Parallels of latitude
Latitude is the angle of a place north or south of the Equator, measured from the Earth's centre. Lines joining places of the same latitude are parallels.
- The Equator (0°) is the longest parallel. Parallels get shorter towards the poles, which are just points (90°N and 90°S).
- There are 89 parallels in each hemisphere at 1° gaps, plus the Equator: 179 in all (the poles are points).
- Important parallels: Tropic of Cancer (23½°N), Tropic of Capricorn (23½°S), Arctic Circle (66½°N), Antarctic Circle (66½°S).
- Latitude helps tell climate zones: tropical, temperate and frigid.
Drawing parallels
Draw a circle for the Earth. Draw a horizontal diameter as the Equator. From the centre, use a protractor to mark angles (say 30°, 60°) above and below it. Join matching points on both sides with straight lines: these are parallels. Label them N or S.
Meridians of longitude
Longitude is the angle of a place east or west of the Prime Meridian. Lines joining places of the same longitude are meridians; each is a half-circle from pole to pole.
- The Prime Meridian (0°) passes through Greenwich near London; this was agreed in 1884.
- There are 180° east and 180° west; 0° and 180° together make one full circle. So there are 360 meridians.
- All meridians are of equal length. The distance between two meridians is largest at the Equator (about 111 km per degree) and becomes zero at the poles.
Drawing meridians
On a circle seen from above the North Pole, draw a vertical line for 0° and 180°. With a protractor at the centre, mark 30°, 60°, 90°... on both sides and join each point to the centre (the pole).
Longitude and time
The Earth rotates from west to east, turning 360° in 24 hours.
- 360° ÷ 24 h = 15° per hour.
- 1° = 60 min ÷ 15 = 4 minutes; 1′ (minute of arc) = 4 seconds.
Places to the east see the Sun earlier, so their time is ahead. Places to the west are behind.
Local time
When the Sun is highest over your meridian, it is 12 noon local time. Every meridian has its own local time.
Solving a time sum
- Find the longitude difference (subtract if both are on the same side of 0°, add if on opposite sides).
- Multiply by 4 minutes.
- Add the time if the place is east, subtract if west.
Standard time and IST
If every town used its own local time, trains and offices would be confused. So a country picks one meridian as its standard meridian, and its local time becomes the standard time for the whole country.
India's standard meridian is 82°30′E, passing near Mirzapur. 82.5 × 4 = 330 minutes = 5 h 30 min ahead of GMT. So when it is 12:00 at Greenwich, it is 17:30 IST.
Standard meridians are usually chosen in multiples of 7°30′, so time differences come in half-hours or hours. Very wide countries such as Russia or the USA use several time zones. The world has about 24 main time zones.
International Date Line
Going east from Greenwich, time runs 12 hours ahead by 180°E. Going west, it runs 12 hours behind by 180°W. At 180° there is a 24-hour difference, one whole day.
So the International Date Line (IDL) is drawn roughly along 180°. Crossing it:
- Westward (from America side towards Asia): the date goes forward by one day (skip a day).
- Eastward (from Asia side towards America): the date goes back by one day (repeat a day).
The line is not straight. It bends around island groups so that no island or group has two different dates at the same time.
Try it
Predict first: when it is noon at Greenwich, what is the local time at 90°E? (90 × 4 = 360 min = 6 h ahead → 18:00.) Now check with the slider in the last 3D step. Try 75°W too.
Key formulas and definitions
- 360° in 24 h → 15° = 1 h
- 1° of longitude = 4 min; 1′ = 4 s
- Time difference = longitude difference × 4 min
- East → add time; West → subtract time
- IST = GMT + 5 h 30 min (82°30′E)
Worked examples
1. How many minutes of time equal 30° of longitude?
30 × 4 = 120 min = 2 hours.
2. It is 12:00 noon at Greenwich. What is the local time at 60°E?
Difference = 60°. 60 × 4 = 240 min = 4 h. East, so add: 16:00 (4 pm).
3. It is 12:00 noon at Greenwich. What is the local time at 45°W?
45 × 4 = 180 min = 3 h. West, so subtract: 09:00 am.
4. It is 8:00 am IST (82°30′E). What is the time at Greenwich?
IST is 5 h 30 min ahead of GMT. 8:00 − 5:30 = 2:30 am GMT.
5. Place A is at 30°E and place B is at 45°W. When it is 10:00 am at A, what is the time at B?
Opposite sides of 0°: 30 + 45 = 75°. 75 × 4 = 300 min = 5 h. B is west of A, so subtract: 5:00 am.
6. The local time at a place is 2:00 pm when it is noon at Greenwich. Find its longitude.
It is 2 h ahead = 120 min. 120 ÷ 4 = 30°. Ahead means east: 30°E.
7. A ship crosses the International Date Line going west on Monday at noon. What day is it after crossing?
Going west across the IDL, add a day: it becomes Tuesday noon.
Common mistakes
- Subtracting longitudes on opposite sides of 0°. If one is east and one is west, ADD them.
- Adding time when moving west. West is behind: subtract.
- Using 1° = 1 hour. It is 1° = 4 minutes; 15° = 1 hour.
- Mixing up the date change: crossing the IDL westward adds a day; eastward repeats a day.