II. Astronomy
III. Hebrew Calendar
IV. Islamic Calendar
VI. Week
The Julian Calendar was introduced by Julius Caesar in 45 BC, and was widespread until the 16th century, when nations started to switch to the Gregorian Calendar. However, some countries such as Greece and Russia kept the Julian Calendar into this century, and the Orthodox Church in Russia still uses it.
The years are 365.25 days in length. This approximate length results in the error of one day roughly every 128 years. To balance out the quarter of a day per year, there is one leap day every four years.
Every four years, there is a leap year. However, in the first fifty-two years of the Julian Calendars existence, there was a leap year every three years due to a counting error. The leap years were:
45 BC, 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC, 8 AD, 12 AD, and every forth year that followed.
There were no leap years between 9 BC and 8 AD, as decreed by emperor Augustus, thus earning him recognition in Calendrology, the eighth month (August) was named after him.
The decision to count the years from the birth of Christ in 0 AD wasn't established until the sixth century AD. The fact that the leap years are always a multiple of four is a mere coincidence.
The Gregorian Calendar is the commonly accepted calendar. It was created by Aloysius Lilius, a physician from Naples, and adopted by Pope Gregory XIII during the Council of Trent (1545-1563). Its purpose was to improve accuracy of the currently accepted Julian Calendar, by changing the length of the tropical year from 365.25 days to 365.2425 days. as a result, it would take 3300 years for an error of one day in the tropical year of the Gregorian Calendar.
In the Gregorian Calendar, there are approximately 97 leap years every 400 years. The leaps years are determined by:
As a result, the years 1700, 1800, 1900, 2100, and 2200 are not leap years, but 1600, 2000, and 2400 are leap years.
There is a myth that there are double leap years, with 367 days (With the possible exception of Sweden in 1712).
The astronomer John Herschel (1792-1871) suggested that a better approximation of the tropical years would be 365.24225. This would result in 969 leap years every 4000 years, rather than the accepted 970. This could be achieved by dropping a leap year every 4000 years, hence every year divisible by 4000. However, this practice was never officially established yet.
The Greek Orthodox church practice the Gregorian Calendar slightly differently. They have the rule:
Every year that leaves a remainder of 200 or 600 when divided by 900 is a leap year.
This rule replaces the accepted 400 year rule. As a result, the years 1900, 2100, 2200, 2300, 2500, 2600, 2700, and 2800 are not leap years. This won't create conflict with the rest of the world until the year 2800.
As a result of the rule, the tropical year for the Greek Orthodox is an average of 365.24222 days, more accurate than the accepted Gregorian Calendar's tropical year of 365.2425 days. However, this practice isn't official in the nation of Greece.
In February of 1582, the papal bull declared that the ten days be dropped for the transition from Julian to Gregorian. October 15, 1582 would immediately follow October 4, 1582. The Catholic nations of Italy, Poland, Portugal, and Spain followed this transition, but the Protestant countries were reluctant to change, and the Greek orthodox didn't change until the beginning of the 20th century.
The reason that the 16th and 17th century require the same drop is because the start of the 17th century (1600) is divisible by 400, and thus has the extra leap year at the transition of the century.
Sweden has a curious history involving the transition from Julian to Gregorian. Sweden decided on having a gradual transition by dropping every leap year from 1700 to 1740. As a result, eleven days would be dropped. By the date March 1, 1740, Sweden would be in sync with the Gregorian Calendar, after forty years of being in sync with nobody. In 1700, Sweden omitted the leap year (which was needed in the Gregorian Calendar), but left the leap years in 1704 and 1708 by mistake. As a result, Sweden decided to go back to the Julian Calendar by having a double leap year in 1712. The double leap year resulted in the date February 30 that year. Sweden then made the transition to the Gregorian Calendar in 1753 by dropping eleven days.
December 1912
The creation of the system of counting years before and after the birth of Jesus was created by a Scythian monk named Dionysius Exiguus (also known as Denis the Little in English) in the sixth century AD. He was assigned the job by the papal chancellor Bonifatius, to implement the rules from the Nicean council (the so-called Alexandrian Rules).
In 523 AD, Dionysius Exiguus was given the assignment to prepare calculations for Easter. At the time, it years were counted since the reign of emperor Diocletian. Dionysius Exiguus decided to give the honor to Jesus, rather than Diocletian. Dionysius Exiguus tried to fix Jesus' birth at December 25, 753 AUC (ab urbe condita, since the founding of Rome). Therefore, the current era started at 1 AD on January 1, 754 AUC.
