Willebrord Snellius · 1617
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The founding text of modern geodesy. In 1615 Snellius measured the arc of the meridian between Alkmaar and Bergen op Zoom by chaining a network of triangles across the towns of Holland — the first survey to determine the size of the Earth by triangulation rather than by pacing or dead reckoning.
Book I is a critical reconstruction of the ancient measurements, sifting Eratosthenes, Posidonius, Ptolemy and Strabo against one another. Snellius is not paying homage: he is establishing what the ancients actually claimed, on what evidence, and where their figures cannot be reconciled — clearing the ground for a measurement that will settle the question.
Book II is his own survey. It sets out the instruments, the baselines, the observed angles and the computations, and arrives at a degree of the meridian of 55,100 toises. The volume also carries a set of carefully recorded latitudes of places, by which the chorographies of the provinces could be corrected.
The method is the contribution. Rather than measure a long distance directly — impossible over any useful span — Snellius measures one short baseline with care, then determines a chain of triangles by angle alone, carrying the known length across the whole network by trigonometry. The Dutch landscape, flat and studded with church towers, was almost purpose-built for it.
The title is a claim: Eratosthenes had measured the Earth in antiquity, and Snellius means to do it again properly, as a Dutchman. He succeeded well enough that the technique he introduced governed geodesy until the satellite era — national surveys, map projections and the figure of the Earth all rest on triangulation for the next three centuries.
It also belongs beside the astronomy in this corpus for a practical reason: the size of the Earth is the baseline against which every astronomical distance is eventually measured.
The translated source is the 1617 printing, which opens with commendatory verses to the author, including a scazon — a “limping” choliambic poem.