Notes For All Chapters – Geography Class 9th
1. Introduction
What is Time?
- Time helps us track days and hours as Earth moves.
- It’s based on Earth’s rotation, which takes 24 hours for 360°.
Meridians and Time:
- Meridians are imaginary lines from the North Pole to the South Pole.
- 0° Meridian (Prime Meridian): Used for Greenwich Mean Time (GMT) in London.
- 82.5° E Meridian: Used for Indian Standard Time (IST), 5 hours 30 minutes ahead of GMT.
Earth’s Rotation:
- Earth spins from west to east, so the east sees the sun (and new time) first.
2. Welcoming the New Year
New Year 2017 Around the World (IST Table):
- Samoa Island (Apia): Saturday, 15:30 (first to welcome 2017).
- New Zealand (Auckland): Saturday, 16:30.
- Tuvalu (Funafuti): Saturday, 17:30.
- Australia (Sydney): Saturday, 18:30.
- Japan (Tokyo): Saturday, 20:30.
- England (London): Sunday, 05:30.
- USA (New York): Sunday, 10:30.
- USA (Baker Island): Sunday, 17:30 (last to farewell 2016).
Key Points:
- The day changes based on location and the International Date Line (IDL).
- Sydney (Saturday) and London (Sunday) differ because of their positions relative to the IDL.
3. Understanding Time and Meridians
Time Changes with Meridians:
- Earth is divided into 24 time zones, each 15° wide (360° ÷ 24 = 15° per hour).
- East of 0°: Time moves ahead (e.g., 60° E is 4 hours ahead of GMT).
- West of 0°: Time moves back (e.g., 60° W is 4 hours behind GMT).
Activity Example (Sunil and Meenal’s Journey):
- Start: Thursday, 12 noon at 0° (Greenwich).
- Sunil (Eastward): Goes to 180°, gains time, crosses IDL, returns Friday, 12 noon.
- Meenal (Westward): Goes to 180°, loses time, crosses IDL, returns Wednesday, 12 noon.
Table Example (Fig 7.1):
- 0°: Thursday, 12:00.
- 30° W: Thursday, 10:00.
- 60° E: Thursday, 16:00.
- Crossing 180° changes the day due to the IDL.
4. The International Date Line (IDL)
What is the IDL?
- An imaginary line near the 180° meridian where the date changes.
- Decided in 1884 by nations led by Professor Davidson (USA).
How It Works:
- West of IDL: New day starts (e.g., Japan, Australia).
- East of IDL: Old day ends (e.g., USA, Chile).
Crossing Rules:
- East to West (e.g., Japan to USA): Subtract 1 day (Tuesday to Monday).
- West to East (e.g., USA to Japan): Add 1 day (Monday to Tuesday).
Shape of IDL:
- Not a straight line like 180°; it zigzags through the Pacific Ocean.
- Bends to avoid splitting land (e.g., Russia’s islands, Alaska).
- Last changed in 2011 to adjust for some Pacific islands.
5. Why the IDL is Needed
Problem Without IDL:
- Earth is round, so every place has an east side-where does a day start?
- Without IDL, dates would get mixed up for travelers.
Solution:
- IDL sets a clear line: West starts a new day, East ends the old one.
- Example: Shyamrao’s trip:
- Japan to USA (Monday 1st, 1 p.m.): Stays Monday 1st, 1 p.m.
- USA to Japan (Friday 5th, 12 noon): Becomes Saturday 6th, 12 noon.
Real-Life Example:
- Flight UA840: Left Shanghai 1st Jan 2017, arrived San Francisco 31st Dec 2016 (crossed IDL west to east).
6. Importance of the IDL
Global Coordination:
- Helps airlines, ships, and trade keep correct dates and times.
Example: Flight UA 876 (Tokyo to San Francisco):
- Leaves Tokyo 1st April, 12:30 a.m., arrives San Francisco 31st March, 5:45 p.m.
- Note: “Involves a date change” due to IDL.
Daily Life:
- Prevents confusion in places near 180° by keeping them on one date.
- Supports modern global travel and communication.
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