The time before 754 AUC was dubbed Before Christ (BC), with no intervening year 0. However, astronomers often use 1 BC as year 0, and 2 BC as year -1, 3 BC is -2, etc.
Everyone says that Jesus Christ, the son of God in the Christian faith, was born on December 25, 0000. However, this is not true for two reasons.
The concept of the year 0 is a modern myth. Romans numerals have no number zero, therefore it was not popular for one to associate it with other numbers in the 6th century when Dionysius Exiguus established our present year. Dionysius Exiguus had our present year start at AD 1, which was one week after when he believed in Jesus' birthday. There was no 0 AD, as 1 AD immediately followed 1 BC, therefore a person who was born in 20 BC and died on 20 AD would have been 39, not 40, when he or she died.
Secondly, Dionysius Exiguus' calculations of Jesus' birth were wrong. The Gospel of Matthew tells us that Jesus was born under the reign of King Herod the Great, who died in 4 BC. It is most likely that Jesus was born around 7 BC, although the date of his birth is unknown. Whether or not he was born on December 25 or not is an old mystery, as there is speculation amongst numerous parts of the year.
The 21st century starts after 20 centuries passed since the start of our system of years, which has always been January 1, 1 AD (although the myth about a year 0 being the start of the years is a commonly confused myth). As a century is 100 years, then the 21st century starts 2000 years after January 1, 1 AD, which is January 1, 2001 AD.
There is much confusion about when a century starts. It is often believed that a century starts at any year that end in 00. However, a century is only a period of 100 years. For instance, October 14, 1947 (the first time the sound barrier was broke) to October 14, 2047 is just as much a century as January 1, 1900 to January 1, 2000, or better yet, January 1, 1901 to January 1, 2001. Feel free to celebrate the start of a new century any day you like.
January 1, 1901 is the start of the new century, while January 1, 1900 is the start of the 1900's. People had a party on both of these dates, both to start a new century. Let's keep it this way.
Before the year 2000, the leap day was February 24, because of the Roman Calendar! Although numerically February 29 is the leap day, the day of celebration and feasts has traditionally been February 24. For example, the feast of St. Leander has been celebrated on February 27 in non-leap years, and February 28 on leap years.
After the year 2000, the leap year will officially be February 29, in a decision made by the European Union. This decision will affect nations such as Sweden and Austria, who celebrate name days (each day is associated with a name). Also, the Roman Catholic church has apparently started using February 29 as the leap day long before the year 2000.
The relationship between the days of the week and the dates of the year is repeated in cycles of 28 years. This is true in both the Julian and Gregorian calendars, except when the cycle falls through a year divided by 400 in the Gregorian calendars.
The period of 28 years is known as the Solar Cycle. The Solar Number of a year is found as:
Solar Number = (year + 8) % 28 + 1
In the Julian Calendar, there is a one-to-one relationship between the Solar Number and the day on which a particular date falls.
(The leap year cycle of the Gregorian calendar is 400 years, which is 146,097 days, which curiously enough is a multiple of 7. So in the Gregorian calendar the equivalent of the Solar Cycle would be 400 years, not 7*400=2800 years, as one might easily believe.)
d = (day + y + y/4 - y/100 + 7/400 + (31*m) / 12) % 7
d = (5 + day + y + y/4 + (31*m) / 12) % 7
| d = 0 | Sunday |
| d = 1 | Monday |
| d = 2 | Tuesday |
| d = 3 | Wednesday |
| d = 4 | Thursday |
| d = 5 | Friday |
| d = 6 | Saturday |
Before Julius Caesar created the Julian Calendar in 45 BC, the Romans had a calendar that was extremely disorganized, and most of our understanding of it is through guesswork.
The Roman calendar originally consisted of ten months: Matius; Aprilis; Junius; Quintilis; Sextilis; September; October; November; and December. The year consisted of 304 days, and after December was the unnamed and unnumbered winter period. Around 750 BC, the Roman king Numa Pompilius (715-673 BC, although his historicity is disputed) created the months February and January, and inserted them, in that order, between December and March, increasing the length of the year to 354 days. As a result of the dearth of days, and extra month with 22 days called either Intercalaris or Mercedonius was added in. In an eight year period, the lengths of the years were:
This averaged 366.25 days a year. When they realized that this was too long, seven days were dropped from the following year.
