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2 years ago in Geodesy , History of Science By Lalit Mudra

How did geodesists determine the position of the equatorial plane?

I'm researching the history of cartography and the development of global coordinate systems. The concept of the equator as a fundamental reference plane is ancient, but I'm fascinated by the practical methods. Before GPS, how did surveyors on the ground actually establish where "zero latitude" was with sufficient accuracy for mapping?

 

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By Roopa K Answered 3 years ago

Before the space age, this was done through meticulous astronomy. Geodesists used a meridian circle telescope to track the precise moment a star crossed the local meridian (due north to south). By measuring the star's zenith angle at that moment and knowing its declination (its celestial "latitude"), they could compute the observer's latitude. The celestial equator is defined as 0° declination. To locate it physically on Earth, they relied on the fact that at the equinoxes, the Sun is directly over the terrestrial equator. By observing equal day and night and the Sun's transit height, they could infer the equatorial line and extrapolate it via triangulation from multiple observatories.

 

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