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## Why do we move our clocks back in autumn?

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.

## The sky in June (2015)

In two days’ time, Venus will be as far to the East of the Sun in the sky as it can be (what is called “maximum Eastern elongation”), so I thought I would take that event as an excuse to summarise what is happening in the sky this month of June.

For anyone who has been paying even the most cursory attention to the evening sky over the last several months, Venus has been almost impossible to miss as it has dominated the Western sky after sunset. The only objects which can outshine Venus in the sky are the Moon and the Sun; so when Venus is visible it is the first object to appear as the sky darkens after sunset, or the last object to disappear as the sky lightens before dawn. For the last several months, Venus has been shining with a magnitude of about $-4$ (it varies because its distance from the Earth is changing and also its phase is changing). This is some eleven times brighter than Sirius, the brightest star in the sky (which has a magnitude of $-1.44$). It has even outshone Jupiter, the object which has dominated the later evening sky throughout most of the last several months, by a factor of about six.

This Saturday (the 6th), it will get as far to the East of the Sun as it can get as seen from Earth, and when it is at greatest Eastern elongation, the angle between looking towards the Sun and towards Venus is about $45^{\circ}$. This means that Venus will set some three hours after the Sun. For those who wish to be precise, this particular maximum eastern elongation will occur at 19:13 UT on the 6th, and the angle between the Sun and Venus will be $45^{\circ} 24^{\prime}$.

The figure below shows the orbit of Venus (in red) and that of the Earth (in blue). Both planets orbit the Sun in an anti-clockwise direction if one were to look down on the Solar System from above the Earth’s North pole (which is the convention used, sorry Southern Hemisphere people!). Venus is currently heading towards inferior conjunction (the time when Venus and the Sun lie in a straight line as seen from Earth, and so it is not visible). When it is heading towards inferior conjuction it is to the East of the Sun, and hence sets after the Sun and is seen as an “evening star”. The upcoming inferior conjunction happens on the 15th of August, so just a couple of months away.

I diagram of Venus’ and Earth’s orbits. Venus’ orbit is shown in red, the Earth’s orbit in blue. Currently, Venus is to the East of the Sun, and will reach maximum Eastern elongation on Saturday (the 6th), at 19:23 (UT)

After passing inferior conjunction, Venus will lie to the West of the Sun as seen from Earth, and so will slowly re-appear as a “morning star”, but you will have to wait for a few weeks after inferior conjunction for this, as initially it will be too close to the Sun and so lost in the glow of dawn.

However, long before it reaches inferior conjunction, there is a celestial highlight to look out for, which happens towards the end of June. As June progresses, Venus and Jupiter will appear to get closer and closer together in the sky, and by month’s end there will be a spectacular conjunction of the two brightest planets, something not to be missed. The two diagrams below show Venus and Jupiter on the evening of maximum Eastern elongation (the 6th), and then again at the end of the month.

Venus and Jupiter at 21:30 BST (20:30 UT) as seen from London on the 6th of June. On this evening, Venus will be at “maximum Eastern elongation”.

Venus and Jupiter as seen at 21:30 BST (20:30 UT) from London on the 28th of June. The two planets will get even closer over the following few nights, producing a spectacular conjunction of the two brightest planets.

The other planet worth looking out for this month is Saturn. Saturn is currently in Libra, but moving into Scorpio. In the middle of June it will be rising at just before 7pm and transiting at just before 11:30pm, so this month is a good time to see it.

Saturn in the middle of June, at 21:30 BST (20:30 UT) as seen from London. Saturn is currently in Libra, heading into Scorpio.

Saturn is not particularly bright at the moment, but you can use the bright stars Antares (in Scorpio) and Spica (in Virgo) to find it; just look at the diagram above.

The other highlight of June is, of course, the summer solstice (or winter solstice to people in the Southern Hemisphere). This is, of course, the longest day of the year for people living in the Northern Hemisphere, the moment when the Sun reaches its most northernly point in the sky. This year’s solstice will happen on the 21st of June at 16:38 UT, so at that moment the sky will be directly overhead for a person at the correct longitude on the Tropic of Cancer (for anyone on the Tropic of Cancer the Sun will effectively cast no shadow at midday on that day). Here is south Wales, the days around the Summer Solstice are really long, with the Sun rising at about 4:30am and not setting until nearly 9:30pm. It is my favourite time of the year!

## The Analemma (part 1)

About this time of year, presumably because we are at the Autumn Equinox, photographs of analemmae seem to pop up a lot on the internet and on FaceBook. I showed an example of an analemma to one of my Physics classes last week, and found that they were unable to explain what it shows. So, I figured there was a need for a clear explanation of what a solar analemma shows.

A solar analemma is a photograph taken at the same time each day from the same place, showing where the Sun is in the sky at that same time each day. If one were to take a photo at midday each day, then one would get an analemma that looks like this (bear in mind that one would not adjust when the clocks go forward. So, when they do, one would take the photograph at e.g. 13:00 instead of 12:00 as 12:00 is the real, Solar time).

An analemma taken when it is local noon, hence the “figure 8” is vertical. This analemma must have been taken in the Northern Hemisphere at a latitude between the Tropic of Cancer and the Arctic Circle.

This is the easiest type of analemma to understand, so I will start by explaining this one before we move on to more complex ones. The “figure of 8” you can see in the photograph above is due to two effects. The first is one everyone is familiar with, the changing height of the Sun in the sky in the middle of the day depending on whether it is summer, autumn, winter or spring. The Sun will reach its highest point in the sky at midday in the Summer, but at the same time in the Winter the Sun will be much lower in the sky. This explains the vertical change in the analemma.

On the Spring (or Autumn) Equinox, the Sun at midday is overhead as seen by someone on the Equator. By the time of the Summer Solstice, it will have moved to be overhead at midday for someone at the Tropic of Cancer. On the Winter Solstice, it will be overhead for someone in the Southern Hemisphere on the Tropic of Capricorn. This wonderful picture (taken from here) shows the path of the Sun on the Winter Solstice, the Spring Equinox and the Summer Solstice as seen from Bursa, Turkey.

The path of the Sun on 3 important days as seen from Bursa, Turkey. The top is the Summer Solstice, the middle one is the Spring (or Autumn) Equinox, and the bottom one is the Winter Solstice

But, what about the horizontal change? Why is the Sun sometimes to the left of the vertical midpoint, and why is it sometimes to the right? This is more difficult to explain. It involves something called the Equation of Time, and I will explain it in part 2 of this blog.