beginning in july 1877, the astronomer sir david gill and his wife isobel spent six months on ascension island. this was to take advantage of the near approach of mars occurring that year. based on johannes kepler's laws of planetary motion gill conceived that in pioneering the use of a heliometer he would be able to accurately measure the transit of mars on his own, rather than in combination with many observers simultaneously recording the position of the planet as had been the technique during the time. this is because a heliometer is a telescope that uses a split image to measure the angular separation of celestial bodies. in observing this from near the equator a greater observable distance would be visible, hence a temporary observatory being decided upon for ascension.