Mesoamerican Long Count calendar
The Mesoamerican Long Count calendar is a non-repeating, vigesimal (base-20) and base-18 calendar used by several pre-Columbian Mesoamerican cultures, most notably the Maya. For this reason, it is often known as the Maya (or Mayan) Long Count calendar. Using a modified vigesimal tally, the Long Count calendar identifies a day by counting the number of days passed since a mythical creation date that corresponds to August 11, 3114 BCE in the Proleptic Gregorian calendar.[n 1] The Long Count calendar was widely used on monuments.
- 1 Background
- 2 Long Count periods
- 3 Mesoamerican numerals
- 4 Earliest Long Counts
- 5 Correlations between Western calendars and the Long Count
- 6 2012 and the Long Count
- 7 Calculating a full Long Count date
- 8 Piktuns and higher orders
- 9 See also
- 10 Notes
- 11 References
- 12 Bibliography
- 13 External links
The two most widely used calendars in pre-Columbian Mesoamerica, were the 260-day Tzolk'in and the 365 day Haab'. The equivalent Aztec calendars are known in Nahuatl as the tonalpohualli and Xiuhpohualli.
The combination of a Haab' and a Tzolk'in date identifies a day in a combination which does not occur again for 18,980 days (52 Haab' cycles of 365 days times 73 Tzolk'in cycles of 260 days, approximately 52 years), a period known as the Calendar Round. To identify days over periods longer than this, Mesoamericans used the Long Count calendar.
Long Count periods
The Long Count calendar identifies a date by counting the number of days from a starting date that is generally calculated to be August 11, 3114 BCE in the proleptic Gregorian calendar or September 6 in the Julian calendar (or −3113 in astronomical year numbering). There has been much debate over the precise correlation between the Western calendars and the Long Count calendars. The August 11 date is based on the GMT correlation (see Correlations between Western calendars and the Long Count calendar section elsewhere in this article for details on correlations).
The completion of 13 b'ak'tuns (August 11, 3114 BCE) marks the Creation of the world of human beings according to the Maya. On this day, Raised-up-Sky-Lord caused three stones to be set by associated gods at Lying-Down-Sky, First-Three-Stone-Place. Because the sky still lay on the primordial sea, it was black. The setting of the three stones centered the cosmos which allowed the sky to be raised, revealing the sun.
Rather than using a base-10 scheme, like Western numbering, the Long Count days were tallied in a modified base-20 scheme. In a pure base-20 scheme, 0.0.0.1.5 is equal to 25, and 0.0.0.2.0 is equal to 40. The Long Count is not pure base-20, however, since the second digit from the right (and only that digit) rolls over to zero when it reaches 18. Thus 0.0.1.0.0 does not represent 400 days, but rather only 360 days. And 0.0.0.17.19 represents 359 days.
Note that the name b'ak'tun was invented by modern scholars. The numbered Long Count was no longer in use by the time the Spanish arrived in the Yucatán Peninsula, although unnumbered k'atuns and tuns were still in use. Instead the Maya were using an abbreviated Short Count.
Long Count dates are written with Mesoamerican numerals, as shown on this table. A dot represents 1 while a bar equals 5. The shell glyph was used to represent the zero concept. The Long Count calendar required the use of zero as a place-holder, and presents one of the earliest uses of the zero concept in history.
On Maya monuments, the Long Count syntax is more complex. The date sequence is given once, at the beginning of the inscription, and opens with the so-called ISIG (Introductory Series Initial Glyph) which reads tzik-a(h) hab’ [patron of Haab' month] ("revered was the year-count with the patron [of the month]"). Next come the 5 digits of the Long Count, followed by the Calendar Round (tzolk'in and Haab') and supplementary series. The supplementary series is optional and contains lunar data, for example, the age of the moon on the day and the calculated length of current lunation.[n 2] The text then continues with whatever activity occurred on that date.
A drawing of a full Maya Long Count inscription is shown below.
