What is a calendar?
A calendar is a system used to measure and organize periods of time. Using a calendar, it is possible to identify the date of a past or future event, as well as schedule meetings, holidays, and due dates. The term "date" refers to the identifying information for a particular day. For the modern calendar used in the United States and most of the rest of the world, a complete date consists of the month, day of the month, and year. Most calendars are devised to correspond with the cycles of the sun or the moon. The Gregorian Calendar, the most widely used calendar in the world today, is based on the solar year, or the length of time it takes the earth to complete a single orbit around the sun.
What is the origin of the word "calendar"?
The English word "calendar" comes from the Latin word "kalendae," which means "the first day of the month." In turn, the word "kalendae" is derived from the Latin word for accounting book, "kalendrium." This makes a great deal of sense when one considers that, in Roman times, bills, or "accounts," were due on the first day of the month.
What is the Gregorian Calendar?
The Gregorian Calendar is the formal name of the modern calendar that today is almost universally accepted as the official calendar by the international community. It is a solar calendar based on the earth's movement around the sun and first came into use through a papal bull, or decree, by Pope Gregory XIII in 1582. Although most Catholic nations adopted the Gregorian calendar shortly after its introduction, in some cases it took centuries for Protestant and non-Western nations to adopt this new method of telling time. The British Empire, including the colonies that later became the United States did not adopt the Gregorian Calendar until the 1750s. Russian adopted the Gregorian Calendar in 1918, Turkey in1926, and the People's Republic of China in 1929.
What is a solar year?
A solar year is a measurement based on the length of time it takes for the Earth to complete one full orbit or revolution around the sun. A solar year is approximately 365 ¼ days long.
What is a tropical year?
From the vantage point of Earth, the sun appears to move through the sky and appear in different positions as the seasons pass. A tropical year is the length of time that it takes for the sun to move through the sky and return to its starting position. Sometimes the tropical year is defined as the length of time from one vernal equinox (the first day of spring) to the next. The tropical year is roughly equivalent to the solar year, the time that it takes the earth to make one revolution around the sun, though the two measurements can vary in length from fractions of a second to a minute or two. This is largely due to the fact that the length of the tropical year will differ slightly depending upon the point in time and the geographical location from which it is measured. Prior to the Renaissance, it was believed that the sun traveled around the earth, rather than vice versa. Therefore, early calendars based on the cycle of the sun were, strictly speaking, based on the tropical year, rather than the solar year.
What is a lunar year?
A lunar year is the equivalent of twelve lunar cycles, a lunar cycle being the time it takes for the moon to travel all its phases from full moon to quarter moon to crescent moon to new moon and back again to full moon. Lunar calendars are based on the lunar year. However, a lunar year and a solar year are not equivalent in length. A lunar year is approximately 354 days long, while a solar year is approximately 365 days long. Thus, most lunar calendars include adjustments so that the length of the calendar year ultimately corresponds, more or less, with the solar year. Because the lunar year is 11 days shorter than the solar year, a "pure" lunar calendar, one that does not make such an adjustment, has free-floating seasons; in other words, over a period of years, the different seasons of the year will no longer correspond to specific months on the calendar.
What are equinoxes and solstices?
There are two equinoxes and two solstices per calendar year. The vernal equinox and the autumnal equinox are the two days of the year on which the hours of daylight and the hours of nighttime are equal. The summer solstice is the day of the year which has the most hours of sunlight, while the winner solstice is the day of the year which has the least hours of sunlight. The vernal equinox occurs every year on March 20 or 21 and marks the first day of spring. Each day after the vernal equinox, the day gets slightly longer until the summer solstice, the longest day of the year and the first day of summer, which occurs on or around June 21. The summer solstice is the peak of daylight hours; the hours of daylight get shorter and shorter each day after the summer solstice. Three months after the summer solstice, on or around September 22, the hours of day and night again equalize. This event is known as the autumnal equinox and is considered the start of the season known as fall. The day continues to shorten until the winter solstice on or around December 21, the first day of winter. After the winter solstice, the days grow gradually longer leading to the next vernal equinox and the next summer solstice.
How many months, weeks, and days are there in a year?
The modern, or Gregorian, calendar is based on a unit of time known as a "year," which roughly corresponds to the solar year, or the amount of time that it takes for the earth to complete one full revolution around the sun. There are 12 months, 52 weeks, and 365 days in a year.
Why doesn't a particular date, such as December 1st, fall on the same day of the week every year?
A week consists of seven days, and there are 52 weeks in a year. However, 52 weeks of seven days each equals only 364 days, and there are 365 days in a year. Therefore, if December 1st falls on a Monday one year, it will fall on a Tuesday the next year.
What is a leap year and why do we have one?
