ChatGPT: Patokan GMT 0

GMT 0 (Greenwich Mean Time)

GMT 0 is the world reference time located at Greenwich, London, United Kingdom.

• Reference longitude: 0° (Prime Meridian)
• Physical location: Royal Observatory Greenwich
• Function: Global time baseline

Earth Rotation and Time Calculation

Earth rotates 360 degrees in ~24 hours

360° / 24h = 15° per hour

1° longitude = 4 minutes

    

This physical law is the foundation of all time zones.

GMT Offset Based on Longitude

• 0° longitude → GMT 0
• East of Greenwich → GMT + (faster time)
• West of Greenwich → GMT - (slower time)

Example:

15°E  → GMT+1

30°E  → GMT+2

105°E → GMT+7 (Indonesia - WIB)

75°W  → GMT-5 (New York)

    

Indonesia Time Zones (Longitude-Based)

WIB  = GMT+7  ≈ 105°E

WITA = GMT+8  ≈ 120°E

WIT  = GMT+9  ≈ 135°E

    

Why GMT Is Based in Greenwich

British Maritime Dominance (18th–19th Century)

During the 18th and 19th centuries, Britain was the world's dominant naval and trading power.

Focus: MAPS and International Navigation

• Britain produced most global nautical charts via the Royal Navy and Hydrographic Office.
• These maps were used not only by British ships, but also by foreign commercial and military vessels.
• Most widely distributed sea maps already used Greenwich as the zero-longitude reference.

Even before official international agreement, Greenwich was already the de facto Prime Meridian in global cartography.

Map Reference System:

- Longitude measured from Greenwich

- Ship clocks set to Greenwich time

- Position calculated using time difference

    

Navigation accuracy required consistency between:

1. Ship chronometer (time)
2. Astronomical observation (sun/stars)
3. Longitude calculation
4. Greenwich-based nautical maps

Mismatched references would cause massive navigation errors.

Because so many maps, ports, sailors, and routes already used Greenwich, changing the reference meridian would require reprinting and retraining on a global scale.

Royal Observatory Greenwich (Founded 1675)

• Center for astronomical observation
• Accurate time measurement
• Longitude calculation reference

International Meridian Conference (1884)

Location: Washington DC

Result:

- Greenwich chosen as Prime Meridian (0°)

- Approved by 22 out of 25 nations

    

Real-World Time Zones

Modern time zones do not strictly follow longitude. They are adjusted for political and social reasons.

China  → GMT+8 (single zone)

India  → GMT+5:30

Nepal  → GMT+5:45

    

Final Summary

- GMT 0 = Greenwich, 0° longitude

- Time zones based on Earth's rotation

- 15° longitude = 1 hour

- British nautical maps already used Greenwich

- Global navigation depended on Greenwich-based maps

- Officially adopted in 1884