The sun never sets (or rises) at exactly 6 o’clock

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

22 April 2012

Simon Mambo shoots a straight question: “who decided that Kenya will be three hours ahead of Britain”. Well I don’t know who, but I know how that decision was arrived at. It all has to do with the way the Earth rotates.

Now the Earth is a sphere and like all other spherical objects, it can be divided into 360 degrees all the way round. Furthermore, it rotates through these 360 degrees once every 24 hours (actually, 23h:56min – but that’s another story). Thus the planet turns through an angle of 15 degrees every hour (360 divided by 24). The direction of rotation is from west to east and, as a result, we see the sun moving in the opposite manner: east to west.

Suppose you compare two locations, X and Y where Y is to the east of X. It is obvious that Y will always see the sun earlier than X. Now, since our times of day are based on the daily “movement” of the sun, then we expect that the time at Y will be “ahead” of that at X. The size of the difference will depend on how far eastward Y is from X.

Kenya is to the south-eastern direction from Britain, thus we are ahead of them in time of day. More specifically, Nairobi is about 37 degrees to the east of London. This means that the Nairobians see the sun about 2h:28min (37degrees divided by 15 degrees per hour) earlier than Londoners.

But Kisumu is about two degrees to the west of Nairobi, thus Kisumites (or are they Kisumians?) see the sun some 8 minutes after Nairobians. Mombasa is about 12min ahead of Nairobi and so on.

Obviously, it would be quite confusing if each town used its own time depending on its geographical location. So, countries generally prefer to use one common time within their boundaries. For that reason, the world has been divided into 24 time zones separated by one hour.

In the International Waters, these time zones are straight lines, but on land and over territorial waters, they follow the country boundaries. In some regions, several countries can use the same time zone (for example the East African nations). Very large countries may have several time zones within their boundaries (for example, USA and Australia).

Most territories use the full-hour time zones but a few have resorted to the fractions of an hour. Notable examples include India (2h:30min ahead of Kenya) and Nepal (2h:45min ahead of us).

The case of China is also unique. Geographically, it straddles across five time zones, however the whole country uses the same time – it is 5h ahead of Kenya. Thus in China, the sun rises at 6am in some regions and at 3am in others! Compare that to Kenya where the difference is only a few minutes.

For historical reasons, all time-zones are referenced to the town of Greenwich in London. Thus even though geographically we are between 2h:20min and 2h40min ahead of Greenwich, Kenya has chosen to use the 3h time-zone. For that reason, our sunrise and sunset are never at exactly 6 o’clock.