Earliest Long Counts
The earliest contemporaneous Long Count inscription yet discovered is on Stela 2 at Chiapa de Corzo, Chiapas, Mexico, showing a date of 36 BCE, although Stela 2 from Takalik Abaj, Guatemala might be earlier.  Takalik Abaj Stela 2's highly battered Long Count inscription shows 7 bak'tuns, followed by k'atuns with a tentative 6 coefficient, but that could also be 11 or 16, giving the range of possible dates to fall between 236 and 19 BCE.[n 3] Although Takalik Abaj Stela 2 remains controversial, this table includes it, as well as six other artifacts with the eight oldest Long Count inscriptions according to Dartmouth professor Vincent H. Malmström (two of the artifacts contain two dates, and Malmström does not include Takalik Abaj Stela 2). Interpretations of inscriptions on some artifacts differ.
|Archaeological site||Name||Gregorian date
GMT (584283) correlation
|Takalik Abaj||Stela 2||236 - 19 BCE||7.(6,11,16).?.?.?||Guatemala|
|Chiapa de Corzo||Stela 2||December 6, 36 BCE or
October 9, 182 CE
|Tres Zapotes||Stela C||September 1, 32 BCE||126.96.36.199.18||Veracruz, Mexico|
|El Baúl||Stela 1||11 - 37 CE||188.8.131.52.12,
|Takalik Abaj||Stela 5||August 31, 83 CE or
May 19, 103 CE
|Takalik Abaj||Stela 5||June 3, 126 CE||184.108.40.206.11||Guatemala|
|La Mojarra||Stela 1||May 19, 143 CE||220.127.116.11.5||Veracruz, Mexico|
|La Mojarra||Stela 1||July 11, 156 CE||18.104.22.168.7||Veracruz, Mexico|
|Near La Mojarra||Tuxtla Statuette||March 12, 162 CE||22.214.171.124.17||Veracruz, Mexico|
Of the six sites, three are on the western edge of the Maya homeland and three are several hundred kilometers further west, leading some researchers to believe that the Long Count calendar predates the Maya. La Mojarra Stela 1, the Tuxtla Statuette, Tres Zapotes Stela C, and Chiapa Stela 2 are all inscribed in an Epi-Olmec, not Maya, style. El Baúl Stela 2, on the other hand, was created in the Izapan style.
More recently, with the discovery in Guatemala of the San Bartolo (Maya site) stone block text (c.a. 300 BCE), it has been argued that this text celebrates an upcoming time period ending celebration. This time period may have been projected to end sometime between 126.96.36.199.0 and 188.8.131.52.0 — 295 and 256 BCE, respectively. Interestingly, besides this being the earliest Maya hieroglyphic text so far uncovered, it would arguably be the earliest glyphic evidence to date of Long Count notation in Mesoamerica.
Correlations between Western calendars and the Long Count
The Maya and Western calendars are correlated by using a Julian day number (JDN) of the starting date of the current creation — 184.108.40.206.0, 4 Ajaw, 8 Kumk'u.[n 4] This is referred to as a correlation constant. The generally accepted correlation constant is the Modified Thompson 2, "Goodman–Martinez–Thompson", or GMT correlation of 584,283 days. Using the GMT correlation, the current creation started on September 6, 3114 BC in the Julian Calendar or August 11 in the Proleptic Gregorian calendar. The study of correlating the Maya and western calendar is referred to as the correlation question. The GMT correlation is also called the 11.16 correlation.
