 In 1663 Scottish mathematician James Gregory had suggested in his Optica Promota 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 O.S. 1677 from Saint Helena but was disappointed to find that only Richard Towneley in Burnley , Lancashire had made another accurate observation of the event whilst Gallet , at Avignon , simply recorded that it had occurred . Halley was not satisfied that the resulting calculation of the solar parallax at 45 " was accurate . 
 In 1663 Scottish mathematician James Gregory <a href='been_done' title='1'><mark>had</mark></a> suggested in his Optica Promota 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 O.S. 1677 from Saint Helena but was disappointed to find that only Richard Towneley in Burnley , Lancashire <a href='been_done' title='2'><mark>had</mark></a> made another accurate observation of the event whilst Gallet , at Avignon , simply recorded that it <a href='been_done' title='3'><mark>had</mark></a> occurred . Halley <a href='negative_concord' title='4'><mark>was</mark></a> <a href='negative_concord' title='5'><mark>not</mark></a>satisfied that the resulting calculation of the solar parallax at 45 " was accurate . 
In 1663 Scottish mathematician James Gregory done suggested in his Optica Promota dat observations of a transit of da planet Mercury , at widely spaced points on tdasurface of the Earth , could b used to calculate the solar parallax and hence the astronomical unit using triangulation . Aware of dis , a young Edmond Halley made observations of such a transit on 28 October O.S. 1677 from Saint Helena but was disappointed to find that only Richard Towneley in Burnley , Lancashire done made another accurate observation of thdavent whilst Gallet , at Avignon , simply recorded that it done occurred . Halley ain't satisfied tdatthe resulting calculation of the solar parallax at 45 " was accurate .