Showing posts with label latitude. Show all posts
Showing posts with label latitude. Show all posts

Monday, June 3, 2013

Latitude

Latitude is the angular distance of a point on the earth's surface, measured in degrees from the centre of the earth. It is parallel to a line, the equator, which lies midway between the poles. These lines are therefore called parallels of latitude, and on a globe are actually circles becoming

Longitude

Imaginary lines running north/south at right angles to the parallels and passing through the poles are known as lines of longitude or meridians. The line of longitude passing through Greenwich (London) is 0 degrees or the prime meridian (so called because all lines of longitude are numbered east or west from it). The longitude of a place is its angular distance east or west of the Greenwich meridian, measured at the centre of the earth. There are 180 degrees of west longitude and similarly 180 degrees of east longitude. However, since there are 360 degrees in a circle. 180 degrees East and 180 degrees West must be one and the same line. Since the earth is spherical and has a circumference calculated at 40,232.5 km, in linear distance each of the 360 degrees of longitude is 40,232.5/360 or 111.757 km. As the parallels of latitude become shorter polewards, so the meridians of longitude, which coverage at the poles, enclose a narrower space. The degree of longitude therefore decreases in length. It is longest at the equator where it measures 111.318 km. At 25 degrees it is 100.95 km; at 45 degrees it is 78.856 km; at 75 degrees, 28.967 km; and at the poles 0 km. There is so much difference in the length of degrees of longitude outside the tropics, that they are not used for calculating distances as in the case of latitude. But they have one very important function, they determine local time in relation to G.M.T. or Greenwich Mean Time, which is sometimes referred to as World Time.
THE POSITION OF A PLACE
It is necessary to be precise in stating the position of a place in degrees, since there are two latitudes X degrees (X degrees North and X degrees South). Similarly, longitude Y degrees refers to either opposite meridians unless we state it as Y degrees east and Y degrees west.

Longitude and Time

LOCAL TIME. Since the earth makes one complete revolution of 360 degrees in one day or 24 hours, it passes through 15 degrees in one hour or 1 degrees in 4 minutes. The earth rotates from west to east, so every 15 degrees we go eastwards, local time is advanced 1 hour. Conversely, if we go westwards, local time is retarded by 1 hour. We may thus conclude that places east of Greenwich see the sun earlier and gain time, whereas places west of Greenwich see the sun later and lose time. If we know G.M.T., we merely have to add or subtract the difference in the number of hours from the given longitude. A simple memory aid for this will be East-Gain-Add (E.G.A.) and West-Lose-Subtract (W.L.S.). You could coin your own rhymes for the abbreaviations. The local time for Lagos (3 degrees east) will be 12 minutes ahead of London or 12.12 p.m. But the local time for New York (74 degrees west) will be 4 hours 56 minutes behind London or 7.04 a.m. We can put it in another way: when Londoners and Nigerians are having lunch, New Yorkers will have breakfast. This is difficult to believe, but it is true. The rotation of the earth round the sun means that at any point in time different places are experiencing a different time of day.
HOW TO CALCULATE LOCAL TIME
1. Work out the longitude difference.
2. Convert this to a time difference.
3. Adjust the time according to the direction of movement (east or west).
Example. What is the time in Calcutta (longitude 96 degrees east) when it is 9.00 a.m. in Munich (longitude 11 degrees east)?
1. Longitude difference = 85 degrees
2. Time difference 85/15 = 5 hours 40 minutes
3. Calcutta is east of Munich, therefore the time is ahead. Thus 9.00 a.m. plus 5 hours and 40 minutes = 2.40 p.m.
There are many ways of determining the longitude of a place. The simplest way is to compare the local time with G.M.T. by listening to B.B.C. radio. For example, the captain of a ship in the midst of the ocean wants to find out in which longitude his ship lies. If G.M.T. is 8.00 a.m. and it is noon in the local region, it means that he is four hours ahead of Greenwish, and must be east of Greenwich. His longitude is 4 x 15 degrees or 60 degrees east.

