What is a meridian arc?
Q: What is a meridian arc?
A: A meridian arc is the distance between two points with the same longitude. It is also an arc, or segment of a curve, that would be created by an imaginary rope laid over the globe.
Q: How are reference ellipsoids determined?
A: Reference ellipsoids are determined by taking two or more measurements of meridian arcs at different places and using those measurements to get the shape of the reference ellipsoid which most closely resembles the shape of the geoid. This process is referred to as "the determination of the figure of the Earth".
Q: Who was Eratosthenes and what did he do?
A: Eratosthenes was an Alexandrian scientist who lived around 240 BC. He calculated a good value for circumference of Earth by knowing that on summer solstice at local noon, sun goes through zenith in ancient Egyptian city Syene (Assuan). He then measured his own hometown Alexandria and found out that zenith distance there was 1/50th of full circle (7.2°). Assuming Alexandria was due north from Syene, he concluded that distance between them must be 1/50th of Earth's circumference.
Q: When did Newton publish his proof about Earth being an oblate spheroid?
A: Newton published his proof in 1687 in Principia stating that Earth was an oblate spheroid with flattening equal to 1/230.
