15 Fascinating Facts About Time

We measure our lives in seconds, minutes and years, but time is stranger than it looks. Days are slowly getting longer, clocks run at different speeds depending on where they are, and Britain once lost 11 days overnight. Here are 15 fascinating facts about time.

1A second is defined by atoms.

Since 1967, one second has officially been the time it takes for a caesium-133 atom to vibrate 9,192,631,770 times. Before that, a second was defined as 1/86,400 of an average day.

2The days are getting longer.

The Moon’s pull on the oceans acts like a brake on the Earth’s spin, so each day gets slightly longer, by around 2 milliseconds a century. About 70 million years ago, when dinosaurs were alive, a day lasted about 23 and a half hours.

3Leap seconds are being scrapped.

Because the Earth’s spin is not perfectly steady, 27 “leap seconds” have been added to clocks since 1972 to keep them in step with the Sun. They cause problems for computers, so the world’s timekeepers have agreed to stop adding them by 2035.

4The best clocks would not lose a second in billions of years.

The most accurate clocks today are optical atomic clocks, which measure the vibrations of atoms such as strontium. The best of them would not gain or lose a second in tens of billions of years, longer than the universe has existed.

5Greenwich became the centre of the world’s time in 1884.

At the International Meridian Conference in Washington in 1884, countries agreed to use the meridian line at the Royal Observatory in Greenwich, London, as the starting point for longitude and the world’s time zones.

6Britain once skipped 11 days.

When Britain switched from the Julian to the Gregorian calendar in 1752, Wednesday 2 September was followed by Thursday 14 September. It is often said that people rioted to demand their 11 days back, but historians have found little evidence that this happened.

7British Summer Time began during the First World War.

The builder William Willett campaigned for years to move the clocks forward in summer to make better use of daylight. He died in 1915, and British Summer Time was introduced the following year, in 1916, to save fuel during the war.

8Big Ben is adjusted with old pennies.

The Great Clock in Westminster’s Elizabeth Tower is kept accurate by adding or removing old pre-decimal pennies on its pendulum. Each penny makes the clock gain or lose about two fifths of a second a day.

9Sunlight is about eight minutes old.

Light from the Sun takes about 8 minutes and 20 seconds to reach the Earth. If the Sun suddenly vanished, we would not know for more than eight minutes.

10Time really does run at different speeds.

Einstein’s theories of relativity show that time passes more slowly for fast-moving objects and more quickly further from the Earth’s gravity. In 1971, scientists proved it by flying atomic clocks around the world on passenger planes and comparing them with clocks left on the ground.

11Your phone’s map depends on relativity.

The clocks on GPS satellites run about 38 microseconds a day faster than clocks on the ground. Without correcting for this, satnav positions would drift by around 10 km a day.

12A day on Venus is longer than its year.

Venus spins so slowly that one turn on its axis takes 243 Earth days, longer than its 225-day journey around the Sun. On Mercury, the time from one sunrise to the next lasts 176 Earth days, which is two Mercury years.

13The universe is about 13.8 billion years old.

Measurements of the faint glow left over from the Big Bang show that the universe began about 13.8 billion years ago. The Earth formed about 4.5 billion years ago.

14The oldest pieces of Earth are 4.4 billion years old.

Tiny crystals of a mineral called zircon, found in the Jack Hills of Western Australia, are about 4.4 billion years old. They are the oldest known pieces of the Earth’s crust.

15France has more time zones than any other country.

Thanks to its overseas territories, from French Polynesia in the Pacific to Réunion in the Indian Ocean, France covers 12 time zones, more than any other country. China, despite being almost as wide as the United States, uses just one.

FAQs about time

Who invented time?

Nobody invented time itself, but the ancient Egyptians and Babylonians were among the first to divide the day into hours, and the Babylonians gave us 60 minutes in an hour and 60 seconds in a minute.

Why is there a leap year?

The Earth takes about 365 and a quarter days to orbit the Sun, so an extra day is added every four years to keep the calendar in step with the seasons.

What is the most accurate clock in the world?

Optical atomic clocks, developed in laboratories such as the UK’s National Physical Laboratory and the US National Institute of Standards and Technology, are the most accurate clocks ever built.

Sources and further reading

• Royal Museums Greenwich: the Royal Observatory
• National Physical Laboratory
• NIST: time and frequency

Explore more: facts about life.

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