In Breaking the Maya Code, Michael D. Coe writes: "In spite of oceans of ink that have been spilled on the subject, there now is not the slightest chance that these three scholars (conflated to GMT when talking about the correlation) were not right...". The evidence for the GMT correlation is historical, astronomical, and archaeological:
Historical: Calendar Round dates with a corresponding Julian date are recorded in Diego de Landa's Relación de las cosas de Yucatán (written circa 1566), the Chronicle of Oxcutzkab and the books of Chilam Balam. Oxcutzkab and de Landa record a date that is a Tun ending in the Short Count. Regarding these historical references in The Skywatchers Aveni writes: "All the assembled data are consistent with the equation November 2, 1539 = 220.127.116.11.0. Thus for the GMT, or [an alternate term:] 11.16 correlation we find that A = 584,283...". The fall of the capital city of the Aztec Empire, Tenochtitlan, occurred on August 13, 1521. A number of different chroniclers wrote that this was a Tzolk'in (Tonalpohualli) of 1 Snake. Post-conquest scholars such as Sahagún and Durán recorded Tonalpohualli dates with a calendar date. Many indigenous communities in the Mexican states of Veracruz, Oaxaca and Chiapas and in Guatemala, principally those speaking the Mayan languages Ixil, Mam, Pokomchí, and Quiché, keep the Tzolk'in and in many cases the Haab'. These are all consistent with the GMT correlation.
Astronomical: Any correct correlation must match the astronomical content of classic inscriptions. The GMT correlation does an excellent job of matching lunar data in the supplementary series. For example: An inscription at the Temple of the Sun at Palenque records that on Long Count 18.104.22.168.8 there were 26 days completed in a 30-day lunation. This Long Count is also the entry date for the eclipse table of the Dresden Codex [n 5] which gives eclipse seasons when the Moon is near its ascending or descending node and an eclipse is likely to occur. Dates converted using the GMT correlation fall roughly in this eclipse season. The Dresden Codex contains a Venus table which records the heliacal risings of Venus. Using the GMT correlation these agree closely with modern astronomical calculations.
Archaeological: Various items that can be associated with specific Long Count dates have been isotope dated. In 1959 the University of Pennsylvania carbon dated samples from ten wood lintels from Tikal. These were carved with a date equivalent to 741 AD, using the GMT correlation. The average carbon date was 746±34 years. Recently one of these, Lintel 3 from Temple I, was analyzed again using more accurate methods and found to agree closely with the GMT correlation.
If a proposed correlation only has to agree with one of these lines of evidence there could be numerous other possibilities. Astronomers have proposed many correlations, for example: Lounsbury, Fuls, et al., Böhm and Böhm and Stock.
Today, 26 April 2019 (UTC), in the Long Count is 22.214.171.124.17 (GMT correlation).
|Modified Thompson 1||584,284|
|Böhm & Böhm||622,261|
|Fuls, et al.||660,208|
|Long Count||Gregorian date
GMT (584283) correlation
|126.96.36.199.0||Mon, Aug 11, 3114 BCE||584 283|
|188.8.131.52.0||Thu, Nov 13, 2720 BCE||728 283|
|184.108.40.206.0||Sun, Feb 16, 2325 BCE||872 283|
|220.127.116.11.0||Wed, May 21, 1931 BCE||1 016 283|
|18.104.22.168.0||Sat, Aug 23, 1537 BCE||1 160 283|
|22.214.171.124.0||Tue, Nov 26, 1143 BCE||1 304 283|
|126.96.36.199.0||Fri, Feb 28, 748 BCE||1 448 283|
|188.8.131.52.0||Mon, Jun 3, 354 BCE||1 592 283|
|184.108.40.206.0||Thu, Sep 5, 41 CE||1 736 283|
|220.127.116.11.0||Sun, Dec 9, 435||1 880 283|
|10.0.0.0.0||Wed, Mar 13, 830||2 024 283|
|18.104.22.168.0||Sat, Jun 15, 1224||2 168 283|
|22.214.171.124.0||Tue, Sep 18, 1618||2 312 283|
|126.96.36.199.0||Fri, Dec 21, 2012||2 456 283|
|188.8.131.52.0||Mon, Mar 26, 2407||2 600 283|
|184.108.40.206.0||Thu, Jun 28, 2801||2 744 283|
|220.127.116.11.0||Sun, Oct 1, 3195||2 888 283|
|18.104.22.168.0||Wed, Jan 3, 3590||3 032 283|
|22.214.171.124.0||Sat, Apr 7, 3984||3 176 283|
|126.96.36.199.0||Tue, Jul 11, 4378||3 320 283|
|188.8.131.52.0.0||Fri, Oct 13, 4772||3 464 283|
2012 and the Long Count
According to the Popol Vuh, a book compiling details of creation accounts known to the K'iche' Maya of the Colonial-era highlands, we are living in the fourth world. The Popol Vuh describes the first three creations that the gods failed in making and the creation of the successful fourth world where men were placed. In the Maya Long Count, the previous creation ended at the start of a 14th b'ak'tun.