The reason that the calendar is so disorganized is because the priesthood were were responsible to keep track failed miserably, because of their lack of intelligence and bribes. Also, leap years were unlucky, and stricken out during events such as the Second Punic War.
Julius Caesar created his calendar reform in 45 BC. His new calendar had the following months.
| Month | 47 BC | 46 BC |
| January | 29 | 29 |
| February | 28 | 24 |
| Intercalaris | --- | 27 |
| March | 31 | 31 |
| April | 29 | 29 |
| May | 31 | 31 |
| June | 29 | 29 |
| Quintilis | 31 | 31 |
| Sextilis | 29 | 29 |
| September | 29 | 29 |
| October | 31 | 31 |
| November | 29 | 29 |
| Undecember | --- | 33 |
| Duodecember | --- | 24 |
| December |
29 |
29 |
| Total | 355 | 445 |
There is a story about how the months as we know it were created. However, it is only a fabrication from the 14th century.
"Julius Caesar made all odd numbered months 31 days long, and all even months 30 days long, with February having 29 days in the leap year. In 44 BC, Quintilis was named Julius (July) after Julius Caesar, and Sextilis was named Augustus (August) after emperor Augustus. When Augustus had a month named after him, he wanted it to have a full 31 days, so he took a day from February and added it to August."
The Romans didn't have days like we have. They had three fixed points in the month:
Between Kalendae and Nonae, the Romans would name days like, "the 3rd Day before Nonae," "the 2nd Day before Nonae," etc. This method is the same between Nonae and Idus, and Idus and Kalendae. For instance, "the 6th Day before Idus," the 12th Day before Kalendae of next month."
Caesar decided that the leap day would be to double the 6th Day before the Kalendae of March (February 24). The reason for this is because the Roman month of Intercalaris fell between the date. This is why before the year 2000, the leap day is celebrated on February 24.
Ever since Julius Caesar created the modern calendar in 45 BC, the date for the new year has always been January 1. However, the church didn't like the parties that existed every January 1 for New Years, so in 567 AD, the council of Tours declared that having New Years on January 1 was an ancient mistake, and tried to abolish it. Throughout the middle ages, various dates for New Years have been established. However, it was difficult to establish a New Years date, and each nation and religion had their own New Years, which added to the confusion.
For example, the Byzantine Empire had a New Years that started from September 1. However, they counted from when they believe the world was created, in 5509 BC.
By 1600, most nations started to use January 1 as the New Years date. However, England and Italy didn't make January 1 the New Years date since 1750.
In England (but not Scotland), three different dates were used.
- December 25 from 7th to 12th century.
- March 25 from the 12th century to 1751.
- January 1 from 1751 to the present day.
January is Latin for Januarius. It was named after the Roman god Janus.
February is Latin for Februarius. It was named after Februa, the festival of purification.
March is Latin for Martius. It was named after the Roman god Mars, the god of war.
April is Latin for Aprilis. It was named after the goddess Aphrodite, the god of love and beauty.
May is Latin for Maius. It was probably named after the goddess Maia.
June is Latin for Junius. It was probably named after the goddess Juno.
July is Latin for Julius. It was named after Julius Caesar in 44 BC. Before, it was named Quintilis, from the word quintis, meaning fifth, as it was the 5th month in the Old roman calendar.
August is Latin for Augustus. It was named after emperor Augustus in 8 BC. Before, it was Sextilis, from the word sexus, meaning six, as it was the sixth month in the Old Roman calendar.
September is from the word septum, meaning seven, as it was the seventh month in the Old Roman calendar.
October is from the word octo, meaning eight, as it was the eighth month in the Old Roman calendar.
November is from the word novem, meaning nine, as it was the ninth month in the Old Roman calendar.
December is from the word decem, meaning ten, as it was the tenth month in the Old Roman calendar.
In the Christian religion, Easter (and the days following) is the celebration of the death and resurrection of Jesus in approximately 30 AD. It is celebrated on the first Sunday of the first full moon of the vernal equinox. However, the true calculation of Easter is complicated because it is linked to an inaccurate version of the Hebrew calendar.
Jesus was crucified immediately before the Jewish holiday of Passover, the celebration of the Exodus from Egypt. Passover is on the 14th or 15th day of the Jewish month of Nisan, which starts the beginning of spring. Because the Jewish calendar always starts on the new moon, then Passover must always start on a full moon.
