Time and Meridians
When it is morning in our country it is already afternoon in Japan and still night in England. At the same moment different places have different times.
The earth rotates from west to east, so a place to the east is ahead in time of a place to the west. The earth turns 360° in 24 hours, so 15° of longitude = 1 hour.
Key idea 1 : Meridians and time — 360° ÷ 24 hours = 15° per hour. If it is 12 noon at the prime meridian (0°, Greenwich) it is 4 pm at 60° E and 8 am at 60° W. So time increases to the east and decreases to the west.
Key idea 2 : GMT and IST — World Standard Time (GMT) is based on the 0° meridian at Greenwich. Indian Standard Time is based on 82°30′ E. The difference is 82.5° so Indian time is 5 hours 30 minutes ahead of GMT.
Key idea 3 : Different dates at the same moment — At the same moment different places can have different days. At New Year, eastern places like Japan, Australia and New Zealand welcome it first and England and America later. So a line must be drawn somewhere where the day changes.
- Easy : If it is 12 noon at Greenwich what is the time at 45° E? — 3 pm.
- Medium : Time at 75° W? — 7 am.
- Exam-type : How far ahead of GMT is IST? — 5 h 30 min.
In the figure: Time ahead in the east, behind in the west.
15° = 1 hour; east = + and west = −.
Time is not ahead in the west; it is behind in the west and ahead in the east.
Memory clue: 15° = 1 hour. Memory clue: "East ahead, west behind".
Spot the difference
| Direction | Time | Example |
|---|---|---|
| East | Ahead (+) | 60° E = +4 h |
| West | Behind (−) | 60° W = −4 h |
Practice questions
- 1How much time is 15° of longitude?
- 2Why is time ahead at an eastern place?
- 3When it is 12 noon at Greenwich what is the time at 30° E?
- 4When it is 12 noon at Greenwich what is the time at 120° W?
- 5Which meridian determines Indian Standard Time?
- 6What is the difference between IST and GMT and why?
- 7Where will the new year start first and why?
- 8Can two places have different days at the same moment? Give an example.
Show answers (try them yourself first)
- 11 hour.
- 2Because the earth rotates west to east and the sun rises there first.
- 32 pm.
- 44 am.
- 582°30′ E.
- 65 h 30 min; because 82.5° ÷ 15 = 5.5 h.
- 7At the easternmost place, since time is ahead there.
- 8Yes; e.g. Saturday night in Japan and Saturday morning in England.
The Two Travellers Puzzle
Suppose two friends leave the same spot at the same time, one east and one west, and go round the earth and return. Though they meet at the same place they may feel the day is different!
A traveller leaving Greenwich on Thursday noon going east thinks it is Friday on return, and one going west thinks it is Wednesday. A date line is needed to remove this two-day confusion.
Key idea 1 : The eastward journey — Aarav goes east. At 90° E it is Thursday 6 pm. At 180° it is Thursday midnight. As he crosses further it is Friday. At 90° W it is Friday 6 am and on returning to Greenwich it is Friday 12 noon.
Key idea 2 : The westward journey — Neha goes west. At 30° W it is Thursday 10 am, at 60° W 8 am and at 180° Wednesday midnight. At 90° E it is Wednesday 6 pm and on returning to Greenwich it is Wednesday 12 noon.
Key idea 3 : Where the puzzle gets stuck — They meet at the same place but Aarav thinks it is Friday and Neha Wednesday, a two-day difference. In fact it is Thursday there. So an agreement to change day and date near the 180° meridian was needed.
- Easy : Which day would the eastward traveller think on return? — Friday.
- Medium : The westward traveller? — Wednesday.
- Exam-type : The real day? — Thursday.
In the figure: A two-day difference on returning to the same place.
East + 1 day, west − 1 day = a total 2-day difference.
The puzzle is not about travel speed; it is about the earth’s round shape and rotation.
Memory clue: East = Fri; west = Wed; real = Thu. Memory clue: "East ahead, west back, truth in the middle".
Spot the difference
| Traveller | Direction | On return |
|---|---|---|
| Aarav | East | Friday |
| Neha | West | Wednesday |
| Actual | — | Thursday |
Practice questions
- 1How does time change for the eastward traveller?
- 2How does time change for the westward traveller?
- 3When both meet again how many days’ difference is there in their days?
- 4Why does this difference arise?
- 5Which day is actually correct?
- 6What happens at the 180° meridian?
- 7What had to be done to solve this puzzle?
- 8Write such a puzzle of two friends in your own words.
Show answers (try them yourself first)
- 1Time seems to move ahead.
- 2Time seems to move back.
- 32 days.
- 4Because the earth is round and rotates west to east.
- 5Thursday.
- 6The day and date must change there.
- 7An agreement to change the day near the 180° meridian was made.
- 8The student should write in own words.
Rules for Crossing the Date Line
When we go to another country we have to change the time on the clock. Similarly after crossing the date line we have to change the day and date too.