The previous creation ended on a Long Count of 184.108.40.206.19. Another 220.127.116.11.19 occurred on December 20, 2012 (Gregorian Calendar), followed by the start of the 14th b'ak'tun, 18.104.22.168.0, on December 21, 2012.[n 6] There are only two references to the current creation's 13th b'ak'tun in the fragmentary Mayan corpus: Tortuguero Monument 6, part of a ruler's inscription and the recently discovered La Corona Hieroglyphic Stairway 2, Block V.
Maya inscriptions occasionally reference future predicted events or commemorations that would occur on dates that lie beyond 2012 (that is, beyond the completion of the 13th b'ak'tun of the current era). Most of these are in the form of "distance dates" where some Long Count date is given, together with a Distance Number that is to be added to the Long Count date to arrive at this future date.
For example, on the west panel at the Temple of Inscriptions in Palenque, a section of the text projects into the future to the 80th Calendar Round (CR) 'anniversary' of the famous Palenque ruler K'inich Janaab' Pakal's accession to the throne (Pakal's accession occurred on a Calendar Round date 5 Lamat 1 Mol, at Long Count 22.214.171.124.8 equivalent to 27 July 615 CE in the proleptic Gregorian calendar).[n 7] It does this by commencing with Pakal's birthdate 126.96.36.199.0 8 Ajaw 13 Pop (24 March 603 CE Gregorian) and adding to it the Distance Number 10.11.10.5.8. This calculation arrives at the 80th Calendar Round since his accession, a day that also has a CR date of 5 Lamat 1 Mol, but which lies over 4,000 years in the future from Pakal's time—the day 21 October in the year 4772. The inscription notes that this day would fall eight days after the completion of the 1st piktun [since the creation or zero date of the Long Count system], where the piktun is the next-highest order above the b'ak'tun in the Long Count. If the completion date of that piktun—13 October 4772—were to be written out in Long Count notation, it could be represented as 188.8.131.52.0.0. The 80th CR anniversary date, eight days later, would be 184.108.40.206.0.8 5 Lamat 1 Mol.
Despite the publicity generated by the 2012 date, Susan Milbrath, curator of Latin American Art and Archaeology at the Florida Museum of Natural History, stated that "We have no record or knowledge that [the Maya] would think the world would come to an end" in 2012. USA Today writes “‘For the ancient Maya, it was a huge celebration to make it to the end of a whole cycle,’ says Sandra Noble, executive director of the Foundation for the Advancement of Mesoamerican Studies in Crystal River, Florida. To render December 21, 2012, as a doomsday event or moment of cosmic shifting, she says, is ‘a complete fabrication and a chance for a lot of people to cash in.’” "There will be another cycle," says E. Wyllys Andrews V, director of the Tulane University Middle American Research Institute (MARI). "We know the Maya thought there was one before this, and that implies they were comfortable with the idea of another one after this."
Calculating a full Long Count date
As stated, a full Long Count date not only includes the five digits of the Long Count, but the 2-character Tzolk'in and the two-character Haab' dates as well. The five digit Long Count can therefore be confirmed with the other four characters (the "calendar round date").
Taking as an example a Calendar Round date of 220.127.116.11.16 (Long Count) 5 Kib' (Tzolk'in) 14 Yaxk'in (Haab'). One can check whether this date is correct by the following calculation.