Officially, the vernal equinox is on March 21, but this date isn't always completely accurate. The date of the real full moon could differ by a day or two.
The full moon that precedes Easter is called the Paschal full moon. There are two factors that determine the Paschal full moon: the Golden Number, and the Epact. Also, during the Julian calendar, Easter was calculated through tables.
The relationship between the moon's phases and the days of the year repeat themselves every 19 years. Thus is it appropriate to associate a number between 1 to 19 each year, and that number is called the Golden Number. The golden number is:
Golden Number = (year % 19) + 1
The phases of the moon fall on (approximately) the same date in two years with the same Golden Number.
The Epact measures the age of the moon, by the number of days that passed since an official new moon on a given date.
The Epact is linked to the Golden number in both the Julian and Gregorian calendar. In the Julian calendar, the 19 months were believed to be exactly an integral number of synodic months, and the following relationship between the two were;
Epact = (11 * (Golden Number - 1)) % 30
When the formula produced 0, the epact was given the symbol * and its value was said to be 30. The reason for this is unknown, it may be as trivial as the people of the past not liking the number 0.
Since there are only 19 possibly Golden numbers, then there are only 19 possible Epacts: 1, 3, 4, 6, 7, 9, 11, 12, 14, 15, 17, 18, 20, 22, 23, 25, 26, 28, and 30.
Since the Julian calendars methods for calculating the moon were inaccurate, the Gregorian calendar made modifications to the simple Julian relationship between the Golden Number and the Epact.
To calculate the Epact in the Gregorian calendar, use the following formula and all divisions are integers, with the remainder discarded:
What was the Epact for 1992?
Golden Number = 1992%19 + 1 = 17
The Epact for 1992 was 25.
| Epact | Full Moon |
| 1 | April 12 |
| 2 | April 11 |
| 3 | April 10 |
| 4 | April 9 |
| 5 | April 8 |
| 6 | April 7 |
| 7 | April 6 |
| 8 | April 5 |
| 9 | April 4 |
| 10 | April 3 |
| 11 | April 2 |
| 12 | April 1 |
| 13 | March 31 |
| 14 | March 30 |
| 15 | March 29 |
| 16 | March 28 |
| 17 | March 27 |
| 18 | March 26 |
| 19 | March 25 |
| 20 | March 24 |
| 21 | March 23 |
| 22 | March 22 |
| 23 | March 21 |
| 24 | April 18 |
| 25 | April 17 or 18 |
| 26 | April 17 |
| 27 | April 16 |
| 28 | April 15 |
| 29 | April 14 |
| 30 | April 13 |
For an Epact of 25, there are two possible dates. There are two equivalent methods to determine the right date:
This method is based in part on the algorithm of Oudin (1940) as quoted in "Explanatory Supplement to the Astronomical Almanac" in an attempt to simplify the calculations of Easter. All divisions are integer, and the remainders are discarded.
G (Golden Number) = year % 19
For the Julian calendar:
- I = (19*G + 15) % 30
- J = (year + year/4 +I) % 7
For the Gregorian calendar:
- C = year/100
- H (Epact 23) = (C - C/4 - (8*C+13)/25 + 19*G + 15) % 30
- I (number of days from March 21 to Paschal full moon) = H - (H/28)*(1 - (H/28)*(29/(H + 1))*((21 - G)/11)
- J (weekday for Paschal full moon) = (year + year/4 + I + 2 - C + C/4) % 7
For both calendars: L (number of days from March 21 to Sunday on or before Paschal full moon) = I - J
- Easter Month = 3 + (L + 40)/44
- Easter Day = L + 28 -31*(EasterMonth/4)
If you know when the Easter of the current year, you can track when the next Easter will probably occur. If Easters date = X, and there is no leap year, then the next Easter will be either: X - 15, X - 8, X + 13 (rare), or X + 20. If there is a leap year, you need to subtract a day, so you get X - 16, X - 9, X + 12 (extremely rare), or X + 20. Since Easter always falls between March 22 and April 25, the date can be easily narrowed down.
