MAK LABS — Std 9 Geography Chapter 7 International Date Line

1Two boxes in different hemispheres are given in the following diagram. The IDL passes through both the boxes. In one box, the meridian, day and date is given. Find the day and date for the other box.

Eastern Hemisphere (West of IDL)	Western Hemisphere (East of IDL)
(A) 150° E Meridian Monday, 15th August	150° W Meridian Sunday, 14th August
(B) 80° E Meridian Monday, 26th December	170° W Meridian Sunday, 25th December

Explanation: Countries in the Eastern Hemisphere (like Japan and India) are always a day ahead of countries in the Western Hemisphere (like the USA). Therefore, when crossing the IDL from East to West (America to Asia), you add a day. When crossing from West to East (Asia to America), you subtract a day.

2Select the correct option:

(1) While crossing the IDL, a person will have to add one day when travelling from ________.

✓ (a) East to West
(b) West to East
(c) South to North
(d) North to South

(2) If it is Wednesday 10 a.m. at 15° E meridian, then what will be the time at IDL?

(a) Wednesday 6 a.m.
✓ (b) Wednesday 9 p.m.
(c) Thursday 2 p.m.
(d) Thursday 6 p.m.

(3) According to the international convention, at which meridian does the day and date change occurs?

(a) 0°
(b) 90° E
(c) 90° W
✓ (d) 180°

(4) At which direction of the IDL does a new day start immediately?

(a) East
✓ (b) West
(c) North
(d) South

(5) IDL brings coordination in which of the following?

(a) GPS system
(b) Defence departments
✓ (c) Transportation schedules
(d) Determining the hemisphere

3Give geographical reasons.

(1) IDL is proving to be very useful in today's times.

In today's modern era of rapid global developments, the International Date Line (IDL) is essential because it brings coordination between international airlines, transportation services, and global economic and trade activities. It eliminates the confusion of multiple dates globally by providing a universally accepted line where a new day officially begins and ends. Because of the IDL, the schedules of worldwide traffic and communication are organized accurately.

(2) The day starts in Pacific Ocean on the earth.

The International Date Line (IDL) marks the start of a new day, and this line mostly coincides with the 180° meridian. Since the 180° meridian passes almost entirely through the vast Pacific Ocean, the official start of a new day globally occurs there. It was deliberately kept over the ocean so that no single landmass or island would be divided into two different dates.

4Write in brief:

(1) What considerations have been made while deciding the IDL?

While deciding the International Date Line, the following points were taken into consideration:

The direction of travel (whether one is travelling East or West).
The current day and date.
Ensuring that the line passes entirely through the ocean (avoiding landmasses) so that a single country or island group does not experience two different days simultaneously.

(2) While crossing the IDL, what changes will you make?

When crossing the IDL from West to East (e.g., from Japan to America), we continue with the same day (or consider the previous day). Conversely, when crossing the IDL from East to West (e.g., from America to Japan), we must add one day to the date.

(3) Why is the IDL not a straight line like the 180° meridian?

If the IDL were a straight line exactly along the 180° meridian, it would cross over certain landmasses and island groups. This would cause immense confusion, as people living on the eastern and western parts of the same island would have to follow two different dates and times. To prevent this, the IDL turns east or west at certain places to ensure it passes entirely through the ocean.

(4) Why doesn't the IDL pass through land?

The IDL does not pass through land because doing so would split a single region into two different calendar dates. It would be highly confusing and inconvenient for daily administration, businesses, and regular life if crossing a street on the same landmass meant jumping to the next day. Hence, it is drawn exclusively through the ocean.

(5) Why is the IDL considered with respect to the 180° meridian only?

The 180° meridian is exactly opposite to the Prime Meridian (0° at Greenwich). Since the Earth takes 24 hours to complete one rotation, it takes exactly 12 hours for the sun to "move" from the Prime Meridian to the 180° meridian, regardless of whether one travels east or west. Therefore, the 180° meridian is the most logical place to serve as the global boundary for a change in date.

5Using an atlas, tell in which of the following routes the IDL will be crossed and show them on the map.

This is a map-based activity. Based on global geography, here are the routes that cross the IDL:

(1) Mumbai - London - New York - Los Angeles - Tokyo: Yes, the IDL is crossed when flying from Los Angeles across the Pacific Ocean to Tokyo.
(2) Delhi - Kolkata - Singapore - Melbourne: No, the IDL is not crossed. The entire journey remains in the Eastern Hemisphere.
(3) Kolkata - Hong Kong - Tokyo - San Francisco: Yes, the IDL is crossed when flying over the Pacific Ocean from Tokyo to San Francisco.
(4) Chennai - Singapore - Tokyo - Sydney - Santiago: Yes, the IDL is crossed during the flight from Sydney across the South Pacific to Santiago (South America).
(5) Delhi - London - New York: No, the IDL is not crossed. The route crosses the Prime Meridian but not the 180° meridian.

7. International Date Line

Exercise