#50 Second Street, San Juan, Trinidad & Tobago, W.I.
Longtitude and time
Local time is the time of the day on any longitude indicated by the apparent movement of the sun. It is also called sun time or mean solar time. Local time is determined from longitude 0°, which is the Prime Meridian. The noon or midday (12 o’clock) of a place is when the sun is at its highest point in the sky over that place since there are 360° of longitudes, all the longitudes on the globe cannot have noon at the same time. The difference in local time between one longitude and the next is 4 minutes. The local time at Greenwich meridian is called Greenwich Mean Time (G.M.T) or the Universal time. The earth takes 24 hours to complete a rotation. This means, it takes 1 day to turn 360°, 1 hour to make a turn of 15° and 4 minutes to turn 1°.
Since they rotate from West to East, all places in the East will have their time ahead of Greenwich meridian. Places in the West will have their time behind G.M.T. Places 15° East of Greenwich will be 1 hour ahead and those 15° west will be 1 hour behind.
The longitude difference for a town in the East and another in the west is obtained by adding the longitudes.
The 360 longitudes of the globe, has been divided into groups of 15 longitudes. This makes a total of 24 groups. Each group of 15 longitudes forms a time zone or a belt. The world is therefore divided into 24time zones or belts. That is why there are 24 hours in one day. Every time zone of 15° of longitude has only 1 local time.
Standard Time
This is the local time kept within each time zone. It is a common time; it is kept by a group of 15 longitudes found in one time zone. The standard time of each time zone is 1hour different from that of the next zone. Two towns found on different longitudes, will have one standard time, if they belong to the same kind zone.
Some countries have more than one time zone. Eg: U.S.A and Canada cover 5 time zones and Australia 3time zones. They have more than 1 time zone because of their large area.
Step 1: Find out the difference in longitudes between two places given.
Step 2: Convert or change the difference in longitude to time, if the difference in degrees is more than 15°, multiply it by 4 minutes.
Step 3: Adjust the time according to the position of the longitude if it is east, we add but if it is west, we subtract time.
Example 1: What is the local time at Madres in India on longitude 80°E, if local time in Yokadouma in Cameroon on longitude 15°E is 4 p.m.?
Solution
Step 1: Longitudes difference = 80°-15° = 65°
Step 2: Convert longitude difference to time: 65°*4mins = 260mins = 4hrs20mins
Step 3: Add or subtract time
4hrs20mins + 4pm = 8:20pm
Because longitude 80°E is in the East.
Example 2: What is the local time in Baghdad 45°E, 35°N when it is noon in Bombay 75°E, 18°N.
Solution
1) 75°- 45° = 30°
2) 30°/15° = 2hrs or (30°*4mins)/60mins = 2hrs
3) 12 noon – 2hrs = 10 am
Exercices
- If the time in Accra (Ghana) on longitude 0° is 12noon. What will be the local time in Philadelphia (U.S.A) on longitude 70°W.?
- What is the local time of Yaoundé 15°E and 5°N, when G.M.T is 8:00 am?
Exercices
- If the local time of Moutoundou (Cameroon) on longitude 15°E is 12am. What is the local time in Montreal (Canada) on longitude 75°W.?
- What is the local time in Monrovia 10°W, 6°N when local time in Washington DC is 75°W, 35°N is 11:00am.
- The local time of G.M.T is 4pm. What is the longitude of Los Angeles whose local time is 8am?
- The time of G.M.T is 12noon and in Saudi Arabia it is 5pm. On which longitude is Saudi Arabia found?