The line near the 180° meridian where the day and date change is the International Date Line (IDL). While crossing it two things are considered: direction of travel and the present day-date.
Key idea 1 : Going west to east — While going from the west side like Japan to the east side like America the same day and date are kept after crossing. E.g. if it is Thursday 25 December it is still Thursday 25 December after crossing.
Key idea 2 : Going east to west — While going from America towards Japan one day is added just after crossing. If it is Thursday 25 December it becomes Friday 26 December.
Key idea 3 : Special feature of 180° — At 180° when it is midnight the day is the same. To its east (America, Chile) the day is ending and to its west (Japan, Australia) a new day begins. By this idea the day starts in the west of the earth and ends in the east.
- Easy : You cross east to west on Friday 8; new day-date? — Saturday 9.
- Medium : You cross west to east on Monday 3; new day-date? — Monday 3 (same).
- Exam-type : On which side does the day start? — West.
In the figure: Changing day-date by direction.
East to west = +1 day; west to east = same day.
We do not always add a day when crossing; we add only after checking direction.
Memory clue: East→West: +1. Memory clue: "Going west, tomorrow arrives".
Spot the difference
| Travel | Change | Example |
|---|---|---|
| West → East | Same day-date | Thu 25 → Thu 25 |
| East → West | One day added | Thu 25 → Fri 26 |
Practice questions
- 1What is the International Date Line?
- 2Which two things are considered while crossing the date line?
- 3What change is made while going east to west?
- 4What change is made while going west to east?
- 5You cross east to west on Wednesday 12; new day-date?
- 6Where on earth does the day start and where does it end?
- 7At midnight on 180° what state are countries to the east and west in?
- 8Travelling from Kamchatka (north) to New Zealand on one side of the date line it is Monday 22 June in the north; what is the day-date in the south?
Show answers (try them yourself first)
- 1The line near 180° where day and date change.
- 2Direction of travel and the present day-date.
- 3One day is added.
- 4The same day-date is kept.
- 5Thursday 13.
- 6Starts in the west; ends in the east.
- 7East: day ending; west: new day beginning.
- 8The same: Monday 22 June, because the line has not been crossed.
Shape and Importance of the Date Line
If a boundary ran through a village it would be Monday at one house’s front door and Tuesday at its back door! So the date line is bent to avoid land.
The International Date Line follows the 180° meridian opposite Greenwich but is not straight. It mostly passes through the Pacific Ocean and bends east or west as needed.
Key idea 1 : Why the line bends — If the line crossed land or islands the east and west parts of one landmass would have different days and it would be hard to know when it was crossed. So it is bent east at some places and west at others to avoid land. Still most of it coincides with 180°.
Key idea 2 : Importance — The date line brings coordination among international airlines, transport, economic and trade activities. Schedules of world transport, especially flights, can be kept exact. In today’s fast global age its need has grown further.
Key idea 3 : The note on a ticket — The ticket of an international trip shows day, date and time of departure and arrival. If the date line is crossed the date can change, so a special note is given on the ticket. The flight duration is given in hours and minutes; from times and dates we can tell where the trip crosses the line.
- Easy : Why not take the line over land? — To avoid two days on one landmass.
- Medium : Through which ocean does it mostly pass? — Pacific.
- Exam-type : Who benefits? — Air and sea transport, trade.
In the figure: A line bent from 180°.
In 2011 Samoa decided to move from the east side of the date line to the west side. After Thursday 29 December 2011 came Saturday 31 December; Friday 30 December was skipped from their calendar. (The textbook mentions the last change as 2011; check with the book.) Many sources say the line is not fixed by any treaty but is an international convention; the textbook says it was decided in 1884.
Not straight because land must be avoided.
The date line and the 180° meridian are not the same; the date line bends away from it in places.
Memory clue: 180° + bends = date line. Memory clue: "Not straight, a path through the sea".
Spot the difference
| Item | 180° meridian | Date line |
|---|---|---|
| Shape | Straight | Zigzag |
| Land | Crosses some land | Avoids land |
| Work | Just a meridian | Changes day-date |
Practice questions
- 1Why is the date line not straight?
- 2Why should the date line not pass through land?
- 3State the importance of the date line.
- 4Why is the date line based on the 180° meridian only?
- 5Why is there a special note on an air ticket?
- 6What did Samoa do in 2011 and why?
- 7State the difference between the date line and the 180° meridian.
- 8Write an example, giving departure and arrival time-date of a flight, to decide whether the line was crossed.
Show answers (try them yourself first)
- 1It is bent to avoid land and islands.
- 2Because one landmass would have two days.
- 3Coordination and exact schedules in flights, transport and trade.
- 4Because it is exactly opposite the prime meridian with a full 12-hour difference.
- 5Because the date may change after crossing the line.
- 6Moved from east to west side and skipped 30 December; to match trade partners.
- 7180° is straight; the date line is zigzag and avoids land.
- 8E.g. Monday 8 am Tokyo to Monday 8 am San Francisco; the same date shows the line was crossed.