Every fourth year is a leap year. A leap year has an extra day, February 29, a date that occurs only once every four years. Thus, while most years have 365 days, a leap year has 366 days. We need leap years in order to keep our calendar synchronized with the cycle of the earth's revolutions. Although a non-leap year, or a "common" year, has 365 days, it actually takes the earth 365 ¼ days to travel all the way around the sun. That means that every year, our calendar gets one-quarter of a day behind. Every four years, in a leap year, we add the extra quarter days together and add one whole day to the calendar. That way, the calendar "catches up so that the earth's revolution around the sun and the calendar year remain in synch with one another.
What do the abbreviations B.C. and A.D. stand for?
"B.C." stands for "Before Christ" and "A.D." stands for Anno Domini, which, in Latin, means "The Year of Our Lord." These terms are used to classify dates under both the Julian and Gregorian Calendars. Under the Anno Domini system, the birth of Christ is used to designate the start of what we think of as the modern era. The year of Christ's birth is therefore 1 A.D. and the year before is birth is 1 B.C. There is no year 0 B.C. or 0 A.D.
When dealing with B.C. dates, the larger the number used, the further back in time the date is. When we say that an event occurred in the year 2000 B.C., we mean that it took place 1000 years before the historical birth of Christ. An event that occurred in 1000 B.C. predates an event from 200 B.C. which would have taken place only 100 years before Christ's birth. A.D. dates, on the other hand, get larger as time advances. The year 2000 A.D., which stands for 2000 years after the birth of Christ is a more recent date than 200 A.D. Today, we commonly leave the "A.D." off of modern dates and simply write 2000 A.D. as "2000."
Interestingly, most biblical scholars and historians believe that the Anno Domini system is inaccurate because Christ was almost certainly born before the year 1 A.D., sometime in the period between the years 7 B.C. and 4 B.C.
What does the abbreviations B.C.E. stand for?
The initials "B.C.E." can stand for "Before the Christian Era," "Before the Common Era," or "Before the Current Era." In recent years, there has been a move by many scholars and commentators to use the term "B.C.E." in place of "B.C." and the term "C.E." in place of "A.D." This trend represents a desire to show sensitivity and inclusiveness towards peoples of all religions by avoiding references to Christ in the modern dating system.
What is the Roman Calendar?
The term "Roman Calendar" refers to the calendar used by the Romans during the time of the Roman Empire up until the year 46 B.C., when the Julian Calendar came into use. The early Roman calendar had ten months and 304 days, with an extra 61 days that occurred in the middle of the winter and were not assigned to any particular month. It is believed that this winter period was not counted because the Roman Calendar was based on the agricultural season, beginning in the spring with the month of Martius (or March) and ending with December, when the autumn planting was completed. Because nothing of agricultural significance occurred in the 61 days between the end of December and the start of March, those days did not "count" and were not included on the calendar.
In the early eighth-century B.C., Numa, the second king of Rome, revised the calendar by adding two months, January and February, making a twelve-month year. However, while January and February are the first two months in the modern Gregorian calendar, they were the last two months of the Roman calendar, and the new year began on March 1.
What does the term "The Ides of March" mean?
In the Shakespearean play Julius Caesar, Caesar is warned by a fortune-teller to beware the Ides of March. In the Roman Calendar, the "Ides" simply referred to the fifteenth day of a month. Thus, the Ides of March is just another way of saying "March 15th." In Roman times, festivities were held in honor of Mars, god of war (for whom the month Martius or March was named). Originally, the Roman calendar was based on the lunar cycle, and the Ides was intended to correspond with the monthly full moon, which was considered to augur good fortune. In addition to the Ides, the Romans also made special note of the Kalends, or the first day of the month, and the Nones, which was intended to correspond with the new moon in the lunar cycle and fell on either the fifth or seventh day of the month.
What is the Julian Calendar?
The Julian Calendar was introduced in the Roman Empire in 46 B.C. by Julius Caesar and takes its name from him. It was meant as a reform or improvement on its predecessor, the Roman Calendar as revised by Numa. In contrast to the Roman Calendar, which was based on the lunar cycle, the Julian Calendar is based on the tropical year.
Because the Roman Calendar was based on the lunar year, it required the addition of what is known as an "intercalary month" in order to keep the calendar in line with the earth's revolution around the sun and the passage of the seasons. Although Numa had replaced the 61 unassigned days from the early Roman Calendar with the months of January and February, it was still necessary, every other year, to insert an intercalary month of 27 days between the months of February and March. In essence, the Roman Calendar had a leap year every other year, but, instead of having a "leap day" as in the Gregorian Calendar, a Roman leap year had an entire "leap month."
