Posts Tagged ‘Daylight Saving Time’

This last weekend the clocks went back 1 hour in the Disunited Kingdom. It is done on the night of the last Saturday/Sunday in October. We have switched from being on British Summer Time (BST) to being on Greenwich Mean Time (GMT). We will move them forward again by 1 hour on the night of the last Saturday/Sunday in March. In the United States, the clocks go back this coming weekend (the night of the 5/6 November), changing from e.g. Eastern Daylight Time (EDT) to Eastern Standard Time (EST), and they will go forward again on the night of the second Saturday/Sunday in March.

Conversely, in e.g. Sydney (Australia), they moved their clocks forward (as they are in spring) in early October, and will move them back in early April.

Whenever we change our clocks, I tend to get people asking me why we do this? This is asked by many people who have grown up here, but also by people who have come from countries like e.g. Nigeria or India or Saudi Arabia where they don’t change their clocks in spring and autumn. And, for the first time, my youngest daughter, who is now 15, asked me why we did it. So, here is my best attempt to explain it.

It has to do with the varying length of the time between sunrise and sunset during the summer months and the winter months. And, in addition to this variation, trying to shift the time of sunset to a later time during the longer days of summer. The difference between what I will call “the length of the day” (what I really mean is the time between sunrise and sunset) varies most for places far from the Earth’s equator, and varies very little for places near the equator.

The variation in the length of the day

To illustrate this, I have chosen four cities, Reykjavik (in Iceland), London (England), Lagos (Nigeria) and Cape Town (South Africa) which have differing latitudes. As you can see from the map below, Reykjavik is a long way from the equator (which, in this map, goes through e.g. Gabon, Congo Kinshasa, and Kenya), at a latitude of 64 degrees North. London is at a latitude of 53.5 degrees North. Lagos is close to the equator, at only 6.5 degrees North of it, and Cape Town is just outside of the tropics, at a latitude of 34 degrees South of the equator.


The four cities marked are Reykjavik (Iceland), London (England), Lagos (Nigeria) and Cape Town (South Africa).

Of these four cities, Reykjavik will have the biggest variation between the length of the day in late June and late December (the summer and winter solstices), and Lagos will have the smallest difference. Here is a table showing the length of time between sunrise and sunset on the summer and winter solstices for these four cities (to the nearest quarter of an hour).

Length of longest and shortest day for 4 cities

The length of the longest and shortest days in Reykjavik, London, Lagos and Cape Town
City Latitude Longest day Shortest day
Reykjavik (Iceland) 64 degrees N 21h 4h 15m
London (England) 53.5 degrees N 16h 30m 8h
Lagos (Nigeria) 6.5 degrees N 12h 30m 11h 30m
Cape Town (South Africa) 34 degrees S 13h 30m 10h

Changing our clocks in spring/autumn

So, this shows how the length of the day varies between June and December, but why do we move our clocks in e.g. the Disunited Kingdom (or the USA), but not in e.g. Nigeria? Well, for countries nearer the equator, the variation in the length of the day is pretty small. For Lagos, the longest day is only 1 hour longer than the shortest day (12 hours 30 minutes compared to 11 hours 30 minutes). For London, the longest day is 16 hours and 30 minutes, the shortest only 8 hours, a difference of 8 and a half hours between the length of the day in June and the length of the day in December. In Reykjavik the difference is even more extreme. In late June the length of the day is 21 hours, whereas in late December it is only 4 hours and 15 minutes, a massive variation.

Let us now look at what time the sun rises and sets in London on the longest day. It rises at 03:45 GMT, and sets at 20:20 GMT (8:20 PM). Most people are not in bed before about 10pm, and very few people are awake at 3:45 AM. Therefore, by shifting the clocks one hour forward we make the sunrise one hour later (4:45 AM), which is still before most people get up, and gain an hour of extra daylight in the evening instead, with the sun not setting until 9:20 PM, when most people are still awake to take advantage of it.

There has been talk for as long as I can remember of having the Disunited Kingdom be on “double” British Summer Time during the summer months, and be on British Summer Time (GMT + 1 hour) during the winter months. This would mean that the sunrise in late June in London would be at 5:45 AM (still before most people get up), and sunset would be at 10:20 PM. That would be agreeable to a lot of people, but if we were on GMT + 1 hour in the winter months sunrise in late December would not be until after 9 AM, which I think most people would not like at all!

Of the four cities I have used in this illustration, only London changes its clocks. Reykjavik does not, and neither does Lagos nor Cape Town. I guess with Reykjavik, the days are so long in the summer months that it is light before anyone gets up and it is still light when most people go to bed. With Lagos, the change between longest day and shortest day is so short that it is pointless to change the clocks, but I was surprised to see that Cape Town does not utilise daylight saving.

In fact, South Africa does not observe daylight saving. Surprisingly, Namibia, which is closer to the equator than South Africa, does observe daylight saving between early April and early September. Namibia seems to be the only country in southern Africa which observes daylight saving; Botswana, Zimbabwe, Mozambique, Malawi, Angola, Zambia and South Africa do not observe daylight saving. If you want to check whether a particular city observes daylight saving, and when the changes happen, you can follow this link.

Does anyone know of any countries in Europe which do not move their clocks forward and back in spring/autumn? I know in the United States there are states which do not go on to Daylight Savings Time, e.g. Arizona and parts of Indiana. And, of course, Hawaii.

The World’s Time Zones

Here is a map of the time zones in the world, centred on Greenwich Mean Time (GMT), as Greenwich is where zero of longitude is set. The country with the most time zones is, not surprisingly, Russia, as it extends from Europe all the way to eastern Asia, even further to the east than any part of Australia or Japan.

Surprisingly for two countries which are so extended in an east-west direction, both China and India only have 1 time zone respectively. This means that if you live in e.g. Beijing your local sunrise will be about two hours earlier than if you live in a city in the west of China such as Aksu. And, there are parts of China which lie to the west of most of India, but these western parts of China are 2.5 hours ahead on the time used in the two countries (GMT+5.5 in India, GMT+8 in China).


A map of the world’s time zones. The time is centred around Greenwich Mean Time (GMT), as Greenwich is the zero line of longitude on the Earth.

To finish, as a bit of frivolity, I will share this. My favourite is the advice on how to adjust your sundial 😉

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