Transits of Mercury

Abstract

A 12^th-century Moroccan astronomer, Abu Ishaq Al-Bitruji Al-Ishbili — better remembered today as Alpetragius — made the comment that since he had never seen Mercury pass across the face of the Sun, the planet must be transparent. What he did not know was that when in transit, Mercury is too small to be seen with the naked eye — and there were no telescopes in Alpetragius’ day.