In the Julian calendar, the cycle of dates is repeated every 532 years because 532 is the product of the following numbers:
- 19 = The Metonic Cycle, or the cycle of the Golden Number
- 28 = The Solar Cycle
In the Gregorian Calendar, the cycle is repeated every 5,700,000 years, because 5,700,000 is the product of the following numbers:
- 19 = The Metonic Cycle, or the cycle of the Golden Number
- 400 = The Gregorian equivalent of the Solar Cycle
- 25 = The Cycle used in Step 3 of the Epact calculations
- 30 = The Number of different Epact values
The Indiction was used in the middle ages to specify the position of a year on a 15 year taxation cycle. It was introduced by Emperor Constantine the Great on September 1, 312. It may be calculated by the following algorithm:
Indiction = (year + 2) % + 1
The Indiction has no astronomical significance. Also, it didn't always follow the calendar year, and three different indictions are identified:
The Julian Period isn't associated with the Julian calendar, and must not be confused with one another. The Julian Period was created by the French scholar Josef Justus Scaliger (1540-1609). The name Julian Period is probably after the Julian calendar, although it is possible Scaliger named it after his father Julius Caesar Scaliger.
Scaliger's goal was to create a method of determining year with each year being positive, so as not to worry about AD and BC. The period starts at January 1, 4713 BC, and last for 7980 years, where it will repeat on January 1, 3268.
Why January 1, 4713 BC and 7980 years. Because on January 1, 4713 BC, the Indiction, the Golden Number, and the Solar Number all become 1, and this won't repeat for 15 * 19 * 28 (7980) years.
Scientist often use the Julian period for giving a number to each day. 12:00 PM UTC, January 1, 4713 BC for 24 hours marks Julian Day 0, each day that follows adds another number. Therefore, at 12:00 PM UTC, January 1, 2000, the Julian Day would become 2,451,545.
Calculating the Julian Day of 12:00 PM UTC, January 1, 2000 is simple:
- From 4713 BC to 2000 AD, there are 6712 years.
- The year in the Julian calendar is 365.25 days, so 6712 * 365.25 is 2,451,558 days.
- For the Gregorian calendars, subtract 13 days to convert from Julian to Gregorian, so you get 2,451,545
Often the Julian Day is used in fractions to determine the time of day as well.
The Julian Date Number starts at 12:00 PM UTC on the given date.
(The divisions are integers, with the remainders discarded. )
For a date in the Gregorian calendar:
Julian Date Number = day + (306*M+5)/10 +Y*365 + Y/4 - Y/100 + Y/400 + 1721119
For a date in the Julian calendar:
Julian Date Number = day + (306*M+5)/10 +Y*365 + Y/4 + 1721119
This works fine for AD dates. For BC dates, you must convert BC year to a negative year (10 BC = -9), and make sure you round down instead of up (-9/4 = -3, not -2).
Sometimes a Modifies Julian Day number is used in replace of the standard Julian number. In the MJD, 2,400,000.5 days are subtracted to make the number more manageable, and the numbers change at midnight rather than noon. MJD 0 falls on November 17, 1858 of the Gregorian calendar.
Because the Earth's orbit isn't exactly circular, the time between two equinoxes or solstices is slightly longer than a rotation of the earth. As a result, the longitude that the sun is at during these events is slightly different each year, taking 21,000 years to complete the cycle. In the modern calendar, the year is simply viewed as the time between two equinoxes or solstices.
Astronomy is the basis for calendars, and the cycles of the sun and moon are the most important to the creation and understanding of most calendars.
For example, in our modern calendar, a tropical year (a tropical year is the time from one fixed point, such as the solstice and the equinox, to the next) is viewed as one revolution around the sun, which is equivalent to roughly 365.242190 days (this figure varies over time, as the year was 365.242196 days in 1900, an the year will be 365.242184 days in 2100).
A month is equivalent to a revolution to the moon (although our modern calendar rarely follows this pattern, but a month following the moon is called a synodic month). The rotation of the moon is 29.5305889 days (like the sun, this figure varies over time, as the synodic month will be 29.5305886 in 1900, and 29.5305891 days in 2100).
In reality, 29.5305886 is not the length of time for the moon to make one revolution around the earth, it is the length of time between two full moons. This is because the earth is rotating as well, and the moon must travel farther so that it is in the same position relative to the earth. The actual time it takes for one rotation of the moon is 27.32 days. The equation below explains why:
Let the orbital period of the earth be Te = 365.25 days.
The time between full moons is Tf = 29.53 days
The angle (in degrees) that the earth has moved in time Tf is A = 360Tf/Te.
The angle that the moon has moved around the earth in Tf is B = 360 + A (see diagram, parallel lines subtend equal angles).
