distinct
course, governed by considerations quite different from those which
controlled the main direction of my voyage. Thus far I had simply
risen straight from the Earth in a direction somewhat to the
southward, but on the whole "in opposition," or right away from the
Sun. So, at the conclusion of my journey, I should have to devote some
days to a gradual descent upon Mars, exactly reversing the process of
my ascent from the Earth. But between these two periods I had
comparatively little to do with either planet, my course being
governed by the Sun, and its direction and rate being uniform. I
wished to reach Mars at the moment of opposition, and during the whole
of the journey to keep the Earth between myself and the Sun, for a
reason which may not at first be obvious. The moment of opposition is
not necessarily that at which Mars is nearest to the Earth, but is
sufficiently so for practical calculation. At that moment, according
to the received measurement of planetary distances, the two would be
more than 40 millions of miles apart. In the meantime the Earth,
travelling on an interior or smaller orbit, and also at a greater
absolute speed, was gaining on Mars. The Astronaut, moving at the
Earth's rate under an impulse derived from the Earth's revolution
round the Sun (that due to her rotation on her own axis having been
got rid of, as aforesaid), traveller in an orbit constantly widening,
so that, while gaining on Mars, I gained on him less than did the
Earth, and was falling behind her. Had I used the apergy only to drive
me directly outward from the Sun, I should move under the impulse
derived from the Earth about 1,600,000 miles a day, or 72 millions of
miles in forty-five days, in the direction common to the two planets.
The effect of the constantly widening orbit would be much as if the
whole motion took place on one midway between those of the Earth and
Mars, say 120 millions of miles from the Sun. The arc described on
this orbit would be equivalent to 86 millions of miles on that of
Mars. The entire arc of his orbit between the point opposite to that
occupied by the Earth when I started and the point of opposition--the
entire distance I had to gain as measured along his path--was about
116 millions of miles; so that, trusting to the terrestrial impulse
alone, I should be some 30 millions behindhand at the critical moment.
The apergic force must make up for this loss of ground, while driving
me in a directio