From a fresh discussion of the transits of Venus in 1761 and 1769 he deduced (1822-1824) a solar parallax of 8" 57, long accepted as authoritative.
Delisle is chiefly remembered as the author of a method for observing the transits of Venus and Mercury by instants of contacts.
Thus in 1857 he went to Peru in order to determine the magnetic equator; in1861-1862and 1864, he studied telluric absorption in the solar spectrum in Italy and Switzerland; in 1867 he carried out optical and magnetic experiments at the Azores; he successfully observed both transits of Venus, that of 1874 in Japan, that of 1882 at Oran in Algeria; and he took part in a long series of solar eclipse-expeditions, e.g.
Up to about the middle of the 19th century it was supposed that transits of Venus across the disk of the sun afforded the most trustworthy method of making the determination in question; and when Encke in 1824 published his classic discussion of the transits of 1761 and 1769, it was supposed that we must wait until the transits of 1874 and 1882 had been observed and discussed before any further light would be thrown on the subject.
Transits of Venus and observations of Deterrnina- various kinds on Mars are all to be included in this tion.