Notes For All Chapters – Geography Class 8
Local Time and Standard Time
1. Introduction
- The Earth is divided into longitudes, which help determine time differences between places.
- Time is measured based on the rotation of the Earth and the position of the Sun.
2. Rotation of the Earth and Time Calculation
- The Earth rotates 360° in 24 hours, which means it moves 15° in one hour and 1° in 4 minutes.
- This rotation causes different places on Earth to experience day and night at different times.
Time Differences
- Eastern longitudes are ahead in time.
- Western longitudes are behind in time.
- Example: If it is 12 noon at 0° longitude (Prime Meridian), then:
- At 15°E → 1:00 PM
- At 15°W → 11:00 AM
3. Local Time
- Local time of a place is the time based on the position of the Sun in the sky.
- When the Sun is overhead, it is 12 noon at that place.
- Each longitude has a different local time.
Key fact:The difference in local time between two places is determined by their longitude difference.
Formula: Time Difference (in minutes) = Longitude Difference × 4 minutes
4. Standard Time and Time Zones
- Every country follows a Standard Time to avoid confusion due to different local times.
- A standard meridian is selected, and the whole country follows its local time as the Standard Time.
- The world is divided into 24 time zones, each covering 15° of longitude.
Indian Standard Time (IST)
- India’s standard meridian is 82.5°E (passes through Mirzapur, Uttar Pradesh).
- IST = GMT +5:30 hours (Greenwich Mean Time).
- This means that when it is 12:00 noon GMT, it is 5:30 PM IST.
Multiple Time Zones in Large Countries
- Some countries like Canada, USA, Russia, Australia have multiple time zones due to their vast size.
- Example: Canada has 6 standard time zones.
5. Greenwich Mean Time (GMT) and Coordinated Universal Time (UTC)
- Greenwich Mean Time (GMT): The time at 0° longitude (Prime Meridian).
- Coordinated Universal Time (UTC): A more precise, atomic-clock-based time system used globally.
- GMT & UTC are the reference points for all world time zones.
6. International Date Line (IDL)
- The International Date Line (IDL) is at 180° longitude in the Pacific Ocean.
- It helps adjust the date when crossing time zones.
- Rule:
- Moving east to west → Gain a day (+1 day)
- Moving west to east → Lose a day (-1 day)
Example:
- If it is Monday, 10 AM before crossing the IDL from west to east, after crossing, it becomes Sunday, 10 AM.
7. Important Time Calculations
(a) Time at Different Longitudes
Formula:
Time at given longitude = GMT ± (Longitude × 4 minutes)
Example:
- If it is 10:00 PM GMT on 21st June, then:
- At 120°E → 6:00 AM (22nd June)
- At 160°W → 12:40 PM (21st June)
- At 60°E → 2:00 AM (22nd June)
(b) Football Match Time Calculation
Example:
- A match starts in São Paulo, Brazil at 6 AM IST.
- IST = UTC +5:30 and São Paulo = UTC -3:00.
- Time difference = 5 hours 30 minutes + 3 hours = 8 hours 30 minutes.
- Local time in São Paulo = 6:00 AM IST – 8 hours 30 minutes = 9:30 PM (Previous day).
8. Conclusion
- Understanding local time, standard time, and time zones is important for global communication and travel.
- IST (82.5°E) is the official time for India, while GMT and UTC are used for global coordination.
- The International Date Line (IDL) ensures correct date changes when crossing time zones.
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