The problem was that, under Roman law, an intercalary, or "leap" month could not be inserted into the calendar without the official authorization of the College of Pontiffs, an official body of the Roman government. On a number of occasions, especially during times of civil strife in the Roman Empire, this authorization was never given. After a number of missed intercalary months, the Roman Calendar no longer corresponded with the seasons, which was a disaster from an agricultural point of view. The Julian Calendar did away with this problem entirely by lengthening the year to 365 days and including a leap day, which fell between February 23 and February 24, every four years.
Is the Julian Calendar still in use?
Over the last 400 to 500 years, the Julian Calendar has been almost completely replaced by the Gregorian Calendar, although some countries did not switch to the Gregorian method until well into the twentieth century. However, the Julian Calendar is still used by some of the Berbers who inhabit North Africa, and although the Gregorian Calendar is used in the civil context, the Russian Orthodox Church and some other Eastern Orthodox Churches still use the Julian Calendar for religious dates.
What is the Solar Cycle?
Under the Julian Calendar, the Solar Cycle is a 28-year cycle relating to the correspondence between dates and days of the week. All the dates in a Julian calendar year would fall on exactly the same days of the week in a cycle that repeated every 28 years. The same correspondence of dates with days of the week occurs every 28 years rather than every seven years because of the introduction of the leap year every four years. A similar cycle exists with the Gregorian Calendar, but it takes 400 years because the Gregorian Calendar includes years that are numerically divisible by four which are not leap years (1800 and 1900 are examples). These years have the effect of interrupting and prolonging the cycle.
What is the origin of the names of the months?
The names of the months in the Gregorian Calendar derive from the month names of the Roman Calendar. In the earliest Roman Calendar, there were only ten months: Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November, and December. January and February were added later by Numa, second King of Rome.
What do the names of the days of the week mean?
The names of the days of the week derive their names from the names of the heavenly bodies, including the planets Mercury, Venus, Mars, Jupiter, and Saturn, as well as the sun and moon.
Did New Year's Day always take place on January 1?
There was no month of January on the initial Roman Calendar, which started on March 1. When January and February were added to the Roman Calendar, they were actually the last two months of the year. However, when the Julian Calendar was introduced, the calendar was modified so that the year started on January 1. Several hundred years later, though, the Roman Catholic Church called for a restoration of March 1 as the first day of the new year. During medieval times, several different dates were used to mark the start of the yearly calendar, including January 1, March 1, March 25, and December 25. In the Byzantine Empire, the new year began on September 1. From 1600 onward, most calendars were standardized so that January 1 was the first day of the year, although two notable holdouts, England and Italy, celebrated the new year in March until the mid-eighteenth century. In addition, although China has officially adopted the Gregorian Calendar, the traditional Chinese lunar New Year takes place in February, and the Jews celebrate Rosh Hashanah, Jewish New Year according to the Hebrew calendar in mid-September.
How is the date of Easter calculated?
Easter Sunday falls on the first Sunday following the first full moon after the vernal equinox, the first day of spring. Thus, the date of Easter varies from year to year depending on the timing of the phases of the moon with respect to the vernal equinox. There is a special term for the first full moon after the vernal equinox: the Paschal full moon. The date of Easter as celebrated by the Greek Orthodox Church often differs from the date celebrated by Roman Catholics and Protestant denominations. This divergence occurs because the Greek Orthodox Church uses the Julian Calendar, rather than the Gregorian Calendar, to calculate the date. Easter can occur on any date that falls on a Sunday between March 22 and April 25. However, because dates on the Gregorian Calendar and the Julian Calendar do not exactly correspond, dates that fall between March 22 and April 25 on the Gregorian Calendar fall between April 4 and May 8 on the Julian Calendar. Thus, Greek Orthodox Easter can occur as late as May 8 under the Gregorian Calendar. Although the dates for Easter vary from year to year, the sequence of dates repeats, albeit over the course of many, many years. Under the Julian Calendar, the sequence of Easter dates repeats every 532 years. However, under the Gregorian Calendar, the sequence of Easter dates takes an astounding 5,700,000 to repeat.
What is the Hebrew Calendar?
