São Lourenço, i.e., Saint Laurence or Lawrence, was the name given to Madagascar by the Portuguese. According to Wikipedia:
European contact began on August 10, 1500, when the Portuguese sea captain Diogo Dias sighted the island after his ship separated from a fleet going to India. The Portuguese traded with the islanders and named the island São Lourenço (Saint Lawrence).
In William C. Atkinson's translation, the passage quoted in the question is rendered:
While this scene was taking place on Olympus, the Portuguese sailed on and on. They had rounded the Cape now and were heading north again between the mainland and the island of Madagascar. (p. 74)
So also the 1877 translation by William Julius Mickle at Project Gutenberg:
Whilst thus in heaven's bright palace fate was weigh'd
Right onward still the brave Armada strayed:
Right on they steer by Ethiopia's strand
And pastoral Madagascar's verdant land.
Mickle's endnote says:
And pastoral Madagascar.—Called by the ancient geographers, Menuthia and Cerna Ethiopica; by the natives, the Island of the Moon; and by the Portuguese, the Isle of St. Laurence, on whose festival they discovered it.
In Canto 10, Camõens mentions this island twice. In verse 39, relating the adventures of Tristão da Cunha, he writes:
Pelo Cunha também, que nunca extinto
Será seu nome em todo o mar que lava
As ilhas do Austro, e praias que se chamam
De São Lourenço, e em todo o Sul se afamam! (ll. 5–8)
Atkinson renders this as:
This is the doing of Tristão da Cunha, a name that will never be forgotten throughout the sea that washes these southern islands and the far-famed shores of Madagascar in particular. (p. 335)
Green Madagascar's flow'ry dales shall swell
His echo'd fame, till ocean's southmost bound
On isles and shores unknown his name resound.
I do not know why in these earlier instances these English translators have used "Madagascar" in the text. I would imagine that keeping St Laurence and mentioning Madagascar in a footnote would make more sense. Particularly since in stanza 137 Canto 10, Camões identifies the island by both its former and its current name:
De São Lourenço vê a Ilha afamada,
Que Madagáscar é dalguns chamada. (ll. 7–8)
And see here the famous island of São Lourenço, that some call Madagascar. (p. 364)
Mickle's translation of these lines is, well, creative:
And lo, the Island of the Moon displays
Her vernal lawns, and num'rous peaceful bays
His endnote explains:
And lo, the Island of the Moon.—Madagascar is thus named by the natives.
Mickle is mistaken. Samuel Pasfield Oliver notes that Arab sailors sometimes called Madagascar jazirat al-qamar or "Island of the Moon" (p. 3); but there is no indication it was ever called this by its original inhabitants, nor does Camões use this fanciful description. But as Atkinson's more faithful translation shows, Camões was aware that the island he has referred to as São Lourenço was now known by a different name.
- Atkinson, William C, trans. The Lusiads. By Luis Vaz de Camões. London: Penguin Books, 1952.
- Camões, Luís Vaz de. Os Lusíadas. Project Gutenberg. Accessed 29 March 2021.
- Mickle, William Julius, trans. The Lusiad; Or, The Discovery of India. By Luis de Camões. 1776. 5th ed., revised by E. Richmond Hodges. London: George Bell, 1877. Project Gutenberg. Accessed 29 March 2021.
- Oliver, Samuel Pasfield. Madagascar: an Historical and Descriptive Account of the Island and its Former Dependencies. London: Macmillan, 1886. Archive.org. Accessed 29 March 2021.