# Why do the clocks jump back and forwards?

Queen’s Maths and Physics lecturer and researcher, Dr Andrew Brown gets us up to speed on this timely conundrum…

Every March, across the UK and Ireland a whole hour vanishes! But then every October, we get a glorious extra hour in bed when the clocks go back. Smartphones change time as if by magic and we have to remember to re-set the microwave and to whiz round to Granny’s house to diligently wind her mantelpiece clock back. But what would happen if we decided to forgo the annual lie-in and just stayed in Irish/British Summer Time (BST) aka Daylight Savings Time all year?

Well, while we wouldn’t notice any major difference at first, Dr Andrew Brown from Queen’s School of Mathematics and Physics explains that things would start to get bleak during the daily commute in late December. “During the winter solstice, just before Christmas, we get less than eight hours of sunlight. If we didn’t put the clocks back, sunrise would be after 9.00am and sunset before 5.00pm. By putting the clocks back, it's still dark when we're going home, but the morning commute is a bit cheerier.”

## Q: So, which is the ‘real’ time?

A: “You could make the argument that in winter, we’re using ‘real’ time, as we put the clocks back to Greenwich Mean Time (GMT), or Coordinated Universal Time (UTC).”

However, if you want to be truly objective, you’d need a sundial. “In solar time – measured on a sundial - you have an equal amount of sunlight either side of midday (when the sun is at its highest point). But solar time changes relative to clock time every day - that's why we change the clocks, to try and 'catch up' with solar time.”

## Q: Why October and March?

A: On average across a whole year, there will be exactly the same amount of daylight as there is darkness, but on any given day, they will not be the same. “You might have heard the word equinox: it is derived from the Latin for 'equal night'. In March and September every year, there is a particular date with exactly 12 hours of daylight and 12 hours of darkness. In the northern hemisphere between the March and September equinoxes, the day is longer than the night, and vice versa between September and March.”

A (cont): “The clocks go back one hour at 2.00am on the last Sunday in October and forward one hour at 1.00am on the last Sunday in March (unless that’s Easter Sunday). These dates line up roughly with the equinoxes, with a slight bias towards keeping light evenings through September.”

## Q: When did we start moving the clocks back?

A: The man we have to thank for our annual lie-in is William Willet – the great-great grandfather of Coldplay’s Chris Martin - who campaigned for daylight savings time in the early 1900s. “He wanted to increase recreation time in the evenings and save the country a fortune in energy bills.”

While Willet was able to save people money in candles and coal, there have been calls to abandon GMT and remain in summertime mode all year. “More recent proposals for changing the way we do daylight savings have been motivated by energy costs, as well as the associated environmental concerns, road safety (more accidents happen after dark) and even a closer alignment with the rest of Western Europe. However, all of these political and social motivations stem from the simple physical fact that the hours of daylight change throughout the year. Changing the clocks is a way of aligning our working day with the hours of daylight available to us.”

## Q: Why do other countries not change their clocks?

A: Scientifically speaking, it has to do with the latitude. The majority of the world do not need to adjust their clock because they are situated closer to the equator. “The earth is tilted by about 23 degrees relative to its orbital plane. That's why we see seasons: in winter, the northern hemisphere is tilted further away from the sun than in summer, leading to lower temperatures for instance. The reason this affects the hours of daylight is that effectively we have to 'peer over' the equator to see the sun. If the sun has to spend a lot of time rising above the equator from our perspective, that means that the sunrise is later (and equivalently, the sunset is earlier)."

A (cont): If you thought a winter commute home in darkness was bad, spare a thought for the Arctic Circle where there are six months of darkness at a time, followed by six months of daylight. “That's because the sun never rises above the bit of the globe that's in the way when the Northern Hemisphere is tilted away from the sun. Conversely, at the equator, there isn't really much seasonal variation of either temperature or daylight hours because your position relative to the sun doesn't change much. As you move further north (or south) away from the equator the variation becomes more and more pronounced. In the UK, the difference between the longest and shortest day is nearly nine hours and thus, in terms of aligning the clock with the hours of daylight, there is a more noticeable advantage to putting the clocks forward or back.”

About Dr Andrew Brown.

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