It is perhaps easier to find out how many days there are since 4 Ajaw 8 Kumk'u, and show how the date 5 Kib' 14 Yaxk'in is derived.
|9||× 144000||= 1296000|
|12||× 7200||= 86400|
|2||× 360||= 720|
|0||× 20||= 0|
|16||× 1||= 16|
|Total days||= 1383136|
Calculating the Tzolk'in date portion
The Tzolk'in date is counted forward from 4 Ajaw. To calculate the numerical portion of the Tzolk'in date, add 4 to the total number of days given by the date, and then divide total number of days by 13.
- (4 + 1383136) / 13 = 106395 (and 5/13)
This means that 106395 whole 13 day cycles have been completed, and the numerical portion of the Tzolk'in date is 5.
To calculate the day, divide the total number of days in the long count by 20 since there are twenty day names.
- 1383136 / 20 = 69156 (and 16/20)
This means 16 day names must be counted from Ajaw. This gives Kib'. Therefore, the Tzolk'in date is 5 Kib'.
Calculating the Haab' date portion
The Haab' date 8 Kumk'u is the ninth day of the eighteenth month. There are 17 days to the start of the next year.
Subtract 17 days from the total, to find how many complete Haab' years are contained.
- 1383136 − 17 = 1383119
Divide by 365
- 1383119 / 365 = 3789 and (134/365)
Therefore, 3789 complete Haab' have passed, and the remainder 134 is the 135th day in the new Haab', since a remainder of 0 would indicate the first day.
Find which month the day is in. Dividing the remainder 134 by 20, is six complete months, and a remainder of 14, indicating the 15th day. So, the date in the Haab' lies in the seventh month, which is Yaxk'in. The fifteenth day of Yaxk'in is 14, thus the Haab' date is 14 Yaxk'in.
So the date of the long count date 18.104.22.168.16 5 Kib' 14 Yaxk'in is confirmed.
Piktuns and higher orders
There are also four rarely used higher-order periods above the b'ak'tun: piktun, kalabtun, k'inchiltun, and alautun. All of these words are inventions of Mayanists. Each one consists of 20 of the lesser units.
Many inscriptions give the date of the current creation as a large number of 13s preceding 22.214.171.124.0 4Ahau 8 Kumk'u. For example, a Late Classic monument from Coba, Stela 1. The date of creation is expressed as 126.96.36.199.188.8.131.52.184.108.40.206.220.127.116.11.18.104.22.168.0.0.0.0, where the units are 13s in the nineteen places larger than the b'ak'tun. Some authors think that the 13s were symbolic of a completion and don't represent an actual number.
Most inscriptions that use these are in the form of distance dates and Long Reckonings - they give a base date, a distance number that is added or subtracted and the resulting Long Count.
The first example below is from Schele (1987 p.). The second is from Stuart (2005 pp. 60, 77)
Palenque Temple of the Cross, tablet, Schele (1987 p.)
22.214.171.124.0 8 Ajaw 18 Tzek in the prior era
6.14.0 Distance number linking to the "era date"
126.96.36.199.0 4 Ajaw 8 Kumk'u
Palenque Temple XIX, South Panel G2-H6 Stuart (2005 pp. 60, 77)
188.8.131.52. 2 9 Ik' 5 Mol (seating of GI in the prior era)
184.108.40.206.2 9 Ik' 15 Keh (rebirth of GI, this date also in Temple of the Cross)
The tablet of the inscriptions contains this inscription:
220.127.116.11.0 8 Ajaw 13 Pop
The Dresden codex contains another method for writing distance numbers. These are Ring Numbers. Specific dates within the Dresden codex are often given by calculations involving Ring Numbers. Förstemann identified these, but Wilson (1924:24–25) later clarified the way in which they operate. Ring Numbers are intervals of days between the Era Base date 4 Ajaw 8 Kumk’u and an earlier Ring Base date, where the place-holder for the numeral of days in the interval is circled by an image of a tied red band. Added to this earlier Ring Base date is another count of days forward, which Thompson refers to as a Long Round, leading to a final date within the Long Count that is given as an entry date to be used within a specific table in the codex.
