The assumed longitude is usually obtained from the DR or Dead Reckoning position run up from a morning sight taken at around 9.00 am. This is worked out by applying the distance from that position either by log or by the estimated speed over time with the course steered. A sight is taken, that is the distance above the horizon of a heavenly object, in this case nearly always the sun, is measured with a sextant and the exact time noted in UTC. The sextant angle obtained is corrected for dip (the error caused by the observers height above the sea) and refraction to obtain the true altitude of the object above the horizon. This is then subtracted from 90° to obtain the angular distance from the position directly above, the zenith. This is referred to as the True Zenith Distance. The true zenith distance of the object is also the distance (in arc) on the Earth's surface from the observer to where that object is overhead, the geographical position of the object.