The orbital period of the moon is Tm, which we want to find. It took the moon Tf to move through an angle B, and it will take the moon Tm to move through a full rotation (360). The rate of angular rotation of the moon is constant, so these ratios are equal:
B/Tf = 360/Tm
So (360+A)/Tf = 360/Tm,
(360 + 360Tf/Te)/Tf = 360/Tm,
(1+Tf/Te)/Tf = 1/Tm
1/Tf+1/Te = 1/Tm
Tm = 1/( 1/Tf + 1/Te)
Tm = 1/(1/29.53+1/365.25) = 27.32 days
However, early calendrologist based the orbits of the moon on the time it took between two full moons / new moons / quarter moons / etc. As a result, the synodic month is based on this length of time, roughly 29.53 days.
Because the length of the tropical year isn't a multiple of the synodic month, the relationship between the sun and the moon can't be maintained. However, nineteen tropical years is equal to 234.997 synodic months, which is almost an integer. Because of leap years, the phases of the moon fall on the same date every nineteen years. This nineteen year cycle is referred to as the Metonic cycles, after the astronomer Meton of 5th century BC Athens.
The Hebrew year has 353, 354, or 355 days during non-leap years. During leap years, they are 383, 384, or 385 days on a year. The three different years are called deficient, regular, and complete.
| Name | Deficient | Regular | Complete |
| Tishri | 30 | 30 | 30 |
| Heshvan | 29 | 29 | 30 |
| Kislev | 29 | 30 | 30 |
| Tevat | 29 | 29 | 29 |
| Shevat | 30 | 30 | 30 |
| Adar I | 30 | 30 | 30 |
| Adar II | 29 | 29 | 29 |
| Nisan | 30 | 30 | 30 |
| Iyar | 29 | 29 | 29 |
| Sivan | 30 | 30 | 30 |
| Tammuz | 29 | 29 | 29 |
| Av | 30 | 30 | 30 |
| Alul |
29 |
29 |
29 |
| Total | 353 or 383 | 354 or 384 | 355 or 385 |
Adar I is the leap month. In non-leap years, there is no Adar I and Adar II is called Adar.
There is a noticeable alteration between the lengths of months, with 29 and 30 days, with the exception of the added day to Heshvan in complete years, and removing a day from Kislev in deficient years. The alteration between 30 and 29 ensures that when the year starts with a new moon, so does each month.
One can determine a leap year by:
If Year (in the Anno Mundi formula) % 19 = 0, 3, 6, 8, 11, 14, or 17
That is determined by when New Years starts at Tishri 1, and the number of days until Tishri 1 of next year determines the year.
Years are taken from the believed creation of the world, which is 3761 BC. This is 1 AM (Anno Mundi = Year of the World).
In the year 2000 AD, the Hebrew year would be 5761 AM.
The Hebrew day starts at sunset, rather than midnight, when 3 stars are visible.
Sunset marks the start of a 12 night hours, while sunrise marks the start of 12 day hours. As a result, depending on the season, the night hours may be longer or shorter than the day hours.
The Hebrew year starts on Tishri 1 (Rosh Hashanah) in compliance with the following rules:
This rule can only come into play if the first year was supposed to start on a Tuesday. Therefore a two day delay is used rather than a one day delay, as the year must not start on a Wednesday (like it can't start on Sunday or Friday as a result of Yom Kippur and Hoshana Rabba).
A calculated new moon is used. The new moon that started the year Anno Mundi 1 was 5 hours and 204 parts (an hour is divided up into 1080 parts), which is just before midnight on October 6, 3761 BC of the Julian calendar. The new moon is found by extrapolating from this time, using the synodic month of 29 days, 12 hours, and 793 parts.
The Islamic calendar is based on the Koran, and its observance is sacred to the Muslim faith.
The Islamic calendar (also known as the Hijri calendar) is a purely lunar calendar. It contains 12 synodic months, which shift with the moon. A synodic month is 29.53 days, and 12 synodic months are 354.36 days. The Islamic calendar is always shorter than the tropical year, so it shifts with respect to the Christian Calendar.
The Islamic calendar is the official calendar of the Gulf nations, especially Saudi Arabia. However, most other Muslim nations use the Gregorian calendar for civil purposes, and only use the Islamic calendar in religious practice.
There are 12 Islamic months that follow the synodic moon. The month officially changes when a human witnesses a lunar crescent after the new moon.
