agriculture in the 3rd millennium BC, people relied heavily on the cycle of the seasons for planting and harvesting crops. Most humans came to live in settled societies and the whole community relied upon accurate predictions of the seasonal cycle. This led to the development of calendars. Over time, some people came to recognize patterns of the stars with the seasons. Learning astronomy became an assigned duty for certain people so they could coordinate the lunar and solar calendars by adding days or months to the year. At about the same time, sundials were developed, likely marked first at noon, sunrise and sunset. In ancient Sumer and Egypt, numbers were soon used to divide the day into 12 hours; the night was similarly divided. In Egypt there is not as much seasonal variation in the length of the day, but those further from the equator would need to make many more modifications in calibrating their sundials to deal with these differences. Ancient traditions did not begin the day at midnight, some starting at dawn instead, others at dusk (both being more obvious). Since a sundial has only one "hand," a minute probably only meant "a short time." It took centuries for technology to make measurements precise enough for minutes (and later seconds) to become fixed meaningful units—longer still for milliseconds, nanoseconds, and further subdivisions. When the water clock was invented, time could also be measured at night—though there was significant variation in flow rate and less accuracy and precision. With water clocks, and also candle clocks, modifications were made to have them make sounds on a regular basis. With the invention of the hourglass (perhaps as early as the 11th century), hours and units of time smaller than an hour could be measured much more reliably than with water clocks and candle clocks. The earliest reasonably accurate mechanical clocks are the 13th century tower clocks probably developed for (and perhaps by) monks in Northern Italy. Using gears and gradually falling weights, these were adjusted to conform with canonical hours—which varied with the length of the day. As these were used primarily to ring bells for prayer, the clock dial likely only came later. When dials were eventually incorporated into clocks, they were analogous to the dials on sundials, and, like a sundial, the clocks themselves had only one hand. A possible explanation for the shift from having the first hour