Calculating Sun Time
October 12, 2015
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September 27, 2015
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10_1_3_Time_Zones_01

Lines of longitude run from the North Pole to the South Pole. They are “labeled” from the Greenwich Meridian which has the value of 0˚.

  • Lines of Longitude on the eastern side (to the right) of the Greenwich Meridian increase in value until 180˚ E.
  • Lines of Longitude on the western side (to the left) of the Greenwich Meridian increase in value until 180˚ W.
  • Strangely, the Lines of Longitude 180˚ E and 180˚ W are the same line of longitude, and this Line of Longitude is known as the International Date Line. It does not cut through any country and therefore has several parts that are not straight. This is to avoid confusion over the date within a country.
  • For every 15 degrees difference in Longitude, sun time changes by one hour.

This means, for example, that :

PLACE A that is 15 degrees of longitude different to PLACE B will have a time difference of one hour (sun time).

Sun_Time_Calculation_1

PLACE C that is 45 degrees different to PLACE D will have a time difference of three hours (sun time)

Sun_Time_Calculation_2

PLACE X that is 150 degrees different to PLACE Y will have a time difference of ten hours (sun time).

Sun_Time_Calculation_3

You will notice that these places are all on the eastern side of the Greenwich Meridian and the rules are that

  • if the two labels are the SAME (i.e. both are labeled E or both are labeled W), you must SUBTRACT the smaller value from the greater value.
  • if the two labels are DIFFERENT (i.e. one is labeled E and the other is labeled W), you must ADD the two values.

Note the letters in red that will help you to remember whether to subtract or add when getting your answer.

Look at the example below where the places are both on the western side of the Greenwich Meridian, and both have the label W, i.e. they are the SAME. To find the difference in longitude, we must SUBTRACT the smaller value from the greater value:

Sun_Time_Calculation_4

Look at the example below where one place is on the western side of the Greenwich Meridian (and labeled W), while the other is on the eastern side of the Greenwich Meridian (and labeled E). As these labels are DIFFERENT, we need to ADD the two values to find the difference in longitude.

Sun_Time_Calculation_5

Test yourself : Difference in Time

What is the difference in time between the following places?

1. Place A (30˚ E) and Place B (90˚ E)

2. Place C (30˚ E) and Place D (150˚ E)

3. Place E (30˚ W) and Place F (165˚ W)

4. Place G (30˚ W) and Place H (90˚ E)

5. Place I (120˚ E) and Place J (45˚ W)