Magnus Sinus

The Magnus Sinus or Sinus Magnus (Latin; Greek: ὀ Μέγας Κόλπος, o Mégas Kólpos), also anglicized as the Great Gulf, was the form of the Gulf of Thailand and South China Sea known to Greek, Roman, Arab, Persian, and Renaissance cartographers before the Age of Discovery. It was then briefly conflated with the Pacific Ocean before disappearing from maps.

The gulf and its major port of Cattigara had supposedly been reached by a 1st-century Greek trader named Alexander, who returned safely and left a periplus of his voyage. His account that Cattigara was "some days" sail from Zaba was taken by Marinus of Tyre to mean "numberless" days and by Ptolemy to mean "a few". Both Alexander and Marinus's works have been lost, but were claimed as authorities by Ptolemy in his Geography. Ptolemy (and presumably Marinus before him) followed Hipparchus in making the Indian Ocean a landlocked sea, placing Cattigara on its unknown eastern shoreline. The expanse formed between it and the Malay Peninsula (the "Golden Chersonese"), he called the Great Gulf.

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