How to Calculate the Local Time from Longitude and GMT

Example 5.5.1:

Assuming the longitude where Port Harcourt is located is 7°E. It means Port Harcourt is 28 minutes ahead of the Greenwich, taking 1° to represent 4 minutes. The Greenwich is put at 0° longitude. Let’s say that longitude 0° is 0900 hours; it means that GMT is 0900 hours. Hence the local time at Port Harcourt is 0928 hours due to the fact that Port-Harcourt is ahead of GMT.

Example 5.5.2:

When it is 11:00 am in Monrovia, longitude 11°W, what is the time in New York, longitude 74°W?

Solution:

Step 1:

Locate the places involved in the question. From the sketch, we can see that New York is more West than Morovia (Morovia is more East than New York)

Step 2:

The longitudes are of the same hemisphere (West – West) so we subtract.

The difference in location from 11°W to 74°W = 63°

Step 3:

The time difference = \( \frac{63^o}{15^o}\\ \scriptsize = 4\: hours \: 12 \: minutes \)

or

63° × 4 = 252 mins

252 mins = 4 hours 12 minutes

Step 4:

New York is located West of Monrovia; hence its time is behind that of Monrovia.

∴ Time in New York = 11:00 am – 4 hours 12 minutes = 6:48 am

Example 5.5.3:

The time of place A on longitude 45°E is 5 pm. Find the time of another place B of longitude 30°W.

Solution:

Step 1:

Locate the places involved in the question.

Step 2:

The longitudes are of different hemispheres (East-West) so we add the longitudes.

Difference in longitudes = 45 + 30 = 75°

Step 3:

1° = 4 mins

75° = 75 × 4 = 300 mins

300 mins = 5 hours

Step 4:

B is located West of A; hence B’s time is behind that of A. Therefore we have to subtract.

5.00 pm (17 hrs) – 5 hours = 12.00 pm or 12 noon.

Example 5.5.4:

A city X is located at (37°S, 175°E) and city Y is located at (34°S, 150°E)

(a) What is the time difference between these two cities?
(b) What time is it in city X if it is 2 pm in city Y?
(c) What is the time in city Y if it is 9.00 am in city X?

Solution:

(a)

Step 1:

Locate the places involved in the question. Note that City X is further East than City Y. This can be seen in the longitude diagram below:

Hence the sketch below:

Step 2:

The longitudes are of the same hemisphere (East – East) so we subtract.

175 – 150 = 25°

Step 3:

1° = 4 mins

25° = 25 × 4 = 100 minutes

100 mins = 1 hour 40 mins

The time difference between these two cities is 1 hour 40 mins.

(b)

City X is located East of City Y; hence City X’s time is ahead of City Y. So we add

2.00 pm + 1 hour 40 mins = 3.40 pm

If it is 2 pm in city Y, it is 3.40 pm in city X.

(c)

City Y is located West of City Y; hence City Y’s time is behind City Y. So we subtract.

9.00 am – 1 hour 40 mins = 7.20 am

If it is 9.00 am in city X, it is 7.20 am in city Y.

Example 5.5.5:

An important announcement was broadcast from London (0°) at 3:40 pm GMT. This was picked up by the navigator of a ship anchored off at the coast of West Africa on longitude 10°W. What was the local time for the ship’s location?

Solution:

The location of London = 0° longitude

Time in London = 3:40 pm

The local time of the ship’s location =?

In this case (Greenwich Meridian 0°), we can add or subtract to get the longitude difference.

i.e 10 + 0, or 10 – 0 = 10°

Convert to hours and minutes

 If 1° = 4 mins, then 10°W = 40 minutes

Being that the ship’s location is in the western part of London, we subtract from London time

= 3:40 pm – 40 minutes

= 3:00 pm

Example 5.5.6:

Suppose an international golf championship is to be held in Accra, Ghana, Latitude 5° 35’N; longitude 0° and is scheduled to start at 9:00 pm, local time on June 26. The contestants will come from France and Australia and the referee will be coming from Manila in the Philippines (latitude 14° 36’N; longitude 120° 59’E).

Calculate the time and date that listeners in the following cities will have to tune in to their radios for the start of the game;

(a) Sydney, Australia (Latitude 33° 55’S; longitude 151° 10’E)
(b) Paris, France (Latitude 48° 52’N, longitude 2° 20’E)
(c) Manila, the Philippines (latitude 14° 36’N, longitude 120° 59’E)

Solution:

(a)  Sydney, Australia

Location; Latitude 33° 55’S; longitude 151°, 10’E

Time for the start of the game = Difference in longitude between Sydney and Accra, which is 151°

Convert 151° to minutes

= (151 × 4) minutes = 604 minutes

Then convert 604 minutes to hours and minutes

= \( \frac{604\:mins}{60\:mins} = \scriptsize 10\:hours \: 4\:mins \)

Being that Sydney is East of Accra, it is ahead of the time in Accra.

∴ Add 10 hours 4 mins to the time in Accra

= 9:00 + 10 hours 4 mins

= 7:04 am on 27th June

(b) Paris, France

London; Latitude 48°52’N; longitude 2°20’E

Time for the start of the game

= Difference in longitude = 2°

To get the time, convert 2° to minutes = (2 × 4) minutes = 8 minutes

Time at Paris is; add 8 minutes to the time in Accra because Paris is ahead of the time in Accra

= 9:00pm + 8 minutes = 9:08 pm on 26th June.

(c) Manila, Philippines

Location; Latitude 14°36’N; longitude 120° 59’E

Difference in longitude between Manila and Accra = 120°

Convert 120° to minutes = (120 × 4) = 480 minutes

Convert 480 minutes to hours and minutes = \( \frac{480\:mins}{60\:mins} = \scriptsize 8\:hours \)

Being that Manila is in the east of Accra, it is ahead of time in Accra

Add 8 hours to 9:00 pm = 5 am on 27th June.

Another way one can calculate time using longitudinal lines is as below.

If the Globe is made to appear like a spreadsheet of paper, the longitudinal lines appear straight thus:

In this case, what you need to do is to draw the lines to correspond to the lines of longitude required by the question. Knowing that the distance in degrees between one longitudinal line and the other is 15° all the longitudinal lines are of one-hour intervals, and also draw a longitude that could be less than one hour or 15°.

Assign the time to the appropriate longitudes and count using 1-hour intervals until you reach the longitude whose time you are working for. See the spread diagram above used to calculate the time of the given latitude.

The diagram above is used to solve the example below:

Example 5.5.7:

The Super Eagles versus Italy March was scheduled to start in Dallas, USA, longitude 97° 30’W at 5 p.m. local time on Monday, June 20 1994. On what day and what time would people in Nigeria, one hour ahead of GMT, tune their television sets to watch the match?

Note: whichever method you use, it will give you the same answer. The above question was solved thus;

Step 1

Divide the longitudinal distance at an interval of 15° (1 hr.).

Step 2

By adding 1 hour for each 15° from Dallas (Nigeria is East of Dallas, we arrive at 11:28 p.m. at longitude 0° (i.e., GMT). Note 7° = 28 mins, i.e. (4 × 7)

Step 3

Since Nigerian time is one hour ahead of GMT, we add 1 hour to 11:28 p.m. to arrive at 12:30 a.m.

Step 4

We have gone beyond midnight so 12:30 a.m. is the next day Tuesday, June 21 1994