 In 1663 Scottish mathematician James Gregory had suggested in his <unk> <unk> that observations of a transit of the planet Mercury , at widely spaced points on the surface of the Earth , could be used to calculate the solar parallax and hence the astronomical unit using triangulation . Aware of this , a young Edmond Halley made observations of such a transit on 28 October <unk> 1677 from Saint Helena but was disappointed to find that only Richard <unk> in Burnley , Lancashire had made another accurate observation of the event whilst <unk> , at Avignon , simply recorded that it had occurred . Halley was not satisfied that the resulting calculation of the solar parallax at 45 " was accurate . 
