Which Persian polymath calculated Earth's radius from a single mountain measurement?

Al-Biruni, around 1020, using a method requiring one hill and one instrument. Measure the height of the hill, climb it, measure the angle down to the horizon, and trigonometry gives the radius of the sphere you are standing on. His answer was within about one per cent of the modern value.

He was formidably productive across fields that had no business overlapping: a comparative study of India written after learning Sanskrit, in which he described Hindu beliefs accurately and without hostility at a time when he was travelling with a conquering army; measurements of the specific gravity of eighteen substances accurate to three decimal places; an argument that the Indus valley had once been a sea bed, from the rounded stones in its soil.

He also reasoned that there was probably land between Europe and Asia across the Atlantic, on the grounds that the known landmasses did not plausibly account for the whole sphere and that a hemisphere of empty ocean seemed unlikely. He wrote something like 146 works; about 22 survive. He and Ibn Sina corresponded, and the surviving exchange is mostly Al-Biruni pointing out problems in Aristotle and Ibn Sina defending him, with both becoming visibly irritated.