International Date LineClass 9 Geography Chapter 7 Notes – Maharashtra Board

Summary: International Date Line

The Earth spins from west to east, turning 15° every hour. So places to the east are always ahead in time and places to the west are behind. Going 180° east or west from Greenwich, you are 12 hours ahead or 12 hours behind GMT.

If you keep going east, your clock keeps moving forward; if you keep going west, it keeps moving back. A traveller who goes all the way round the globe would therefore gain or lose one whole day. A fixed line is needed where this extra day is adjusted.

By international agreement (the book links it to a meeting led by Professor Davidson in 1884) the date line was fixed opposite Greenwich, along the 180° meridian. A new day starts just west of it and ends just east of it.

The IDL is kept over the open Pacific Ocean. It bends east and west around islands and land so that no country is split into two dates. It was last changed in 2011, but most of it still lies on the 180° meridian.

Crossing the IDL from west to east (Asia → America) you go back one date; crossing from east to west (America → Asia) you add one date. The line helps airlines, shipping, trade and communication keep their times and dates in order.

Key points to remember

• The Earth turns 360° in 24 hours: 15° per hour, 4 minutes per degree
• East is ahead in time, west is behind
• 180° E and 180° W are the same meridian — 12 hours ahead and 12 hours behind GMT
• The International Date Line (IDL) lies roughly along the 180° meridian, in the Pacific Ocean
• A new calendar day begins first on the western side of the IDL (near Japan, Australia, New Zealand)
• The day ends last on the eastern side of the IDL (near the USA, Alaska, Samoa's neighbours)
• Crossing west → east (Japan to America): subtract one day — the same date is repeated
• Crossing east → west (America to Japan): add one day — a date is skipped
• The IDL zig-zags so that no island group or country has two dates at once
• The line was fixed by international agreement (1884 in the book); its latest change was in 2011
• The whole world has the same date only at the instant it is midnight on the 180° meridian
• The IDL helps airlines, ships, trade and communication keep correct schedules

International Date Line – Notes

1. Recall: longitude and time

• World time is counted from the Prime Meridian (0°) through Greenwich — this is Greenwich Mean Time (GMT). Indian Standard Time (IST) is set by 82° 30' E and is 5 h 30 min ahead of GMT.
• Since the Earth turns through 15° every hour, each degree of longitude is worth 4 minutes. Places to the east see the sun earlier, so their clocks are ahead.
• If it is 12 noon at 0°, the clock at 60° W shows 8 am (4 hours behind) and 4 pm (16:00) at 60° E (4 hours ahead).
• This chapter uses the 24-hour clock: 16:00 means 4 pm and 00:00 means midnight.
• Travelling 180° to the east puts you 12 hours ahead; travelling 180° to the west puts you 12 hours behind. Both journeys end on the same 180° meridian — but the two answers are 24 hours, i.e. one full day, apart.

2. Sunil and Meenal's journey — where did Thursday go?

• In the book's activity, Sunil and Meenal start together from 0° at 12 noon on Thursday. Sunil walks east and Meenal walks west, each going all the way round the globe, 30° at a time.
• Sunil (eastward) adds 2 hours every 30°: 14:00, 16:00, 18:00 at 90° E … 24:00 (midnight) at 180°. After that his day becomes Friday: 06:00 at 90° W, and 12 noon Friday when he is back at 0°.
• Meenal (westward) takes away 2 hours every 30°: 10:00 at 30° W, 08:00 at 60° W … midnight at 180°. After that she is in Wednesday: 18:00 at 90° E, and Wednesday 12 noon when she is back at 0°.
• Three day-names — Wednesday, Thursday and Friday — appear in the activity, although the two children are standing at the same place at the same moment.
• The real day at Greenwich is still Thursday. Sunil must take one day back and Meenal must add one day when they pass the 180° line. That is exactly the job of the International Date Line.

3. What the IDL is and why it bends

• Every day begins at midnight, but midnight reaches different longitudes at different times. Since the Earth is a sphere, every place has another place to its east — so a fixed starting line for the date had to be agreed.
• According to the book, representatives of many countries met in 1884 under American Professor Davidson and chose the line opposite the Prime Meridian, i.e. based on the 180° meridian. Changing the date when crossing it is an international convention.
• The line is drawn through the Pacific Ocean, where there is mostly open water. If it cut across land, people on the two sides of one island or country would be living on different dates, and it would be hard to tell where the date changes.
• So the IDL is not straight: it swings east in some places and west in others (Fig. 7.2) to go around islands and land. It has been shifted when needed; the last change came in 2011. Still, most of it lies on the 180° meridian.
• Countries just west of the line (Japan, Australia, New Zealand) are the first to start a new day; countries to its east (USA, Chile) are the last to finish it. The whole world shares one date only at the moment it is midnight on the 180° meridian.
• Walking north–south along the line does not change the date. For example, going from Kamchatka (Russia) to New Zealand along the line, both lie on its western side — if it is Monday 22nd June in the north, it is Monday 22nd June in the south too.

4. Rules for crossing the date line

• Two things matter: the direction of travel and the current day and date.
• West → east (for example Japan → USA): you cross into the 'earlier' side, so the date goes back by one. If it is Thursday 25th December when you cross, you keep Thursday 25th December on the other side — that day is lived twice.
• East → west (for example USA → Japan): you cross into the 'later' side, so you add one day. Thursday 25th December becomes Friday 26th December — a day is skipped.
• Real flights show this. The book mentions flight UA840 which left Shanghai on 1st January 2017 and reached San Francisco on 31st December 2016 — it 'landed in the past'.
• On an airline ticket for UA 876 shown in the book, the plane leaves Tokyo (Haneda) at 12:30 am on Friday, 1st April and reaches San Francisco at 5:45 pm on Thursday, 31st March, after 9 h 15 min. The ticket warns that the flight involves a date change, because it crosses the IDL from west to east.
• Memory trick: going towards America (east), you get a day back; going towards Asia (west), you lose a day.

5. New Year around the world and the importance of the IDL

• The book's table gives the Indian time when places welcomed 2017. Apia (Samoa) was first, at 15:30 IST on Saturday; Auckland (New Zealand) followed at 16:30, then Funafuti (Tuvalu) at 17:30, Sydney at 18:30 and Tokyo at 20:30 — all on Saturday.
• London reached midnight at 05:30 IST on Sunday and New York at 10:30 IST on Sunday. Baker Island (USA), just east of the line, was the last in the table, at 17:30 IST on Sunday — exactly one full day after Funafuti, which lies just west of the line.
• So Sydney (far east of Greenwich) celebrates many hours before London: Sydney is ahead in time, London behind.
• The IDL brings order to international airline timetables, shipping, trade and business. Without it, flight and ship schedules across the Pacific would be confusing.
• In today's fast-moving, connected world, the IDL lets travellers and companies calculate dates and times correctly everywhere on the globe.

Date line at a glance

Key terms

International Date Line – Flashcards

Read the question, answer it in your head, then tap the card to check. 20 cards.

International Date Line – MCQ Quiz

15 questions. Tap an option — you will see at once if it is right and why.

How to study International Date Line

⏱ About 40 minutes

1. Recall (10 min)
   Revise 15° per hour, 4 minutes per degree, and east = ahead, west = behind. Fill in the Sunil–Meenal tables.
2. Understand the line (15 min)
   Learn why the IDL follows 180°, why it bends round land, and the two crossing rules with the flight examples.
3. Practise (15 min)
   Answer the 'Try it yourself' questions, take the quiz and play 🎮 Play.