October 25, 2021

I, Science

The science magazine of Imperial College

Keeping time: Kruti Shrotri looks at the different timekeepers our civilisation has used over the last five thousand years


The sundial is the earliest known timekeeping device. The first known sundial dates to over five thousand years ago, and consisted of only a vertical stick. The shadowcasting part of a sundial is known as the gnomon; in traditional sundials, the gnomon tends to be a triangle, such that its hypotenuse is parallel to the Earth’s axis.

Sundial 800w
CandleClock 800w

Candle Clock

The earliest mention of the candle clock dates back to the 6th century, but it is usually attributed to Alfred the Great who lived a few centuries later. His candle clock comprised of six candles, each twelve inches high. One inch of the candle would burn in approximately 20 minutes such that six candles would burn in 24 hours.


A clepsydra measures time through the gradual flow of liquid. The oldest specimens found were Egyptian, dating back to the 14th century BC. These used water. However, since water freezes at 0°C, later versions used mercury instead, which freezes at a much lower temperature of -38°C. Galileo used a mercury clepsydra in as late as the 16th century in his experiments with falling bodies.

WaterClock 800w
Mechanical Clock 800w

Mechanical Clock

Mechanical clocks have been used since the 8th century. They tended to be driven by water, and later by weights. Soon after 1600, Galileo discovered that the motion of a pendulum could be used to regulate clocks, but it wasn’t until 1656 that Chistiaan Huygens built the first pendulum clock. These increased timekeeping accuracy from around 15 minutes per day to 15 seconds per day.

Pocket Watch

The first pocket watch, invented around the 15th century, had an accuracy of only several hours per day. However, in the 17th century, Robert Hooke and Christiaan Huygens developed the balance spring. The balance spring, which may be thought of as a watch’s equivalent of a clock’s pendulum, decreased inaccuracy from several hours to just ten minutes per day

PocketWatch 800w

Atomic Clock

The first atomic clock was designed by Louis Essen and built in 1955. These clocks keep time using the properties of a caesium atom and are accurate to one second every few hundred million years. The invention of the atomic clock resulted in a new time standard, one that we still use today. According to this time standard, one second is equal to 9,192,631,770 cycles of a light wave emitted by a caesium atom.

Quartz Clock

The first quartz clock was built in 1927. These clocks use a quartz crystal, made from silicon dioxide, to keep time. An electric current, from a battery, makes the quartz crystal oscillate at 32,768 times per second. These oscillations regulate the gears that make the clock tick. They are accurate to one second per two days.

QuartzClock 800w

Looking Forward

Scientists have succeeded in building quantum clocks, which keep time by counting the vibrations of an electrically charged aluminium atom. This atom vibrates at 1.1 quadrillion times per second, meaning the clock is accurate to one second per few billion years. The quantum clock offers the potential for a new time standard, more precise than ever before. However its design is complicated and needs further development before it can be considered a serious contender.

Illustrations by Rosie Woodcock

Kruti Shrotri is studying for an MSc in Science Communication; Rosie Woodcock is studying for an MSc in Science Media Production