National Maritime Museum
You are here: NMM Home / Learning / Online resources / GCSE Astronomy / Planet Earth / Our place on Earth

Our place on Earth

Latitude and longitude

You can describe your location on Earth using latitude and longitude.

Latitude states how far north or south you are - it is zero at the equator, 90º N at the North Pole and 90ºS at the South Pole. Lines of latitude run parallel to the equator.

Lines of longtitude
Lines of longtitude
Lines of latitude
Lines of latitude

Longitude tells you your east-west position, measured from the 0º line of the prime meridian of Greenwich. Lines of longitude run from the North Pole to the South Pole. Moving eastwards, longitude increases to a maximum value of 180ºE (also 180ºW of Greenwich as you move around the sphere of the surface of the Earth). Moving further east, you start to move back towards Greenwich. Points west of Greenwich are given in units of ºW.


Time zones

The rotation of the Earth from west to east causes the apparent motion of the Sun across the sky from east to west.

Apparent solar time (or apparent local time) is measured by the changing position of the Sun in the sky and is the time shown on a sundial. In practice, the length of the solar day varies slightly, so a standardised mean Sun is used which assumes that the day is a constant length of 24 hours. Over a year, the average position of the Sun at a given time defines mean solar time.

In the UK standard time is co-ordinated Universal Time (UTC), very close to and commonly referred to as Greenwich Mean Time (GMT). This is mean solar time along the Greenwich meridian. East of this line mean solar time is ahead of GMT - the Sun reaches noon earlier than at Greenwich. To the west of this line mean solar time is behind GMT.

In a country as small as the UK, it is not practical to have many local times in use, so clocks are synchronised to a standard or civil time - GMT - across our local time zone. This zone is shared with the Republic of Ireland.

Moving east or west away from the UK, countries have different time zones where clocks run ahead of or behind time in the UK. In general, neighbouring time zones have a time difference of 1 hour. In 24 hours the Earth rotates through 360° with respect to the Sun. Each time zone is then theoretically 15° wide, corresponding to a 1 hour difference in mean solar time. In practice the shape of time zones is changed to match internal and international borders.
Time zones
A map of international time zones. Click to enlargeCourtesy of HM Nautical Almanac Office


Calculating zone time

For every 15° east or west of the Greenwich meridian civil time changes by 1 hour forwards and backwards respectively. To find the appropriate time zone, in hours, divide the longitude, in degrees, by 15.

For example:

  • at 150° W longitude, the time should be 150° divided by 15° = 10 hours behind GMT
  • at 75° E longitude, the time would be 75° divided by 15° = 5 hours ahead of GMT

If you travel around the world, changing standard time by 1 hour each time you enter a new time zone, then a complete circuit would mean that you adjusted your clock or watch time by 24 hours. This would lead to a difference of 1 day between the date on your clock and the real calendar date.

To avoid this, countries are on either side of the International Date Line which runs down the middle of the Pacific Ocean. If you cross the date line moving east, you subtract a day, whereas if you are moving west you add on a day.


Questions to think about

1. Use an atlas to find the latitude and longitude (to the nearest degree) of the following cities and then calculate the time zone:

i.

Johannesburg

ii.

Paris

iii.

San Francisco

Example
Berlin is at longitude 13°23' E. To calculate the time zone, use the following equation:

15° of longitude is equivalent to 1 hour

Therefore, the number of hours difference= 13°23' / 15° = 54 minutes.

This is rounded to the nearest hour, so Berlin could be expected to be one hour ahead of GMT.


2. What do you think the time zone should be for the following cities:

City Longtitude Time zone
Dublin 6.25° W 0
Paris 2.3° E +1
New York 74° W -5
Warsaw 21° E +1
Bangkok 100° 35' E +7
Honolulu 157° 52' W -10
Millennium Island 150° 3' W +14
Time zone is expressed as the difference from GMT in hours


3. Explain how a sundial may be used to measure local solar time. How does this differ from mean time and why?


4. In what direction does the gnomon or style of a sundial point?


5. Why does time measured by the Sun at Greenwich differ from time measured by the Sun at Redruth (in Cornwall)?

Example
The equation of time tells us whether sundial time is ahead or behind local mean time.

On 15 February, the time on a sundial reads 9.08 am. What is the local mean time?

Dial time is 14 minutes behind clock time. Therefore local mean time is 9:08 + 0:14 = 9:22 am.


6. A sundial reads 3.01 pm on 14 November. Find the local mean time.


7. A clock (showing local mean time) reads 17.03 on 3 September. What will a sundial indicate?


8. On 6 April a sundial reads 12.10 pm. What is the local mean time?

Example
On 15 July a sundial in Glasgow reads 3.12 pm. Glasgow is at a longitude of 4°15' W. What would a GMT clock read at that time?

Firstly, the equation of time states that sundial time is 6 minutes behind local mean time. Therefore local mean time is 3.18 pm.

Secondly the longitude difference needs to be considered; 1° is equivalent to 4 minutes. Therefore Glasgow local mean time is 4.25 x 4 minutes = 17 minutes behind GMT.

So GMT is 3:18 + 0:17 = 3:35 pm.



9. On 20 December an observer at Redruth finds that her sundial reads 10.50 am. Redruth is at longitude 5°14' W. What is the GMT at this moment (hint: use the equation of time graph)?


© NMM London
http://www.nmm.ac.uk/server?show=conWebDoc.5461&setPaginate=No