Longitude and time difference

2. Longitude and time difference At this time of the year, Australia has 3 time zones, Eastern, Central and Western Standard Times. This is because as the sun rises over Sydney it is still dark in Adelaide and Perth. Approximately 1 hour later, the sun rises in Adelaide, but it is still dark. It is approximately another hour before the sun rises in Perth. Today we will “ignore time zones”, but look at how we can work out time differences. Melbourne, Canberra, Sydney and Brisbane have approx. the same longitude so there is almost no time difference between the cities. Time difference occurs if there is a difference in longitude. As the earth completes one full revolution (360º) in one day (24h): 360º = 24h 15º = 1h 1º = 4min ÷ 24 ÷ 15 also 1hour = 60 min

3. Longitude and time difference Around the world local times are measured in relation to the time along the Greenwich Meridian. This is called Greenwich Mean Time, GMT, or Universal Time, UT. Locations to the east of the Greenwich Meridian are ahead of GMT. This is because the sun rises in the east. Locations to the west of the Greenwich Meridian are behind GMT. Therefore if you head EAST you ADD the time difference between two locations. If you head WEST you SUBTRACT the time.

4. Example 1 a) Sydney is 10 hours ahead of GMT, or GMT +10h. i) If it is 8am in London, what time is it in Sydney? ii) If it is 5pm Saturday in London, should you ring Sydney? i) 0800 + 1000 = 1800 or 6pm As Sydney is east of London add the time ii) 1700 + 1000 = 2700 = 3am Sunday morning  don’t ring mum at this time. b) New York is GMT 5h and Moscow is GMT +3h. i) If it is 4pm in Moscow, what time is it in New York? ii) If it is 2am Friday in Moscow, what time is it in New York? As New York is west of Moscow subtract the time difference. As the 2 cities are on different sides of the Greenwich meridian ADD the time differences.  5 + 3 = 8 hour difference i) 1600  0800 = 0800 or 8am ii) 0200  0800 = 0600 = 1800 Thursday or 6pm Thursday

5. Example 2 Rio de Janeiro is GMT 3h and Los Angeles is 8h. i) If it is 7am in Rio, what time is it in LA? ii) If it is 9pm Tuesday in LA, what time is it in Rio? iii) What are the approximate longitudes of Rio and LA As the 2 cities are on the SAME sides of the Greenwich meridian so SUBTRACT the time differences.  8  3 = 5 hour difference i) Going from Rio to LA you are heading west subtract the time difference. 0700  0500 = 0200 or 2am ii) Going from LA to Rio you are heading east add the time difference. 2100 + 0500 = 2600 = 0200 Wednesday or 2am Wednesday iii) As 1h = 15º Rio: 3 × 15 = 45 Rio is approximately 45ºW and GMT xh means you are west of Greenwich LA: 8 × 15 = 120 LA is approximately 120ºW

6. Example 3 If it is 10am in Hong Kong (22ºN, 114ºE), what time is it in Durban (29ºS, 31ºE), ignoring time zones. As the 2 cities are on the SAME sides of the Greenwich meridian  SUBTRACT the angles.  114  31 = 83º As 1º = 4min 83 × 4 = 332 min = 5h 32min From Hong Kong you head west to Durban  Subtract the time difference  1000  0532 = 0428 or 4:28am

7. Example 4 New Orleans is (30ºN, 90ºW) and Karachi is (24ºN, 67ºE). i) If it is 7:27am in New Orleans what time is it in Karachi? ii) If it is 2:14am in Karachi what time is it inNew Orleans? As the 2 cities are on different sides of the Greenwich meridian  add the angles.  90 + 67 = 157º As 1º = 4min 157 × 4 = 628 min = 10h 28min i) From New Orleans you head east to Karachi  add the time difference  0727 + 1028 = 1755 or 5:55pm the same day ii) From Karachi you head west to New Orleans  subtract the time dif.  0214  1028 = 0814 = 1546 the previous day or 3:46pm the previous day

8. Today’s work Exercise 7-04 Page 265 → 266 Q1 → 5, 7 → 10, 12 → 15 & 17