A calendar is a system of reckoning the time over extended
intervals by combining days into longer groupings which are
linked to the way in which we live. The groupings often have
religious significance and some of the groups are linked to
Our calendar is made up of days, weeks, months and years. The
day is the rotational period of the Earth; the week is a purely
artificial period linked to the Biblical story of creation; the month
is linked to the period of the Moon and the year is linked to the
Earth's period of rotation around the Sun.
The Jewish calendar and the Moslem calendar are intimately
connected with the Moon. The Jewish calendar is now a fixed
calendar with rather complex rules for its construction. The
length of the Jewish year may be 353, 354 or 355 days or 383, 384
or 385 days. Each month has 29 or 30 days.
The Moslem calendar is also a fixed calendar, but the religious
festivals depend on visual sightings of the New Moon. The length
of the Moslem year is 12 months of alternate lengths 30 and 29
days, except for the 12th month which can have either 29 or 30
days. The calendar is kept in adjustment with the Moon using a
cycle of years of different length.
The definition of the day seems very simple. It should be the time
taken for one passage of the Sun across the meridian to its next
crossing. Unfortunately the Earth's movement around the Sun is
not circular and this causes the time indicated by a sundial to be
different from that measured by a clock. (The difference is called
the Equation of Time and is described in a separate pamphlet.)
Nor is the Earth's rotation period constant, although the variations
are very small. So we use a length for the day which represents
the Earth's average rotation period. Occasionally we have to
insert `leap-seconds' to correct from this exact measure to the
slightly irregular rotation of the Earth.
Mosaic law forbidding work on every seventh day established a
seven-day period as a measure of Jewish time. This passed over
into the Christian church and gradually became established in the
Roman calendar. The astrological practice of naming the days
using the names of the Sun, Moon and the five known planets also
yielded a seven day period.
The names that we give to the days of the week are still based on this nomenclature. In English Sunday and Monday are named after the Sun and Moon while the other days of the week are named after the teutonic versions of the gods that correspond to each planet. Tiu = Mars, Woden = Mercury, Thor = Jupiter and Freya = Venus, Saturday keeping its latin connection with Saturn.
Many ancient calendars were based on the lunar cycle. The lunar
month is 29.530589 days and the number of lunations in a year is
12.368267. The most common scheme was to have 12 months of
30 days with either arbitrary or calculated additional days or
months to bring the system into accord with the solar year.
Originally the Roman calendar had 10 months Martius (named
after Mars and indicating the time for resumption of war), Aprilis
(after the word for `to open', hence Spring), Maius (after Maia, the
goddess of growth), Junius (after juvenis, meaning youth),
Quintilius, Sextilis, Septembris, Octobris and Decembris (meaning
the fifth to tenth months). The Romans had a dislike of even
numbers as these symbolized death and so the months had either
29 or 31 days. King Numa increased the number of months to 12
by introducing Januarius (after the god Janus, the protector of
doorways and hence the opening of the year) and Februarius (after
Februalia, the time for sacrifices at the end of the year). The order
of the months was later changed.
In order to reconcile the solar and lunar years,at the
recommendation of Sosigenes Julius Caesar altered the lengths of
the months and the change to our present format was completed
by Caesar Augustus. They both claimed the names of one of the
months and this gave rise to Julius and Augustus which replaced
Quintilius and Sextilis.
The number of days in one revolution of the Earth around the Sun
is 365.2422 days. Unfortunately, this is not a whole number and
so if we wish to keep the calendar in track with the seasons we
must adopt some form of variable year length.
The first major attempt to accomplish this was by Julius Caesar.
Besides pinching a day from the last month of the year (then
February) to make `his' month, July, have 31 days he introduced
the Julian calendar. February was further despoiled by Augustus
Caesar who also purloined a day so as to make `his' month,
August, also have 31 days. As some sort of compensation the
leap-day was given to February.
The Julian calendar introduced a year of length 365 days with a
leap-year every 4 years. It also moved the start of the year from
March 25 to January 1, supposedly the longest day. The adopted
length of the year, 365. 25 days, is only slightly different from the
actual length, 365. 2422 days, but over the centuries the
difference mounts up and by the 16th century had become
noticeable. The effect is to move Spring and the date of Easter,
which is related to the vernal equinox, closer to the date of
The Gregorian reform to the calendar altered the rule for
determining if a year should be a leap-year by stating that
centenary years should only be leap-years if they were divisible by
400. It also dropped several days from the calendar so that the
vernal equinox was brought back to March 21. The mean length
of the calendar year is now 365. 2425 days and the error
compared with the true value amounts to only 3 days in 10,000
The year is defined as being the interval between two successive
passages of the Sun through the vernal equinox. Of course, what
is really occurring is that the Earth is going around the Sun but it
is easier to understand what is happening by considering the
apparent motion of the Sun in the sky.
The vernal equinox is the instant when the Sun is above the
Earth's equator while going from the south to the north. It is the
time which astronomers take as the definition of the beginning of
Spring.The year as defined above is called the tropical year and it
is the year length that defines the repetition of the seasons. The
length of the tropical year is 365.24219 days.
In 46 BC Julius Caesar established the Julian calendar which was
used in the west until 1582. In the Julian calendar each year
contained 12 months and there were an average of 365.25 days in
a year. This was achieved by having three years containing 365
days and one year containing 366 days. (In fact the leap years
were not correctly inserted until 8 AD).
The discrepancy between the actual length of the year, 365.24219
days, and the adopted length, 365.25 days, may not seem
important but over hundreds of years the difference becomes
obvious. The reason for this is that the seasons, which depend on
the date in the tropical year, were getting progressively out of
kilter with the calendar date. Pope Gregory XIII, in 1582,
instituted the Gregorian calendar, which has been used since then.
The change from the Julian calendar to the Gregorian involved the
change of the simple rule for leap-years to the more complex one
in which century years should only be leap-years if they were
divisible by 400. For example, 1700,1800 and 1900 are not
leap-years whereas 2000 will be.
The net effect is to make the adopted average length of the year
365.2425 days. The difference between this and the true length
will not have a serious effect for many thousands of years. (The
error amounts to about 3 days in 10,000 years.)
The adoption of the Gregorian calendar was made in Catholic
countries in 1582 with the elimination of 10 days, October the 4th
being followed by October 15th. The Gregorian calendar also
stipulated that the year should start on January 1. In non-Catholic
countries the change was made later; Britain and her colonies
made the change in 1752 when September 2nd was followed by
September 14 and New Year's Day was changed from March 25
to January 1.
A millennium is a period of 1000 years. The question of which
year is the first year of the millennium hinges on the date of the
first year AD.
Unfortunately the sequence of years going from BC to AD does
not include a Year 0. The sequence of years runs 3 BC, 2 BC, 1
BC, 1 AD, 2 AD, 3 AD etc. This means that the first year of the
first millennium was 1 AD. The one thousandth year was 1000
AD and the first day of the second millennium was 1001 AD.
It is thus clear that the start of the new millennium will be 1 Jan
The year 2000 AD will certainly be celebrated, as is natural for a year with such a round number but, accurately speaking, we will be celebrating the 2000th year or the last year of the millennium, not the start of the new millennium. Whether this will be an excuse for more celebrations in the following year will have to be seen!