Ring number (12) 18.104.22.168.1 13 Imix 9 Wo (22.214.171.124 before (13) 126.96.36.199.0)
distance number (0) 10.13.13.3.2
Long Count 10.6.10.6.3 13 Ak'bal 1 Kank'in
Ring number (portion of the DN preceding era date) 188.8.131.52
Add Ring number to the ring number date to reach 184.108.40.206.0
- The correlation between the Long Count and Western calendars is calculated according to the one used by a majority of Maya researchers, known as the (modified) GMT or Goodman-Martinez-Thompson correlation. An alternate correlation sometimes used puts the starting date two days later. August 11, 3114 BCE is a date in the proleptic Gregorian calendar, which equates to September 6, 3114 BCE in the Julian calendar and −3113 in astronomical year numbering. See Correlations between Western calendars and the Long Count calendar section elsewhere in this article for details on correlations.
- Notable in this sequence is the glyph with nine variant forms labeled G by early epigraphers. It has been connected with the cycle of Lords of the Night known from colonial era sources in Central Mexico but alternate explanations have also been offered. See Thompson.
- To clarify, there are Long Count inscriptions which refer to dates earlier than the 1st century BCE, but these were carved much later in a retrospective fashion.
- All extant Maya inscriptions that represent this base date wrote it with a “13” bak'tuns, not “0”. But when using “220.127.116.11.0” as a base date in calculations, the “13” bak'tuns has the numerical value 0, as if it were written as “0.0.0.0.0”. This is easily confused when the “13” bak'tuns has the actual value 13 in the current bakt'un, as in the Maya date for today: 18.104.22.168.17 (=09:37, Friday, April 26, 2019 (UTC)).
- According to Thompson "The point from which the moon age is counted is not surely known. The possibilities are disappearance of the old moon, conjunction, or appearance of the new moon... Beyer (1973a) believed that the calculation was made from the disappearance of the old moon. The latter method of counting (disappearance of the old moon) is still current in some Tzeltal, Chol, and Tzotzil villages in Chiapas..." Using the third method the new moon would have been the first evening when one could look to the west after sunset and see the thin crescent moon. Given our modern ability to know exactly where to look, when the crescent Moon is favorably located, from an excellent site, on rare occasions, using binoculars or a telescope, observers can see and photograph the crescent moon less than one day after conjunction. Generally, most observers can't see the new Moon with the naked eye until the first evening when the lunar phase day is at least 1.5. If one assumes that that the new moon is the first day when the lunar phase day is at least 1.5 at six in the evening in time zone UTC−6 (the time zone of the Maya area) the GMT correlation will match many lunar inscriptions exactly. In this example the lunar phase day was 27.7 (26 days counting from zero) at 6 pm after a conjunction at 1:25 am October 10, 755 and a new Moon when the lunar phase day was 1.7 at 6 pm on October 11, 755 (Julian calendar). This works well for many but not all lunar inscriptions. Modern astronomers refer to the conjunction of the Sun and Moon (the time when the Sun and Moon have the same ecliptic longitude) as the new moon. In order for the Maya to have been able to calculate this they would have had to have a complete understanding of orbital mechanics, spherical trigonometry, etc. so this seems impossible. The first method seems to have been used for other inscriptions such as Quirgua stela E (22.214.171.124.0). Using the third method it should have a moon age of 26 days when in fact it records a new moon. Using the GMT correlation at six AM in time zone -six, this would be 2.25 days before conjunction, so it could record the first day when one could not see the waning moon. Fuls Analysed these inscriptions and Concluded: "Analysis of the Lunar Series shows that at least two different methods and formulas were used to calculate the moon’s age and position in the six-month cycle…"
- Various sources place this on other dates, notably on December 23.
- Gregorian, using GMT correlation JDN=584283.
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- Mayan versus our calendar: astronomy supports the Bohm not the GMT correlation
- Anton Stock. "Dating the Eclipse Table of the Dresden Codex and the Correlation Problem". baktun.de. Retrieved 2015-09-06.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
- Schele & Freidel (1990), pp.429–430
- Schele and Friedel (1992).
