This document provides examples of calculating time differences between locations based on their longitude. It explains that time is calculated by subtracting 1 hour for every 15 degrees of longitude west of Greenwich or adding 1 hour for every 15 degrees east. For locations not on the major 15 degree lines, the number of minutes to add or subtract is calculated by multiplying the number of degrees difference by 4. The document works through multiple examples, such as determining the time would be 5:08 am at 17 degrees east if it is 4 am at the prime meridian.

1. 0◦
(Sun seen after) WEST EAST (Sun seen first)
Subtract 1 hr for every 15◦ or 4 mins. for Add 1 hr. for every 15◦ or 4 mins. for
every 1◦westwards every 1◦ eastwards
Time is calculated on a time line with 15 degree lines of longitude drawn east and
west of the prime meridian:
W E
60 45 30 15 0 15 17 30 45 60
? 4 a.m ? ?
Question: What time will it be at the 45 East line if it is 4 a.m. at Greenwich?
Answer: Since the 45 East line is to the east of the line you were given and is a
major 15 degree line of longitude, you simply add 1 hour forward to get 7 a.m.
Question: If it is 4 a.m at Greenwich, what time will it be at 30 degrees west?
Answer: Since the 30 degree west line is west of the time zone you were given
and is a major 15 degree line, you simply subtract 1 hour backward to get 2 a.m.
Question: What time will it be at 17 degrees east if it is 4 a.m at Greenwich?
Step 1: Please note, in this case, the 17 degree line is not a major 15 degree line of
longitude. Therefore, you have to either find the time for the 15 degree line in
front of or behind it. Eg. The 17 degree line would be located between the 15 and
30 degree line.
Step 2: At 15 east it would be 5 a.m. after adding one hour.

2. Step 3: To find how many 1 degree lines exist between 15 and 17 degrees
subtract. 17 – 15 = 2 degrees.
Step 4: Next, you need to find out how long it will take the earth to rotate through
2 degrees of longitude. It takes the earth 4 minutes to rotate through every 1
degree line of longitude, therefore ( 2 degrees multiplied by 4 minutes). Answer 8
minutes.
Step 5: Add the 8 minutes to the 5 a.m you got in step 2 at 15 degrees east. Your
answer is now 5:08 am for 17 degrees east.
Question: What time will it be at the following places if it is 7 p.m. at 30 degrees
west?
1. 22 degrees west
2. 60 degrees west
3. 32 degrees east
4. 55 degrees east
5. 0 degrees