or some other planet, but home whence you came."
Mitchell wrinkled his brow in puzzlement. "I don't understand what you're driving at. There is home; there is Earth behind us." He pointed back to the globe they were leaving.
"You do not comprehend the Sacred Duality," was the Cultist's answer. "Let us join your companion in the control room and it shall be made clear."
The two slipped through to the front chamber, Nadir Khan addressed the silent Congreve. "Your fellow space-man wished enlightenment, and now that we are far from the world behind us, it is time you tell him of our discovery."
Congreve nodded, thought awhile, then began:—
"The universe is a strange place. Mitchell. There are more incredible things, queer things, than ever any of us conceived." He stopped for a moment, then peered ahead as if searching for something.
"You recall our stay on Mercury. Our clocks were worthless, for, as soon as we had left Earth and the gravity ceased its pull, their springs threw them all out of gear, as we should have expected.
"On Mercury, we just couldn't compute time; the planet had neither day nor night and the motion of the stars was too slight to use as a criterion. We could only guess at time by the rising and setting of the planets.
"Earth is a vivid planet: it was close enough to give an unmistak-able point. No one could fail to recognize it. When we landed on the little planet, Earth was a green spot high in the heavens. When we finished our many days of work and repair and exploration, we saw our Earth again high in the heavens. Naturally, we assumed that a Mercurian year had passed; our little planet had revolved once about the sun, we thought, and, thus, we had passed about eighty or so Terrestrial days there. So we set flight again, headed for that green globe so clearly placed in the sky.
"When we reached it, it was Earth; it had Earth's moon, Earth's continents, and in all respects was the same planet that we had left. But we were wrong. . . as we soon found out, although we did not realize it then. It was not our Earth, Mitchell. We were not on Mercury for the duration of one of the little planet's years; we were there only for a Mercurian half-year."
Mitchell's bewilderment grew. "But we saw Earth high in the sky; had we stayed only about forty days, it would have been behind the sun and out of sight."
"Precisely," answered Congreve. "So it was. Our Earth was behind the sun and out of sight. There are two Earths, Mitchell. Our world is one of twins. When, untold ages ago, the planets were cast off the sun, there was a dual explosion when it came to our planet's turn. Two bodies were cast off, of equal size and mass, each composed of similar elements. Each body was hurled an equal distance from the sun so that they exactly counterbalanced.
"Thus have they been through the ages. When they finally settled and hardened, the solar gravitational complex forced each into the same orbit; each pursued the same course about the sun, separated only by 180 degrees of their circle. They follow eternally in each other's footsteps, Mitchell; the sun is always between them so that one cannot be seen from the other."
