 By comparing the angle between the moon and the sun for the day one left for Britain, the "proper position" (how it would appear in Greenwich, England, at that specific time) of the moon could be calculated.By comparing this with the angle of the moon over the horizon, the longitude could be calculated.During Harrison's second trial of his 'sea watch' (H4) the Reverend Nevil Maskelyne was asked to accompany HMS "Tartar" and test the Lunar Distances system.