midnight
The apergic force must make up for this loss of ground , while driving me in a direction , so to speak , at right angles with that of the orbit , or along its radius , straight outward from the Sun , forty odd millions of miles in the same time . If I succeeded in this , I should reach the orbit of Mars at the point and at the moment of opposition , and should attain Mars himself . But in this I might fail , and I should then find myself under the sole influence of the Sun 's attraction ; able indeed to resist it , able gradually to steer in any direction away from it , but hardly able to overtake a planet that should lie far out of my line of advance or retreat , while moving at full speed away from me . In order to secure a chance of retreat , it was desirable as long as possible to keep the Earth between the Astronaut and the Sun ; while steering for that point in space where Mars would lie at the moment when , as seen from the centre of the Earth , he would be most nearly opposite the Sun , -- would cross the meridian at midnight . It was by these considerations that the course I henceforward steered was determined . By a very simple calculation , based on the familiar principle of the parallelogram of forces , I gave to the apergic current a force and direction equivalent to a daily motion of about 750,000 miles in the orbital , and rather more than a million in the radial line . I need hardly observe that it would not be to the apergic current alone , but to a combination of that current with the orbital impulse received at first from the Earth , that my progress and course would be due .