
In 1953, this calendar gained a good deal of attention when the representative of India presented a proposal to the United Nations to reform the calendar and institute the World Calendar beginning January 1, 1956. [23] His argument stated that the Gregorian was "unsatisfactory for the economic, social, educational, scientific and other activities of man. Modern progress demands the change." [23] Six years after the debate began at the United Nations, the Vatican issued a response called the "Declaration of the Vatican II Ecumenical Council Concerning Calendar Reform", which stated that they were not opposed to a reform of the calendar, as long as the newly adopted version retained a seven day week that started with Sunday. Their objective of course, was to continue the domination of the Christian agenda over the United Nations calendar system. [25]
New World Reform Peace Act - For decades, the matter was silent, until another proposal emerged in 1991. This time, the approach was to institute the Perpetual 13 week calendar, a system based on the Mayan lunar calendar, as devised by Dr. Jose' and Lloydine Arguelles. Although this had incorporated the Vatican's earlier requirements, the Catholic Church resisted the plan based on the pagan origins of the new system. In 1998, the calendar was placed before the United Nations once again, at which time it became known as the New World Reform Peace Act. Although tentatively proposed to take effect on July 26, 1998, the reform once again fell to the wayside, only to reemerge in 1999 as the primary focus of the World Summit on Peace and Time in Costa Rica. [29] This coalition included individuals from around the world that sought to bring about world peace through calendar reform. Their proposal stated in part:
"Article I. Resolution of the Commission on Time: In order to return to an original wisdom and safeguard the future evolutionary unfoldment of the human species, we, the people of planet Earth, call for the establishment of a harmonious measure of time. To prevent the further desecration of the biosphere, this measure must reflect the natural pulse of the planet." [29]
All things considered, this is what calendars are all about, finding a natural pulse and timing in the scheme of things, by which humans can organize and manage their lives. In a world of ever increasing complexity, the need to simplify, and retain a feeling of harmony with the universe is a growing need among humankind. Although the verdict is still out on adopting this ancient Mayan calendar system, perhaps some day soon, we shall find ourselves turning our calendar pages in tune to a cosmic rhythm marked out thousands of years ago by ancient astronomers, and wishing one another a Happy Worldsday.
[1] Swinburne University, History of Astronomy
[2] Worldwide Leap Year Festival, http://astro.nmsu.edu/~lhuber/leaphist.html
[3] The Julian and Gregorian Calendars, http://serendipity.magnet.ch/hermetic/cal_stud/cal_art.htm#Adoption
[4] The Zoroastrian Calendar, http://www.ahura-mazda.com/calendar.htm
[5] The Greek Calendar, http://ourworld.compuserve.com/homepages/khagen/Greek.html
[6] Lunar Calendars, http://serendipity.magnet.ch/hermetic/cal_stud/lunarcal/otherlun.htm
[7] Maya Civilization, http://www.civilization.ca/membrs/civiliz/maya/mmc06eng.html
[8] Aztec Calendar Wheels, http://www.smith.edu/hsc/museum/ancient_inventions/hsc08b.htm
[9] Ancient Calendars, http://physics.nist.gov/GenInt/Time/ancient.html
[10] Pathfinder: An Ancient American Calendar, http://www.archaeoastronomy.com/Pathfinder/
[11] Calendar Reform, http://personal.ecu.edu/mccartyr/calendar-reform.html
[12] History of the Gregorian Calendar and Leap Year
[13] The Cambridge Illustrated History of Astronomy
[14] Farmers Almanac 2002
[15] Prehistoric Astronomy and Ritual, Aubrey Burl
[16] Calendars Through the Ages, http://webexhibits.org/calendars/calendar.html
[17] The Most Known Representation of the Moon, http://web.tiscali.it/liguriapaleoart/pagina16.html
[18] The Egyptian Calendar System, http://emuseum.mnsu.edu/prehistory/egypt/dailylife/calendar.html
[19] Rediscover Ancient Egypt, http://www.egypt-tehuti.org/articles/gregorian-calendar.html
[20] CalendarReform.org, http://www.calendarreform.org/home.html
[21] CalendarZone, http://www.calendarzone.com/Reform/
[22] CalendarHome.com, http://www.calendarhome.com/clink/reform.html
[23] Calendar Reform at the United Nations, http://www.geocities.com/peacecrusader888/un.htm
[24] Calendar Reform, http://www.time-travel.com/reform.htm
[25] New World Calendar Reform, http://www.earthascending.com/time/reform.htm
[26] Lunar Calendars, http://serendipity.magnet.ch/hermetic/cal_stud/lunarcal/luncal.htm
[27] Calendar a History, http://www.ernie.cummings.net/calendar.htm
[28] The Pleiades, http://www.scivis.com/AC/stor/pleiad.html
[29] World Summit on Peace and Time, http://www.tortuga.com/college/red/worldsummit/
[30] World Calendar, http://www.ship.edu/~cgboeree/worldcalendar.html
[31] A Brief History of Our Calendar, http://www.calendarreform.org/IndexContents/calendarhistory.html
[32] Sun, Moon, & Earth, Robin Heath, Walker & Company, NY @1999
[33] The Hebrew Calendar, http://www.tondering.dk/claus/cal/node4.html
[34] Islamic Calendar, http://www.ummah.org.uk/ildl/index.html
[35] Islamic Calendar, http://www.treasure-troves.com/astro/IslamicCalendar.html
[36] Astro-Archaeology, John Michell, Thames & Hudson, 1977
[37] Maya Cosmogenesis, John Major Jenkins, Bear & Company, 1998
[38] Popul Voh, Dennis Tedlock, Simon & Schuster, 1985