Standard Time and Time Zones

If each town were to keep the time of its own meridian, there would be much difference in local time between one town and the other. At 10.00 a.m. in Kota Bharu (a difference of 2 and half degrees in longitude). In larger countries such as canada, U.S.A., China and U.S.S.R. the confusion arising from time differences alone would drive people mad. Travellers going from one end of the country to the other would have to keep changing their watches if they wanted to keep their appointments. This is impracticable and very inconvenient.
To avoid all these difficulties, a system of standard time is observed by all countries. Most countries adopt their standard time from the central meridian of their countries. The Nigerian government has accepted the meridian of 15 degrees east for the standard time which is one hour ahead of Greenwich Mean Time. The whole world has in fact been divided into 24 Standard Time Zones, each of which differs from the next by 15 degrees in longitude or one hour in time. Most countries adhere to this division but due to the perculiar shapes and locations of some countries, reasonable deviations from the Standard Time Zones cannot be avoided.
Larger countries like U.S.A., Canada and U.S.S.R., which have a great east-west stretch, have to adopt several time zones for practical purposes. U.S.S.R., which extends through almost 165 degrees of longitude, is divided into eleven time zones. When it is 10.00 p.m. on Monday night in Leningrad, it will be almost 7.00 a.m. the following Tuesday morning in Vladivostock. Travellers along the Trans-Siberian Railway have to adjust their watches almost a dozen times before they reach their destination. Both Canada and U.S.A. have five time zones - the Atlantic, Eastern, Central, Mountain and Pacific Time Zones. The difference between the local time of the Atlantic anad Pacific coasts is nearly five hours.

The International Date Line

A traveller going eastwards gains time from Greenwich until he reaches the meridian 180 degrees east when he will be 12 hours ahead of G.M.T. Similarly in going westwards, he loses 12 hours when he reaches 180 degrees west. There is thus a total difference of 24 hours or a whole day between the two sides of the 180 degrees meridian. This is the International Date Line where the date changes by exactly one day when it is crossed. A traveller crossing the date line from east to west loses a whole day (because of the loss in time he has made); and while crossing the date line from west to east he gains a day (because of the gain in time he encountered). Thus when it is midnight, Friday on the Asiatic side, by crossing the line eastwards, he gains a day; it will be midnight Thursday on the American side, i.e. he experiences the same calender date twice! When Magellan's ship eventually arrived home in Spain in 1522 after circumnavigating the world from the Atlantic Ocean to the Pacific Ocean and westwards across the International Date Line, the crew knew nothing about adding a day for the one they had missed. They thought they had arrived on the 5 September. They were shocked to be told that the date was 6 September. A modern aircraft leaving Wellington at 5.00 p.m. on Friday reaches Hawaii, 6601 km away, at 2.00 p.m. the same Friday. The same aircraft on its return journey from Hawaii at 11.00 a.m. on Sunday. Can you explain this?
The International Date Line in the mid-Pacific curves from the normal 180 degrees meridian at the Bering Strait, Tonga and other islands to prevent confusion of day and date in some of the island groups that are cut through by the meridian. Some of them keep Asiatic or New Zealand standard time, others follow the American date and time. To find local time in two places on opposite sides of the International Date Line remember that crossing the 'line' going eastwards a whole day is gained. Crossing the 'line' going westwards a whole day is lost.
Example. If someone in Tokyo (time zone 135 degrees east) telephones a friend in Vancouver on 4 December at 10.00 a.m. what time will his friend receive the call in Vancouver (time zone 120 degrees west)?
1. Longitude difference 180' - 135'E = 45'; 180' - 120'W = 60'; 45' + 60' = 105'
2. Therefore time difference = 105/15 = 7 hours.
3. Vancouver is east of Tokyo, therefore time goes on 7 hours.
4. Going eastwards across I.D.L. a whole day is gained. Therefore the time in Vancouver is 5.00 p.m. on 3 December.