The Captive Moon - Part 3 Origins of Modern Calendar
The Roman Calendar
By 47BC the lunar calendar of the Roman Empire was out of step with the solar year by three months. Julius Caesar appointed the Greek astronomer Sosigenes to sort it out. the next year - 46BC the new calendar was introduced. Julius inserted not only the usual extra 23 days but two more months. So the year was 445 days long. It became known as "The Year of Confusion".
From then on, the current calendar now accepted intern ationally had come into use. And it explains why September, October, November and December whose names derive from Seven, Eight, Nine and Ten, are actually the ninth, tenth, eleventh and twelfth months.
Our months date from this time when they were changed from the lunar months into divisions of the solar year.
The Romans had three reference dates in the month:
- The first day was Kalends
- The 4th or 5th day was Nones
- and the 13th or 15th day was Ides.
The other days were numbered backwards from them. In the Julian leap year, the extra day was not placed at the end of February, as we do now, but between the 24th and 25th which was the 7th and 5th days before the Kalends of Mars. the extra day became the extra 6th day before Kalends.
Fixing Christmas
The Emperor Constantine combined Christianity - a popular cult amongst the civil service, and the Sun God religion popular with the Roman army, in his ambition to create a re-united Roman Empire with himself as Emperor. His claim to see a vision of a cross on the Sun's disk could have meant something to either cult. Symbols and festivals merged into the new state religion. Christmas was taken from the Sol Invicta (Unconquered Sun) religion was supposed to be the winter solstice. The 25th March was fixed as the Spring Equinox. Both dates were not exact astronomically. There is more separately on the Guardian of the Sun, World-Surveyor-Man.
Minutes and seconds come from the Roman - pars minuta (small part) and pars secunda -(second part) They also used parte tertia etc. These were still used in the 16th century.
Fixing the actual date of the birth of Christ and numbering the years from this as the Christian Era was invented by the Scythian (from South Russia, or south Siberia) monk Dionysius Exiguus (Dennis the Little). In 532 AD he was in Rome trying to calculate the date for Easter - a contentious issue for the church at the time. He calculated that Christ's birth must have occurred in 753 A.U.C. that is 753 years from the founding of Roman (Ab Urbe Condita). He called the years after Anno ab Incarnatione (Year of Incarnation).
AD (Anno Domini) and the use of the century as a unit were not invented until the 16th century. the use of BC came about in the 17th century. More recently BCE (before the Christian Era) is more acceptable to the majority who are not Christian, and BP (before present) is more relevant to archaeologists and geologists.
Dennis the Little's years had been counted in 15 year cycles called the Indiction, which was based on the Roman Tax system which had begun under the Emperor Diocletian on 1st September 297 AD. 1992 was 15, 1993 was 1, 1999 was 7, The year 2000 or Y2K is 8, 2001 is 9, etc.
The Julian calendar of 365.25 days to a year was too long by 11 minutes and 14 seconds. So over the centuries the seasons shifted until the Equinox, when the legal and administrative year began, was on 11th March in 1582. In this year Pope Gregory XIII introduced a new calendar, decreeing that the 4th October would be followed by the 15th October.
Countries with other state religions were reluctant to adopt a Catholic idea. In England the Gregorian calendar was not introduced until 1752. This caused a lot of controversy, upset and even riots, with some deaths in London and Bristol, as workers demanded the 11 days wages they had lost. It is a bother for historians especially as although New Years Day was January 1st, years tended to be numbered from April to April - still the financial and administrative year. And although this calendar system is now global, countries adopted it at different times.
Russia did not use this calendar until 1924. The Russian calendar used letters for numbers with a ~ over the top. The years were dated from the Creation.
Astronomers' Time
The solar year is the time taken for the Earth to make a complete orbit of the sun from noon - 00.00 hours on March 21st the Spring Equinox, back again.
The sidereal year is measured by the stars. Each day as measured by the stars is nearly 4 minutes (3 minutes, 56.5 seconds) shorter than the solar day. So after one year the sidereal year is a day behind the solar year. This is corrected on September 21st, the autumn Equinox. The sidereal year is measured from September 21st, the autumn equinox to the next September 21st, from 00.00 hours, noon.
Ephemeris time is used to provide accurate timekeeping with reference to the positions of the moon, planets, Jupiter's satellites, etc. An Ephemeris is a table of coordinates which enables the prediction of position and movements of celestial objects.
Ephemeris time is measured by the most efficient clocks available.
Since 1984, the precision of clocks has replaced ephemeris time. This is dynamical time, produced by atomic clocks. The most accurate is the hydrogen maser. Rubidium clocks are the cheapest and used most in industry.
The international time standard is set by the Caesium clock, which uses Caesium 133. In 1967 the standard international time unit was set as the hyperfine transition from the ground state upwards of the electrons in a caesium atom which is about 9.92 giga Hertz.
Dates are written with the year first, then month, day, hour, minute, second. or the decimal fraction of the day: e.g. 2001 January 1d 2h 34m 4.8s or 2001 January 1.107.
Astronomers use the Julian Date. This was invented by Joseph Scaliger, a 16th century French scholar, who named it after his father whose name was Julius Caesar Scaliger. They were friends of Nostradamus.
It is a cycle of time. Scaliger calculated the least common multiple of the 218-year solar cycle, the 19-year Metonic cycle (of the Moon's orbit) and the Roman indiction (based on the Roman Tax system which began with the Emperor Diocletian, on 1st September 297AD). This came to 7,980 Julian years (that is of the Roman Emperor Julius Caesar) of 365.25 days each. Scaliger thought this long enough to cover all human history from the beginning.
The epoch of the Julian Era is the year when the beginning of the four cycles coincided, which is 4713BC. The Julian Day adopted by astronomers began at noon GMT on January 0, 4713BC. The Julian count begins again on January 0, 3267. So...
The Julian date (JD) counts the number of days that have elapsed since Greenwich noon on 4713BC January 1. Any time less than 12h (0.5d) belongs to the Julian day preceding the civil date.
The standard epoch is a set date and time used for comparing star coordinates and other data. Since 1984 the standard epoch has been 2000 January 1.5, or J2000.0 which is the Julian epoch based on the Julian year of exactly 365.25 days. It is exactly one Julian century (36525 days) removed from the standard epoch of 1900 January 0.5.
- Return to first page on the moon
- return to previous page on different calendars.
Copyright Heather Hobden. Much of the information originally published in Clocks Magazine in the 1980s, and updated.
Website written, designed and maintained by Heather Hobden The Cosmic Elk 