Thomas Harriot (c. 1560 – 1621) was the senior English mathematician of his generation. He had been Walter Raleigh’s astronomical-navigational adviser on the 1585 Roanoke expedition (his Briefe and True Report of the New Found Land of Virginia, 1588, is the first English-language ethnographic account of any part of North America). He had developed substantial original algebraic notation (his system of letters for unknowns is the direct ancestor of modern symbolic algebra; his sign for inequality is the modern one). He had carried out detailed optical-refraction experiments in the 1590s that anticipated Snell’s law of refraction by approximately three decades. From 1597 he lived under the patronage of Henry Percy, 9th Earl of Northumberland, at the Earl’s London estate at Syon House.

He was, by any reasonable measure, the best English mathematician of his generation and probably one of the best European ones.

He published almost nothing.

What he saw before Galileo

The historically substantial sequence runs through the second half of 1609 and the first half of 1610.

26 July 1609: Harriot turned a Dutch-style refracting telescope (approximately 6× magnification, acquired through his contacts in the Northumberland circle) on the Moon from his Syon House workroom and made a substantially detailed pen-and-ink drawing of the lunar surface. The drawing shows the terminator (the line between lunar day and night), four large lunar features that are identifiable with modern crater-and-mare features, and an inscribed date in Harriot’s hand. The drawing survives in the West Sussex Record Office. It is the earliest known telescopic drawing of the Moon by approximately five months.

Late 1609 – early 1610: Harriot continued lunar observations, accumulated additional drawings showing the lunar surface at different phases, and produced a outline of what would become his (unpublished) lunar map. He shared the drawings with William Lower at Trefenty in Wales and with several other members of the Northumberland mathematical circle.

March 1610: Galileo published Sidereus Nuncius in Venice. The work announced his discovery of the moons of Jupiter, the mountains of the Moon, and the dense star fields of the Milky Way. The first copies reached England by approximately late April 1610.

Probably early 1610, before learning of the Galileo publication: Harriot independently observed the moons of Jupiter and recorded their motions. The dating is uncertain because Harriot did not date all his observation papers consistently; the historical consensus is that the Jovian observations were at least roughly contemporaneous with Galileo’s and may have been earlier.

December 1610: Harriot observed sunspots — the same observation Galileo would publish in 1613 and that the Jesuit Christopher Scheiner would dispute Galileo’s priority on. Harriot’s sunspot observations continued through 1611–1613 and produced approximately 200 detailed drawings.

Why he did not publish

The historical question of Harriot’s non-publication has produced approximately four centuries of speculation and no definitive answer.

The standing explanations cluster into three groups. The first is the political: the Northumberland circle was under royal suspicion in the years after the Gunpowder Plot (Henry Percy himself was imprisoned in the Tower of London from 1605 to 1621 on suspicion of complicity with his cousin Thomas Percy, one of the Plot conspirators), and publication of philosophically or theologically sensitive work would have increased Harriot’s exposure to royal-conciliar attention. The second is the temperamental: Harriot was substantively a private scholarly figure with no documented appetite for the public-intellectual role that European publication required. The third is the practical: Harriot had been in poor health from approximately 1610 onward (he had a skin cancer of the nose, probably caused by his smoking, that progressively worsened through the next decade and incapacitated him in his final years).

Whatever the explanation, the non-publication left his discoveries substantively without effect on the broader European scientific record. The 5,000 pages of his unpublished manuscripts — now divided between the British Library, the West Sussex Record Office, and Petworth House — were unknown to subsequent generations until the late 18th century, and were not subjected to comprehensive scholarly analysis until the 20th. The standing modern judgement is that Harriot was substantively a first-rank scientific mind whose institutional and personal circumstances prevented him from mattering to the European scientific tradition in his own time.

He died at Syon House on 2 July 1621, aged about 61, of the long-standing nasal cancer.