Longitude or Meridian: The Vertical Mapping of Earth

Now, imagine peeling an orange 🍊. The natural segments of the orange run from top to bottom, meeting at the stems—just like longitudes, which run vertically from pole to pole. They are also called meridians and help us measure how far east or west a place is from the Prime Meridian (0° longitude).

What is Longitude?

• Longitude is the angular distance of a place east or west of the Prime Meridian, measured in degrees from the Earth’s center.
• These lines are semi-circles that meet at the North and South Poles and cross the equator perpendicularly.
• Unlike latitudes (which are parallel), all longitudes are equal in length.

The Prime Meridian (0° Longitude)

• In 1884, geographers decided to use the meridian passing through the Royal Astronomical Observatory in Greenwich, England, as the Prime Meridian (0°).
• Longitudes east of Greenwich are labeled E (East), and those west of Greenwich are labeled W (West).
• The maximum longitude is 180°, meaning the total range is 0° to 180° E and 0° to 180° W.

Longitude and Time: Why Does It Matter?

Longitudes are not just for mapping locations; they help determine time zones.

Earth’s Rotation and Time Calculation

• The Earth completes one full rotation (360°) in 24 hours.
• That means it moves 15° per hour or 1° every 4 minutes.
• Since Earth rotates from west to east, places east of Greenwich have later times than GMT, and places west of Greenwich have earlier times.

Practical Example: Time Change with Longitude

• If you move 15° east of Greenwich, local time increases by 1 hour.
• If you move 15° west, local time decreases by 1 hour.
• At 180° E, the time is GMT+12, while at 180° W, the time is GMT-12.
• The total time difference between 180° E and 180° W is 24 hours, or one full day.

Standard Time and Time Zones

To avoid confusion, countries adopt Standard Time based on a central meridian.

Standard Time

• Standard Time is the official time adopted by a country based on a selected longitude.
• Example: Indian Standard Time (IST) is based on 82.5° E longitude (near Prayagraj, Uttar Pradesh).
• IST = GMT+5:30 (5 hours 30 minutes ahead of Greenwich).

Time Zones in Large Countries

• Countries with large east-west extents cannot follow a single time zone.
• Russia has 9 time zones, and Canada & USA have 6 time zones each.
• If India followed multiple time zones, Assam’s sunrise would be too early compared to Gujarat.

The International Date Line (IDL): When a New Day Begins

The International Date Line (IDL) is an imaginary line at 180° longitude in the Pacific Ocean.

What Happens When You Cross the IDL?

• If you travel eastward across the IDL, you gain a day.
• If you travel westward across the IDL, you lose a day.
• This ensures that calendar dates remain consistent worldwide.

Why is the IDL Zigzagged?

• The IDL is not a straight line; it is zigzagged to avoid splitting countries and island groups into different date zones.
• Example: Kiribati moved its time zone to be the first to enter the New Year!

Who Celebrates the New Year First & Last?

• First places: Samoa, Christmas Island (Kiribati), Tonga.
• Last places: Baker Island (USA), Howland Island (USA).

Final Thought: Why Are Longitudes Important?

Longitudes are more than just mapping tools. They define time zones, business hours, and even historical events. From the Greenwich Prime Meridian to the zigzagging International Date Line, longitudes shape how we experience time on Earth!