This means we have longer evenings in the summer and lighter mornings in the winter. The shifted time in use during the summer is known as Daylight Saving Time (DST), and in the UK we call it British Summer Time (BST). In March, clocks move forward by one hour and in October, they move back by one hour.
For most devices connected to the internet, and for radio-controlled clocks, this will happen automatically. After you’ve enjoyed an extra hour in bed, your analogue clocks, ovens and microwaves will need to be manually changed.
But the time isn’t really changing – we just change our watches and devices between BST and standard time.
Standard time in the UK is based on Coordinated Universal Time (UTC). This is maintained with incredibly precise atomic clocks here at the National Physical Laboratory (NPL). UTC is not adjusted – we simply add one hour to the standard time to get UTC+1 in Spring, and then remove that hour to take us back to UTC in Autumn.
Lots of countries observe some kind of DST practice, but it varies globally. The United States, Canada, lots of countries in Europe and many in the Middle East all follow some kind of clock change routine annually. Some countries used to observe it but now stay on permanent time, such as Turkey.
Most of Asia, Africa and South America don’t have any DST schedule, and of course equatorial countries don’t need to observe it as the daylight hours don’t vary much throughout the year.
Radio-controlled clocks synchronised to NPL’s radio time service, known as MSF, automatically change to or from DST.
NPL's Network Time Protocol (NTP) Internet Time Service allows a computer to set its internal clock by connecting over the Internet to a server at NPL that transmits a time code. The computer then uses an internal look-up table to tell it when to change its clock.
Each device manufacturer or software provider uses its own programs to make sure this happens, whether that’s your smartphone/network provider, your smartwatch, or even some modern kitchen appliances that are paired to apps on our phones. But UTC always remains unadjusted – the core time that we relate everything back to.
Synchronising time across the UK is no mean feat. As well as enabling us to tell the time of the day, accurate timekeeping is key to satellite navigation systems, underpins the functioning of the internet and facilitates timestamping for transactions in financial trading.
Even fractional errors or changes to these time signals can result in huge differences. With the Global Positioning System (GPS), the signals travel at the speed of light to determine your location. Just a 1 millionth of a second of error would mean your position is off by 300 m.
The atomic clocks at NPL ensure our time is accurate to within a few nanoseconds (billionths of a second).
The second is defined by taking the fixed numerical value of the caesium frequency, ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9,192,631,770 Hz.
Caesium atomic clocks apply microwave radiation to atoms to generate this transition between two hyperfine levels – when caesium atoms are exposed to the radiation tuned to exactly 9,192,631,770 Hz, they make the ‘jump’. The transition is extremely stable and reproducible – this makes it perfect for timekeeping.
This transition is governed by quantum mechanics. Much like goldilocks, the transition only happens if the microwave frequency is ‘just right’. This is how we are able to use atomic clocks for incredibly precise timekeeping. They would only lose or gain a fraction of a second over the course of millions of years.
For thousands of years, the Earth's rotation was our most stable timekeeper. However, atomic clocks invented during the 1950s were even better timekeepers and showed that the Earth does not rotate steadily, but "wobbles". This wobbling is caused by various factors, such as the movement of water and ice on the Earth’s surface and movements in the Earth’s core. The melting of the polar ice caps due to climate change has been shown to impact these changes, too.
Looking to the future, we are developing even more accurate timekeeping at NPL in the form of optical atomic clocks. Using the same principles we talked about above to ‘count’ seconds using transition frequencies, optical clocks use lasers to probe atoms at much higher frequencies than could be achieved with microwaves.
Because their frequency is higher, the timekeeping is more stable. Optical frequencies are about 100,000 times higher than microwave frequencies, so it’s a bit like getting a much higher resolution ‘photograph’. More pixels mean more accuracy in images, and a higher frequency gives us a much more accurate ‘picture’ of what one second is.
These clocks are becoming so accurate they can act as quantum sensors, and help us test fundamental physics theories. We will also eventually be able to redefine the SI unit of the second using an optical transition.
Check out how accurately you can keep time with a pendulum by trying our “Give me a second” experiment and share your results with us by tagging @npldigital on Instagram.