Qualla: Prime Meridian (Greenwich)

Prime Meridian (Greenwich): Introduction

Wed, 20 May 2026 00:00:00 GMT

Photo credit Daniel Case, CC BY-SA 3.0. Stand on the brass strip outside the Royal Observatory and check your phone's GPS. It will read 0.0014 degrees east, not zero. The line in the cobblestone, drawn through Sir George Airy's transit circle telescope, is about 102 metres west of where modern satellites place the actual zero meridian. Most tourists never notice. They straddle the strip, one foot in each hemisphere, take the photo, and walk on. The discrepancy is not an error - it is the residue of how Earth's shape was understood in 1851 versus how satellites measure it now. But the older line, the one drawn by Airy, is the one that mattered most. For exactly one hundred years, it was the line from which the world told time.

Image: Daniel Case | CC BY-SA 3.0

Prime Meridian (Greenwich): Forty-One Delegates, Twenty-Five Nations

Wed, 20 May 2026 00:00:00 GMT

Photo credit Encyclopædia Britannica, Volume 8, Slice 9., available freely at Project Gutenberg, Public domain. In October 1884, at the invitation of the President of the United States, forty-one delegates from twenty-six nations gathered in Washington, D.C. for the International Meridian Conference. They had come to settle an awkward question: from which spot on Earth should longitude be ...

Image: Encyclopædia Britannica, Volume 8, Slice 9., available freely at Project Gutenberg | Public domain

Prime Meridian (Greenwich): What Made an Observatory Worthy

Wed, 20 May 2026 00:00:00 GMT

Photo credit Daniel Case, CC BY-SA 3.0. The Spanish geodesist Carlos Ibáñez e Ibáñez de Ibero had laid out the criteria the year before. To host the world's prime meridian, an observatory needed three things: a first-rate astronomical instrument, direct astronomical links to neighboring observatories, and a network of ...

Image: Daniel Case | CC BY-SA 3.0

Prime Meridian (Greenwich): A Line Through Earth

Wed, 20 May 2026 00:00:00 GMT

Photo credit Daniel Case, CC BY-SA 3.0. The meridian runs from pole to pole, slicing through eight countries on its way south: the United Kingdom (only England), France, Spain, Algeria, Mali, Burkina Faso, Togo, Ghana. It touches Antarctica only at Queen Maud Land, claimed by Norway. In Accra, Ghana, the line crosses t...

Image: Daniel Case | CC BY-SA 3.0

Prime Meridian (Greenwich): Why the Line Drifted

Wed, 20 May 2026 00:00:00 GMT

Photo credit Daniel Case, CC BY-SA 3.0. The explanation involves the deflection of the vertical, a concept that sounds esoteric until you think about it. If you hang a plumb line at Greenwich, where does its straight-down direction point? Not exactly toward Earth's center, because Earth is not a sphere - it is an oblat...

Image: Daniel Case | CC BY-SA 3.0

Prime Meridian (Greenwich): Time as Well as Space

Wed, 20 May 2026 00:00:00 GMT

Photo credit en:User:Markhamilton, Public domain. The 1884 conference did one more thing. Having agreed on a zero for longitude, it gave the world a zero for time. Greenwich Mean Time became the universal reference, and time zones - those orderly hour-wide bands marching east and west around the globe - were defined relative to ...

Image: en:User:Markhamilton | Public domain