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- Schele (1992, pp.93–95)
- Schele & Freidel (1990, p.430 n.39)
- MacDonald, G. Jeffrey (March 27, 2007). "Does Maya calendar predict 2012 apocalypse?". USA Today. Gannett Company.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
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- Thompson (1960 Appendix IV pp.314, 316, 148, 149) "I have throughout assumed that the baktuns were grouped, not in 13's, but in 20's, for the evidence supporting a vigesimal count of baktuns in Dresden and at Palenque and Copan is too strong to be overridden."
- Grofe, Michael John 2007 The Serpent Series: Precession in the Maya Dresden Codex page 55 "On occasion, the Maya also recorded intervals of time even greater than 13 B’ak’tuns, such as one Piktun, composed of 20 B’ak’tuns. This is relevant to the current discussion concerning the Serpent Series."
- Martin Time Kingship and the Maya Universe "…we have clear evidence that the current Bak’tun cycle does not conclude at 13, as the last one did, but advances to 20. In other words, 126.96.36.199.0 will be followed by 188.8.131.52.0, 184.108.40.206.0, and so on to 220.127.116.11.0. A text at the site of Palenque, Mexico, makes this very plain when it records the completion of 1 Piktun, the next unit above the Bak’tun, in 4772 CE.
- Fig. 444 in Wagner (2006, p.283)
- Schele and Freidel (1992, p.430).
- D. Freidel, L. Schele And J. Parker, Maya Cosmos: Three Thousand Years On The Shaman's Path, 1993:62, Fig. 2:1
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- Förstemann, Ernst Commentary on the Maya Manuscript in the Royal Public Library of Dresden - Peabody Museum of American Archaeology and Archaeology and Ethnography, Harvard University Vol. IV. - No.2 pp. 222-264
- Eric Thompson (1972:20–21)
- Grofe, Michael John 2007 The Serpent Series: Precession in the Maya Dresden Codex page 55
- Thompson 1972 pp.20-22
- http://www.traditionalhighcultures.org/Distances_across_Era_Date.pdf table from Thompson
- Beyer, Hermann 1933 Emendations of the ‘Serpent Numbers’ of the Dresden Maya Codex. Anthropos (St. Gabriel Mödling bei Wien) 28: pp.1–7. 1943 The Long Count Position of the Serpent Number Dates. Proc. 27th Int. Cong. Of Amer., Mexico, 1939 (Mexico) I: pp. 401–405.
- Grofe, Michael John 2007 The Serpent Series: Precession in the Maya Dresden Codex page 63
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- Graham, John A. (1992). "Escultura en bulto Olmeca y Maya en Tak'alik Ab'aj: Su desarrollo y portento" (PDF online edition). IV Simposio de Investigaciones Arqueológicas en Guatemala, 1990 (edited by J.P. Laporte, H. Escobedo and S. Brady) (in español). Guatemala: Museo Nacional de Arqueología y Etnología: 325–334. Retrieved 2013-03-16.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
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- Pérez de Lara, Jorge; John Justeson (2005). "Photographic Documentation of Monuments with Epi-Olmec Script/Imagery". The Foundation Granting Department: Reports Submitted to FAMSI. Foundation for the Advancement of Mesoamerican Studies, Inc. (FAMSI). Retrieved 2007-04-04.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
- Saturno, William A.; David Stuart; Boris Beltran (2006). "Early Maya Writing at San Bartolo, Guatemala". Science. Science. Retrieved 2013-03-15. <templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
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- Coba Stela 1 (Schele #4087), partial illustration from the Linda Schele Drawings Collection of the monument from Coba with an expanded Long Count date
- Maya calendar on michielb.nl, with conversion applet from Gregorian calendar to Maya date (Uses the proleptic Gregorian calendar.)
- The Dresden Codex Lunar Series and Sidereal Astronomy
- Day Symbols of the Maya Year at Project Gutenberg 1897 text by Cyrus Thomas.