Adam Hart-Davis

BIOLOGICAL CLOCKS

ANIMALS FOLLOW CONSISTENT PATTERNS of behaviour when it comes to eating, sleeping, mating, hibernating, and migrating. Bears and squirrels know when to hibernate and when to wake up; deer have clock telling them to mate in the autumn. Women have monthly clocks that control their menstrual cycles, and all humans have high-speed clocks in the cerebral cortex that tick 100 times a second.
Some of these cycles recur every day, but others come round only once a year, so there must be several types of biological clocks running at the same time. Some of these clocks are well understood, but others are not. The best-known cycle is the circadian rhythm, which encourages us to sleep at night and stay awake during the day. “Circadian” comes from the Latin circa meaning about, and diem meaning a day.

WATER CLOCKS

IN ANCIENT INDIA THE STANDARD WATER CLOCK was the ghati. Similar timing devices were also used in many parts of Africa. To make a simple ghati, drill a small hole in the middle of a half coconut shell, and float it in a bowl of water. Gradually the water will leak in through the hole, and in due course the shell will sink. In an Indian temple, the monk would no doubt have had a finely engraved copper ghati, perhaps 20 cm (8in) in diameter, ratherthan a half coconut shell, and the hole would have been drilled to just the right size so that the ghati would sink in 24 minutes, which was the length of the Indian hour. The day comprised 60 hours of 24 minutes, so the minutes were the same length as they are today.

PLANT RHYTHMS

APPARENTLY THE SAP RISES WHILE THE MOON IS WAXING, but falls while it is waning, so that the timber is less wet. This practice has been around for many generations: Z300 years ago Arist0tle’s successor Theophrastus of Eresos wrote in his Causes of Plants that there is an appropriate season for felling trees, and wood cut from those felled at the beginning of the waning Moon will be harder and less liable to rot. Similar practices seem to be common across the world today: in India, Sri Lanka, Brazil, Guyana, and the Middle East, as well as in Europe. There is plenty of superstition and folklore attached to the tradition, but scientific investigation backs it up.

THE CHINESE CALENDAR

EACH CHINESE MONTH BEGINS AT MIDNIGHT on the day of the dark Moon, and traditionally the Chinese New Year began on the second dark Moon after the winter solstice, which comes anywhere between late Ianuary and mid~February. New Year has been celebrated with firecrackers, feasting, and other festivities for at least 3500 years. Families gather together, sweep out the bad luck and old business with the dust, give presents, including rnoney in red paper envelopes, display tangerines and oranges, and offer dried fruit to guests.

MOST ANIMALS, INCLUDING HUMANS, use the Sun as a basic timekeeper, hunting, working, and otherwise living actively by day, and sleeping by night. A few creatures are nocturnal; badgers, burglars, and bats work by night, partly because this allows them to prey on the sleepy, partly because the world is quieter and cooler, and partly because there is less competition. In any case, each day and night marks one complete cycle, after which life begins again.