General
Patterns of Moon, Patterns of Sun
Taken from Saudi Aramco World Magazine
The hijri calender
In AD 638, six years after the death of the Prophet Muhammad,
Islam's second caliph Umar recognised the necessity of a
calendar to govern the affairs of the Muslims.
This was first of all a practical matter. Correspondence
with military and civilian officials in the newly conquered
lands had to be dated. But Persia used a different calendar
from Syria, where the caliphate was based; Egypt used yet
another. Each of these calendars had a different starting
point, or epoch. The Sasanids, the ruling dynasty of Persia,
used June 16, AD 632, the date of the accession of the last
Sasanid monarch, Yazdagird III. Syria, which until the Muslim
conquest was part of the Byzantine Empire, used a form of
the Roman Julian calendar, with an epoch of October 1,312
BC. Egypt used the Coptic calendar, with an epoch of August
29, AD 284. Although all were solar, and hence geared to
the seasons and containing 365 days, each also had a different
system for periodically adding days to compensate for the
fact that the true length of the solar year is not 365 but
365.2422 days.
In pre-Islamic Arabia, various other systems of measuring
time had been used. In South Arabia, some calendars apparently
were lunar, while others were lunisolar, using months based
on the phases of the moon but intercalating days outside
the lunar cycle to synchronize the calendar with the seasons.
On the eve of islam, the Himyarites appear to have used
a calendar based on the Julian form, but with an epoch of
110 BC. In central Arabia, the course of the year was charted
by the position of the stars relative to the horizon at
sunset or sunrise, dividing the ecliptic into 28 equal parts
corresponding to the location of the moon on each successive
night of the month. The names of the months in that calendar
have continued in the Islamic calendar to this day and would
seem to indicate that, before Islam, some sort of lunisolar
calendar was in use, though it is not known to have had
an epoch other than memorable local events.
There were two other reasons Ullman rejected existing solar
calendars. The turban, in Chapter 10, Verse .5, states that
time should be reckoned by the moon. Not only that, calendars
used by the Persians, Syrians and Egyptians were identified
with other religions and cultures. He therefore decided
to create a calendar specifically for the Muslim community.
It would be lunar, and it would have 12 months, each with
29 or 30 days.
This gives the lunar year 354 days, 11 days fewer than
the solar year. Umar chose as the epoch for the new Muslim
calendar the hijrah, the emigration of the Prophet Muhammad
and 70 Muslims from Makkah to Madinah, where Muslims first
attained religious and political autonomy. The hijrah thus
occurred on 1 Muharram 1 according to the Islamic calendar,
which was named "hijri" after its epoch. (This
date corresponds to July 16, AD 622 on the Gregorian calendar).
Today in the West, it is customary, when writing hijri dates,
to use the abbreviation AH, which stands for the Latin anno
hegirae, year of the hijrah.''
Because the Islamic lunar calendar is 11 days shorter than
the solar, it is therefore not synchronized to the seasons.
Its festivals, which fall on the same days of the same lunar
months each year, make the round of the sea- sons every
33 solar years. This 1 l-day difference between the lunar
and the solar year accounts for the difficulty of converting
dates from one system to the other.
The Gregorian calendar
The early calendar of the Roman Empire was lunisolar, containing
355 days divided into 12 months beginning on January 1.
To keep it more or less in accord with the actual solar
year, a month was added every two years. The system for
doing so was complex, and cumulative errors gradually misaligned
it with the seasons. By 46 BC, it was some three months
out of alignment, and Julius Caesar oversaw its reform.
Consulting Greek astronomers in Alexandria, he created a
solar calendar in which one day was added to February every
fourth year, effectively compensating for the solar year's
length of 36.5.2422 days. This Julian calendar was used
throughout Europe until AD 1.582.
In the Middle Ages, the Christian liturgical calendar was
grafted onto the Julian one, and the computation of lunar
festivals like Easter, which falls on the first Sunday after
the first full moon after the spring equinox, exercised
some of the best minds in Christendom. The use of the epoch
AD 1 dates from the sixth century, but did not become common
until the 10th. Because the zero had not yet reached the
West from Islamic lands, a year was lost between 1 BC and
AD 1.
The Julian year was nonetheless 11 minutes and 14 seconds
too long.
By the early 16th century, due to the accumulated error,
the spring equinox was falling on March 11 rather than where
it should, on March 21. Copernicus, Christophorus Clavius
and the physician Aloysius Lilius provided the calculations,
and in 1582 Pope Gregory XIII ordered that Thursday, October
4, 1582 would be followed by Friday, October 15, 1582. Most
Catholic countries accepted the new Gregorian calendar,
but it was not adopted in England and the Americas until
the 18th century. It's use is now almost universal worldwide.
The Gregorian years is nonetheless 25.96 seconds ahead of
the solar year, which by the year 4909 will add up to an
extra day.
Converting Dates
The following equations convert roughly from Gregorian to
hijri and vice versa. However, the results can be slightly
misleading: They tell you only the year in which the other
calendar's year began. For example, 2007 Gregorian spans
both 1427 and 1428, but the equation tells you that 2007
"equals" 1428, when in fact 1428 merely began
during 2008.
Gregorian year = [(32x hijri year)/33] + 622 hijri year
= [(Gregorian year - 622) x33] / 32
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