midnight
I had to reach this line , and having reached it , to remain for some time above it . To do both , I must attain it , if possible , at the same moment at which I secured a westward impulse just sufficient to counterbalance the eastward impulse derived from the rotation of the Earth ; -- that is , in the latitude from which I started , a thousand miles an hour . I had calculated that while directing through the main bar a current of apergy sufficient to keep the Astronaut at a fixed elevation , I could easily spare for the eastward conductor sufficient force to create in the space of one hour the impulse required , but that in the course of that hour the gradually increasing apergic force would drive me 500 miles westward . Now in six hours the Earth 's rotation would carry an object close to its surface through an angle of 90 ° ; that is , from the sunset to the midnight meridian . But the greater the elevation of the object the wider its orbit round the Earth 's centre , and the longer each degree ; so that moving eastward only a thousand miles an hour , I should constantly lag behind a point on the Earth 's surface , and should not reach the midnight meridian till somewhat later . I had , moreover , to lose 500 miles of the eastward drift during the last hour in which I should be subject to it , through the action of the apergic force above-mentioned . Now , an elevation of 330 miles would give the Astronaut an orbit on which 90 ° would represent 6500 miles .