How is time measured?

Nature’s clocks

Long ago, people noticed that many events in nature repeat after fixed intervals: the Sun rises and sets, the Moon goes through its phases, and the seasons return. They used these cycles to keep track of time. The day, set by the rising and setting of the Sun, and the calendar came first. Then people looked for ways to tell the time within a day.

Sundials, water clocks, hourglasses and candles

• Sundial: the shadow of an object, cast by the Sun, changes position through the day, and its place on a scale gives the time.
• Water clock: in one type, water flows out of a vessel with time marks on it. In the other, a bowl with a fine hole in its bottom floats on water, slowly fills, and sinks after a fixed time; it is then lifted and floated again.
• Hourglass: sand flows from one bulb to the other in a fixed time.
• Candle clock: a candle marked with lines; each part burns in a fixed time.

Timekeeping in ancient India

In ancient India, time was measured with both shadows and water clocks. The Arthashastra of Kautilya, composed and revised between the 2nd century BCE and the 3rd century CE, mentions shadow-based timekeeping and the outflow water clock. Around 530 CE, Varahamihira gave an accurate rule for time from the shadow of a vertical stick. Outflow clocks were not very accurate, because water flows out more slowly as the level drops. That led to the sinking-bowl clock, the ghatika-yantra, first described by Aryabhata. Its bowl took 24 minutes to fill and sink, a unit called a ghatika or ghati, so a 24-hour day had 60 ghatis. It was used in monasteries, palaces and town squares, where each sinking was announced with drums, conch shells or a gong, until pendulum clocks replaced it in the late nineteenth century.

The Samrat Yantra at Jantar Mantar in Jaipur, built about 300 years ago, is the world’s largest stone sundial. It is 27 m tall, its shadow moves about 1 mm every second, and its scale marks intervals as short as 2 seconds. Like every sundial, it shows local solar time, which needs a correction to give Indian Standard Time.

Make a water clock

Cut a used plastic bottle into two halves, make a small hole in the cap with a drawing pin, and fit the top half upside down on the bottom half. Fill the top with coloured water and mark the level in the bottom half every minute using a watch. To use the clock, pour the water back into the top: each time the level reaches a mark, one more minute has passed.

Pendulum, quartz and atomic clocks

As people began to travel long distances, accurate time became very important. From the fourteenth century, mechanical clocks were driven by weights, gears and springs, and in 1656 Christiaan Huygens invented the pendulum clock, based on the steady swing of a pendulum (see simple pendulum and time period). Modern clocks use the same idea with tiny, very fast vibrations: of a quartz crystal in quartz clocks, or of certain atoms in atomic clocks. Huygens’ early clocks could gain or lose 10 seconds a day; atomic clocks lose only about one second in millions of years.

The second and other units

The SI unit of time is the second (s). Larger units are the minute (min) and the hour (h): 60 s = 1 min and 60 min = 1 h. Unit names and symbols are written in lowercase (except at the start of a sentence) and in the singular, with a space between the number and the symbol and no full stop after it. So write 5 s and 2 h, never 5 sec or 2 hrs. The smallest interval a wall clock with a seconds hand can measure is 1 second.

Many tasks need tinier intervals. Race timing records hundredths or even thousandths of a second (milliseconds), ECG heart monitors measure changes of milliseconds, and phones and computers process signals in microseconds, millionths of a second.

Why it matters

Accurate time runs our daily lives, from school bells and train timetables to sports, medicine and space exploration. Time is also what lets us measure motion: read speed and uniform motion. Answers to the textbook questions are in the NCERT solutions for Chapter 8; see all of Chapter 8 notes.