Topic: What is the diameter of the Earth and does it increase over time?

Eratosthenes

"Now the distance from S [Syrene] to A [Alexandria] was known by measurement to be 5,000 stades; it followed that the circumference of the earth was 250,000 stades. This is the figure given by Cleomedes, but Theon of Smyrna and Strabo both give it as 252,000 stades. The reason for the discrepancy is not known; it is possible that Eratosthenes corrected 250,000 to 252,000 for some reason, perhaps in order to get a figure divisible by 60 and, incidentally, a round number (700) of stades for one degree. If Pliny is right in saying that Eratosthenes made 40 stades equal to the Egyptianskoinos, then, taking theskoinosat 12,000 Royal cubits of 0.525 metres, we get 300 such cubits, or 157.5 metres, i.e. 516.73 feet, as the length of the stade. On this basis 252,000 stades works out to 24,662 miles, and the diameter of the earth to about7,850 miles." (Heath 1921)

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Theon of Smyrna

"Thus the whole diameter of the Earth would be approximately 80,182 stades. For three times this number plus a seventh of it was the perimeter of 252,000 stades."

Quoted from On Mathematics Useful for Understanding of Plato.

Also7,850 miles.

Claudius Ptolemy

Ptolemy in theGeographiaimproved on the work of Eratosthenes. He calculated 1 degree as 500 stades, which using Heath's math works out to an Earth diameter of5,607 miles.

Plus, according to one author, "he tended to overestimate the breadth of the whole known world." (Evans 1998)

Modern Measurements

Modern measurements are all over the place (as usual there is no scientific concensus). See here: http://hypertextbook.com/facts/1999/...Martinez.shtml

However, The North American Datum of 1983 (NAD83) [Schwarz, Ed., NOAA, 1989] was determined by the NGS to be 12,756.274 km (7926.75 miles) on the major axis (equatorial), and 12,713.504 km (7900.17 miles) on the minor axis (polar), with an average diameter of 12,734.889 km (7913.46 miles).

So what is the diameter of the Earth and does it increase over time?