(This spelling can change in the different translations of the months)
Although predicting the new moon is easy, there are numerous factors involved to determine when the first crescent is visible by an observer, such as weather, atmosphere, and the location of the observer. It is therefore difficult to give accurate information in advanced as to when a new moon will start.
Some Muslims determine the new moon by a local observer, while others depend on an authority in the Muslim world. Although both are valid Islamic practices, they can lead to different starting dates for the months.
The Muslim calendar isn't exact, yet predictions are made (the purpose of calendars is to plan) that could be a day early or late.
One technique in predicting the full moon is to have all even numbered months have 29 days, and odd numbered months have 30 days, with an extra day on the last month for leap year (a concept otherwise unknown in the Muslim calendar). The leap years are determined by the following algorithm:
If leap year: Year % 30 = 2, 5, 7, 10, 13, 16, 18, 21, 24, 26, 29
Some calendars use a month of 29.53056 days, which is close to but not exactly a synodic month (29.53059 days).
In the Muslim calendar, years are counted since the Hijra (Mohamad's flight to Medina). It is assumed to have taken place on July 16, 622 AD of the Julian calendar. On this date 1 AH (Anno Hegirae = Year of the Higra) started.
In the year 1997 AD, the Muslims witnessed the start of the Islamic year 1418 AH. Although 1997 - 622 = 1376 years have passed in the Christian calendar, because the Muslims year is 12 synodic months, the year is always shorter than the tropical year by eleven days.
The Mayans created a calendar of extreme accuracy and complexity, which was used by many other mesoamerican civilizations, including the Aztecs and Toltec (They changed the names of the days, months, and years).
The Mayans had three methods of dating, the Long Count, the Tzolkin (divine calendar), and the Haab (civil calendar).
A typical Mayan calendar looks like:
12.18.16.2.6, 3 Cimi 4 Zotz
The Long Count is a mixture of base 20/base 18 of the numbers since the start of the Mayan era, similar to the Julian Day Number.
The final number of the Long Count represents the kin (day). Going from right to left, the components are:
The kin, tun, and katun are numbered 0 to 19
The unial are numbered 0 to 17
The baktun are numbered 0 to 13
Although not officially part of the Long Count, the Mayan had names for other lengths of time:
The alautun is probably the longest named period in any calendar
The Tzolkin is a combination of two week lengths:
0) Ahau
1) Imix
2) Ik
3) Akbal
4) Kan
5) Chicchan
6) Cimi
7) Manik
8) lamat
9) Muluc
10) Oc
11) Chuen
12) Eb
13) Ben
14) Ix
15) Men
16) Cib
17) Caban
18) Etznab
19) Caunac
Both the named week and the smallest figure in the Long count are 20 days in length, and there is synchrony in them. If the last digit of today's Long Count is 0, then the day is Ahau, and so forth.
There are both the numbered week and the named week, both moving at the same pace. So if a day is Oc on the named calendar and 5 on the numbered calendar, the next day would be 6 on the numbered calendar, and Chuen (not Oc again) on the named calendar.
There is a 260 day cycle until the named week and the numbered week have the same day together. This cycle has good-luck or bad-luck associated with each day. The 260 day period is known as the divinatory year.
Authorities agree that 13.0.0.0.0 corresponds to 4 Ahau.
The Habb was the civil calendar. It had 18 months of 20 days each, followed by 5 extra days known as Uayeb. The year was a legnth of 365 days.
The months were:
| 1) Pop | 7) Yaxkin | 13) Mac |
| 2) Uo | 8) Mol | 14) Kankin |
| 3) Zip | 9) Chen | 15) Muan |
| 4) Zotz | 10) Yax | 16) Pax |
| 5) Tzec | 11) Zac | 17) Kayab |
| 6) Xul | 12) Ceh | 18) Cumku |
Each month was 20 days in legnth, with the days numbered from 0 to 19.
The use of a 0 instead of a 1 for the first day is unique in Mayan calendars. The use of the 0 and where it can be put, shows that the Mayans discovered the number 0 centuries before the Europeans.
The 5 Uayeb days were known for bad luck. They were called days without names, or days without souls. People spent the days with prayer and mourning. Fires were exstinguished, and the people refrained from hot food. It was believed that people born on these days were doomed a miserable life.
The athorities agree that 13.0.0.0.0 corresponds to 8 Cumka.
