Keeping time before clocks

Prerna is the fastest runner among the girls in her district. She is amazed that Olympic timing can tell two sprinters apart even when they seem to cross the line together. Around her she sees a stopwatch, her mother's wristwatch, her sister's mobile phone, her uncle's Braille watch and talking watch, and the big clock at the school gate. How did people measure time before any of these existed?

Long ago, people noticed that many things in nature repeat after fixed intervals: the Sun rising and setting, the phases of the Moon, the changing seasons. They used these cycles to keep track of time.

First came calendars, with a day measured from one sunrise to the next. Then people wanted to know the time within a day, and they built devices for it:

• Sundial: the time is read from the moving shadow of an object in sunlight.
• Water clock: uses water flowing out of, or into, a vessel. In one kind, water drips out of a pot marked with time lines. In the other, a bowl with a tiny hole floats on water, slowly fills up in a fixed time and sinks; then it is lifted out and floated again.
• Hourglass: sand runs from one bulb into the other.
• Candle clock: a marked candle shows how much time has passed as it burns down.

The world's largest stone sundial, the Samrat Yantra, was built about 300 years ago at the Jantar Mantar in Jaipur, Rajasthan, now a UNESCO World Heritage site. It is 27 metres tall; its shadow moves about 1 millimetre every second and falls on a finely marked scale, so it can measure intervals as short as 2 seconds. Like every sundial it shows local 'solar time', which needs a correction to give Indian Standard Time.

India used both shadows and water. The Arthasastra describes shadow-based timing and the outflow water clock, and Varahamihira gave an accurate way to find the time from the shadow of a vertical stick around 530 CE.

The outflow clock had a problem: as the water level dropped, the water came out more slowly, so the clock slowed down. So the sinking-bowl clock, the Ghatika-yantra, was developed, first mentioned by Aryabhata. Its hole was made so the bowl took 24 minutes to fill and sink. That unit was called a ghatika or ghati, and it divided the day into 60 equal ghatis (60 × 24 minutes = 24 hours).