Longitudes and Time Zones

What are Longitudes?

Longitudes are imaginary vertical lines running from the North Pole to the South Pole. The Prime Meridian at 0° longitude passes through Greenwich, England, and is the basis for Greenwich Mean Time (GMT).

• Measured in degrees (0° to 180° East and West)
• All longitudes meet at the poles
• Used to calculate time zones and standard time

Time Zones

As the Earth rotates 360° in 24 hours, it rotates 15° in 1 hour. Hence, every 15° longitude represents one time zone (1 hour difference).

• 360° ÷ 24 hours = 15° per hour
• Places east of the Prime Meridian are ahead of GMT
• Places west are behind GMT
• India follows IST (Indian Standard Time) = GMT + 5:30
• IST is based on 82.5° E longitude (near Mirzapur, UP)

Flow Diagram: Longitudes and Time Zones

Key Terms

• Prime Meridian (0°): Reference line for GMT
• Standard Time: Uniform time for a country
• Local Time: Time at a place based on sun’s position
• GMT (Greenwich Mean Time): Reference time
• International Date Line: At 180°, separates days

Previous Year Questions (PYQs)

• SSC CHSL 2020: What is the time difference between two longitudes 15° apart?
• 1 hour
• RRB NTPC 2019: What is the standard meridian of India?
• 82.5° E
• HSSC 2018: What is the time difference between IST and GMT?
• 5 hours 30 minutes ahead
• SSC GD 2021: Through which country does the Prime Meridian pass?
• United Kingdom
• SSC MTS 2021: Where is the International Date Line located?
• 180° Longitude
• SSC CGL 2017: The Earth rotates from?
• West to East
• RRB Group D 2019: The line of 0° longitude is called?
• Prime Meridian
• SSC JE 2020: Which city is closest to the standard meridian of India?
• Mirzapur
• SSC CGL 2020: If it is 12 noon at Greenwich, what will be the time at 75° E?
• 5:00 PM
• HSSC CET 2022: India’s time zone is based on which meridian?
• 82.5° East