Although there were only 365 days in the Haab year, the Mayas wereaware that a year is slightly longer than 365 days, and in fact, manyof the month-names are associated with the seasons; Yaxkin, forexample, means "new or strong sun" and, at the beginning of the LongCount, Yaxkin 1 was the day after the winter solstice, when the sunstarts to shine for a longer period of time and higher in thesky. When the Long Count was put into motion, it was started at 7.13.0.0.0, and Yaxkin 0 corresponded with Midwinter Day, as it did at13.0.0.0.0 back in 3114 BC. The available evidence indicates that the Mayas estimated that a 365-day year precessed through all the seasonstwice in 7.13.0.0.0 or 1,101,600 days.
We can therefore derive a value for the Mayan estimate of the year bydividing 1,101,600 by 365, subtracting 2, and taking that number anddividing 1,101,600 by the result, which gives us an answer of365.242036 days, which is slightly more accurate than the 365.2425 days of the Gregorian calendar.
(This apparent accuracy could, however, be a simple coincidence. The Mayas estimated that a 365-day year precessed through all the seasons twice in 7.13.0.0.0 days. These numbers are only accurate to 2 to 3 digits. Suppose the 7.13.0.0.0 days had corresponded to 2.001 cycles rather than 2 cycles of the 365-day year, would the Mayas have noticed?)
The origin of the seven day week is a very complex matter. Different authorities say that it may have had one of many possible origins. The only thing we know for certain is that we have no certain knowledge as to where the seven day week came from.
Much is assumed, with no real proof. We know that the first pages of the bible tell us that the earth was created in six days, and god rested on the seventh. Extra biblical locations such as Egypt, Babylon, and Persia could also be the birthplace of the seven day week. Finally, it is known that the Romans had the seven day week before the advent of Christianity.
The meaning of the days is different amongs the different languages. Some nations like to number their days, while others use the planets. For instance:
| English |
Portuguese |
Russian |
Russian Translation |
| Monday | segunda | ponedilnik | After do-nothing day |
| Tuesday | terca | vtornik | Second day |
| Wednesday | quarta | sreda | Center |
| Thursday | quinta | chetverg | Four |
| Friday | sexta | pyatnitsa | Five |
| Saturday | sabado | subbota | Sabbath |
| Sunday | domingo | voskresenya | Resurrection |
Jews number all of their weekdays, except the sabbath.
Some latin-based nations named their days by the planets, as each planet was meant to rule a day.
| English |
French |
Planet |
| Monday | lundi | Moon |
| Tuesday | mardi | Mars |
| Wednesday | mercredi | Mercury |
| Thursday | jeudi | Jupiter |
| Friday | vendredi | Venus |
| Saturday | samedi | Saturn |
| Sunday | dimanche | Sun |
The link with the sun has been broken in French, but Sunday was called dices solis (day of the sun) in Latin.
English has retained the original planets in the name for Saturday, Sunday, and Monday. For the other four days, the names of Anglo-Saxon or Nordic gods were used, in replace of the Roman gods that named them. Tuesday is named after Twiv, Wednesday is named after Woden, Thursday is named after Thor, and Friday is named after Freya.
There is no record whatsoever of the seven day cycle ever being interrupted. Even through calendar changes and reforms, the week remained unbroken. It is most likely that the seven day cycle remained ever since the time of Moses in 1400 BC.
Some sources claimed that the ancient Jews used a calendar with an extra Sabbath was occasionally introduced, but this is probably not true.
For the Jews, the Sabbath (Saturday) is the day of rest and worship. The reason for this is because it is believed that god created the world in six day, and rested on the Sabbath (Saturday).
For most Christians, Sunday is the day of rest and worship. The reason for this is because it is believed that Jesus rose from the dead on Sunday
For Islam, Friday is the day of rest and worship. The reason for this is because Muhammad was born on a Friday.
In Jewish and Christian practices, Sunday is considered the first day. However, the fact that Russia calls Tuesday the second day proves that some nations view Monday as the first day. The International Organization for Standardization has decreed that the Monday shall be the first day of the week.
The International Standard assigns a number to each week in a year. A week that falls between two years is given to the year it had more days in. Therefore:
Week 1 of any year has to contain January 4
or equivalently:
Week 1 of any year contains the first Thursday in January
Most years have 52 weeks. However, some weeks that start on a Thursday (or Wednesday on a leap year) has 53 weeks.
Obviously a week that you considered to be a seven day interval couldn't have a different length. However, if a week is considered a period of time greater than a day and less than a month, there are several examples.