Unique scientific observations are made on the rare occasions when Venus 'transits' - or crosses - the sun. Tonight's programme marks the bicentenary of Captain Cook's voyage to the South Pacific to observe a transit - on June 3, 1769 - during which he claimed Australia for Britain on his way home. Patrick Moore talks to Cdr. Derek Howse, R.N. about Cook's voyage and to Dr. Vinicio Barocas about the first astronomer ever to observe a transit, in 1639 - a twenty-year-old Lancashire curate, the Rev. Jeremiah Horrocks