166 calendars and clocks: The Islamic World They compensated for the drift by creating a 30-year cycle during which one day was added to Dhil-Hijjih in the second, fifth, seventh, 10th, 13th, 16th, 18th, 21st, 24th, 26th, and 29th years The years of the Christian calendar sometimes are preceded by the abbreviation a.d., which stands for anno Domini, meaning “in the year of our Lord,” which is itself short for the original phrase, anni Domini nostri Jesu Christi, meaning “in the years of our Lord Jesus Christ.” The years of the Islamic calendar are often preceded by a.h., an abbreviation for anno Hegirae, meaning “in the year of the migration,” referring to Muhammad’s move to Medina The designation a.h probably was not used by Islam in the medieval era but came into use in the Western world to distinguish Islamic dates A few names of the months on the Islamic calendar seem to have come from an older Arabian lunar calendar, now apparently lost Other lunar calendars developed in the Near East occasionally added a month to keep certain months within certain seasons Because Muhammad forbade that common practice, Islamic months drifted 11 days at a time from one year to the next, relative to the solar year, which meant that the months did not correspond to seasons This created a significant problem for farmers, whose work depended on keeping track of the seasons so that they could plant crops when they should and know when harvests should take place Keeping track of rainy and dry seasons was especially important in many areas of the Islamic world The result of this problem was that the Islamic calendar was closely followed in cities, but in rural areas other calendars were followed for everyday life and the Islamic calendar for tracking holidays and religious observances In some parts of the Near East and North Africa people found that the Christian calendar, only about 100 years old, was the easiest to use Elsewhere farmers employed lunar calendars such as those of the Sumerians, Greeks, and Jews The Sumerian year had 360 days divided into 12 months of 30 days each; the Babylonians added the 24-hour day The Greeks had a 354-day calendar to which they added 90 days every eight years The additional 90 days were added haphazardly Many Islamic farmers chose to use calendars similar to the Jewish one, in which months and seasons were linked Developed to meet farmers’ need to track the seasons, the Jewish calendar had a 19-year cycle, with each year having a normal 12-month cycle into which an extra month was added during the third, sixth, eighth, 11th, 14th, 17th, and 19th years The extra month came after Adar, the sixth month of the year, and was called Second Adar Although a given day of the month could drift by about 28 days from one year to the next, days of the month remained roughly in the same season from year to year, enabling most Muslim farmers to track seasons Measuring the hours of the day has always been important to Muslims because the Koran requires them to pray five times a day at certain hours Timepieces therefore became a special interest to Muslim engineers Unfortunately for the modern historian, almost every timing device used in the medieval Islamic world has been lost, with the remains of a couple of water clocks in Morocco being the only exceptions Historians disagree greatly about which cultures invented what device or important enhancement, but there was probably cross-pollination between the Islamic world and China Indian devices most likely became known to the Islamic world when much of India was conquered by Muslims, and Muslim engineers credited ancient Greek scientists for providing essential knowledge in developing timepieces In general there were sundials, water clocks, candle clocks, and possibly mechanical clocks and sand clocks The issue of who created the first mechanical clock is a murky one, with credit usually going to European inventors of the 1300s, although some Islamic water clocks included mechanical works in their movements What is known about Islamic clocks comes mostly from written sources and pictures made of the clocks Many medieval Muslim engineers made their discoveries public or published treatises on how to make devices such as clocks The most important of these writers was Ibn Ismail ibn al-Razzaz al-Jazari (1136–1206), usually called simply al-Jazari He invented or adapted about 60 devices, including water clocks and candle clocks The most important clock for the medieval Islamic world was probably the water clock, also called a clepsydra, a Greek word meaning “steal water.” A water clock uses the regulated flow of a liquid to keep track of time The liquid was usually water but could be other liquids, and late in the medieval era Muslim engineers may have developed a clock that used the flow of mercury The clock that was the most famous in the medieval Islamic world was al-Jazari’s elephant clock, which may have stood five feet high and was made in the shape of an elephant Al-Jazari credited the writings of Archimedes for inspiring his elephant clock, which tracked the daylight hours in 12 parts, regardless of how long the day was Thus in the summer, with its long days, hours on the clock lasted longer than in winter It had two cisterns inside, one above the other, with floats that acted as stoppers on the openings through which water flowed The passage of water from the upper cistern to the lower one regulated the time, while the floats occasionally plugged the holes, thus keeping water pressure steady in the upper tank Every half hour the water in the lower cistern was heavy enough to pull a string, releasing a ball that would fall into the image of a snake, which would tip forward, pulling other strings that moved the clock’s hands By adjusting a flow regulator at the