Now in elliptical orbits attracting bodies always occupy one of the foci of the ellipsis. The satellite is, therefore, nearer the body round which it gravitates at one moment than it is at another. When the earth is nearest the sun she is at her perihelion, and at her aphelion when most distant. The moon is nearest the earth at her perigee, and most distant at her apogee. To employ analogous expressions which enrich the language of astronomers, if the projectile remained a satellite of the moon, it ought to be said that it is in its "aposelene" at its most distant point, and at its "periselene" at its nearest.
In the latter case the projectile ought to attain its maximum of speed, in the latter its minimum. Now it was evidently going towards its "aposelene," and Barbicane was right in thinking its speed would decrease up to that point, and gradually increase when it would again draw near the moon. That speed even would be absolutely nil if the point was coexistent with that of attraction.
Barbicane studied the consequences of these different situations; he was trying what he could make of them when he was suddenly interrupted by a cry from Michel Ardan.
"I'faith!" cried Michel, "what fools we are!"
"I don't say we are not," answered Barbicane; "but why?"
"Because we have some very simple means of slackening the speed that is taking us away from the moon, and we do not use them."
"And what are those means?"
"That of utilising the force of recoil in our rockets."
"Ah, why not?" said Nicholl.
"We have not yet utilised that force, it is true," said Barbicane, "but we shall do so."