The Hebrew Calendar is the official calendar for the nation of Israel and is used by Jews outside of Israel to calculate the dates of religious holidays. It dates back to the year 359 A.D. The Hebrew year, like the Julian and Gregorian year, is based on the tropical year. However, the months in the Hebrew Calendar are based on the lunar cycle. There are 12 months to a year in the Hebrew Calendar, corresponding to the twelve complete lunar cycles that occur within the space of a tropical year. However, reconciling the solar and lunar aspects of the Hebrew Calendar requires the use of a leap year which includes a thirteenth month. The rules that govern the Hebrew Calendar are particularly complicated because the years vary in length A non-leap year can have 353, 354, or 355 days, while a leap year can have 383, 384, 385. Leap years occur every two or three years, and a total of seven leap years occur during every period of 19 years. The first day of each month on the Hebrew Calendar falls on or very close to the new moon. Years under the Hebrew Calendar system are not counted from the birth of Christ or the beginning of the Current Era. Instead of the Anno Domini system, Hebrew dating uses the Anno Mundi (In the Year of the World) system, and official Hebrew dates are followed by the abbreviation A.M., rather than A.D. The reference date for the Hebrew Calendar is the date traditionally believed to be the date of creation in Judaism, which, rendered in Gregorian terms is approximately 3761 B.C, and the first year in the Hebrew system is the year before creation. Thus, the earliest Hebrew date is given as 1 Tishrei 1 AM, the first day of the first month in the year before the date of creation.
What is the Islamic Calendar?
The Islamic Calendar, also known as the Hijiri Calendar, is the only purely lunar calendar in wide modern use. There are twelve months to each year on the Islamic Calendar, and each month starts on the day of the first sighting of the crescent moon, or hilal, in the sky after sunset. Days under the Islamic Calendar start at sunset rather than at midnight. So if the hilal is spotted in the sky just after sunset, the day that has just begun is the first day of the month. Months in the Islamic Calendar have 29 or 30 days depending on when the hilal is first sighted. Thus, it is not certain at the start of a month whether that particular month will have 29 or 30 days. That is only determined once the hilal is sighted, and such a sighting must be officially verified and acknowledged. Each Islamic state that follows the Islamic Calendar makes its own official monthly declaration regarding the sighting of the hilal and the start of the new month. If, due to weather conditions of poor visibility, the hilal is not spotted shortly after sunset following the thirtieth day of the month, the Islamic Calendar will move on to the next month rather than extend the previous month to 31 days. Because the Islamic Calendar is a lunar calendar, rather than a solar calendar, it is approximately 11 days shorter than the Gregorian Calendar. Over a period of years, therefore, it falls out of synch with the calendar system used by most of the rest of the modern world. It also falls out of synch with the seasons, and thus has no practical use in agriculture.
What is the Maya Calendar?
As early as the sixth century B.C., the Maya of Mexico and Guatemala developed a very sophisticated calendar system which became the model for calendars used by peoples and cultures throughout the ancient Americas. In actuality, the Maya Calendar is three separate calendar systems that operated both separately and in tandem. Perhaps the most important of these calendars to Maya religion and culture, and the oldest of the three, was the Tzolkin, which was based on a period of 260 days and used for religious and ceremonial purposes. The Haab, a secular calendar, was based on the 365 day tropical year. It consisted of 18 months of twenty days each, followed by a period of five "nameless days." This five-day period was known as the Wayeb and was considered a potentially dangerous time when the barriers between the realms of the living and dead were opened. The Maya combined the Tzolkin and the Haab into a single system known as a Calendar Round, which paired Tzolkin dates with Haab dates. Because the Tzolkin and the Haab were of unequal lengths, it took 52 years for the sequence of 18980 possible paired Tzolkin and Haab dates to complete itself. At that point, the 52-year sequence would begin anew.
The Maya did not "count" years, but because life expectancy in Maya civilization was significantly lower than 52 years, the sequence created by the Calendar Round generally sufficed for the purpose of dating events within the lifetime of the average Maya individual. For events further away on the time horizon, the Maya had a third calendar known as The Long Count which measured time from the start of the Maya era, which the Maya believed to be the date of creation, roughly 3114 B.C. under the Gregorian Calendar. Unlike the Tzolkin and the Haab, the Long Count is non-repeating, and it uses a base of twenty rather than the base of ten that is commonly used in modern calendars and mathematical systems.
The Long Count is divided into periods of time that include the kin, roughly equivalent to a day; the winal, equal to 20 days; the tun, which consists of 360 days; the katun, which is slightly less than 20 solar years; and the baktun, equal to 144,000 days or a little more than 394 years. The Long Count is the source of the belief, embraced in some New Age circles, that the year 2012 will witness a cataclysmic event that will perhaps signal the end of civilization. The Maya believed that the date of creation of the world they inhabited was preceded by the destruction of an earlier created state. That earlier creation met its end on a Long Count date of 12.19.19.17.19. A Long Count date of 12.19.19.17.19 will also occur on December 21, 2012, marking the beginning of a new baktun, the fourteenth since the start of the Maya calendar. This coincidence of dates has led some to theorize that the Maya predicted thousands of years ago that civilization as we know it would end in the year 2012. However, scholars point out that there is absolutely no evidence that the Maya believed that the end of the world was coming in 2012 and that they were more inclined to equate the end of one cycle of the Long Count and the beginning of another as a cause of celebration than a precursor of disaster.