Meridian Longitude - [top]

Greenwich Mean Time (GMT), based on the Prime Meridian, was long used as the international time standard. Today, it has been succeeded by Coordinated Universal Time (UTC), which is kept precise by atomic clocks but remains anchored to the Prime Meridian grid. The International Date Line

Meridian and Longitude: Mapping the Earth’s East-West Grid

A is an imaginary semicircle on Earth’s surface that runs from the North Pole to the South Pole. Each meridian represents a line of constant longitude. When you combine two opposite meridians (e.g., the prime meridian and the 180th meridian), they form a great circle that divides Earth into two equal hemispheres. meridian longitude

Telescopes must track objects across the sky using coordinates that include hour angles derived from longitude. Similarly, space agencies choose launch sites near the equator (low latitude) but must calculate the Earth’s rotational velocity—which varies by longitude—to slingshot rockets into orbit efficiently.

Every system of measurement needs a zero point. For meridian longitude, that zero point is the . Historically, different civilizations used various reference meridians—for example, the Romans used the meridian through the Canary Islands, while Arab astronomers used a meridian through the western edge of the known world. Greenwich Mean Time (GMT), based on the Prime

However, 18th-century pendulum clocks could not keep accurate time on a rocking, humid ship. Astronomers argued that the answer lay in the skies—painstakingly mapping the movement of the moon against the stars (the Lunar Distance Method).

This public link is valid for 7 days and shares a thread, including any personal information you added. This link or copies made by others cannot be deleted. If you share with third parties, their policies apply. Can’t copy the link right now. Try again later. Each meridian represents a line of constant longitude

When looking at a map or a globe, you will see a grid of lines that help us pinpoint exact locations on Earth. While horizontal lines (parallels) measure latitude, vertical lines represent . Meridians are fundamental to navigation, timekeeping, and geographical studies, connecting the North and South Poles and defining how far east or west a location is from a designated starting point. What is a Meridian?

While Greenwich is the global standard, other meridians hold historical or scientific importance.

As mentioned, the 180th meridian is the basis for the International Date Line. It is also used as a reference for some national boundaries and time zone offsets (UTC+12 to UTC-12). Countries like Fiji, New Zealand, and Russia have territories near this meridian.