constellations told him only that it was spring in the northern
hemisphere of Earth.
But spring of what year?
Then, suddenly, it occurred to Garrard that he had a
method of finding the answer. The Moon causes tides in the
Earth, and action and reaction are always equal and op-
posite. The Moon cannot move things on Earth without
itself being affectedand that effect shows up in the moon’s
angular momentum. The Moon’s distance from the Earth
increases steadily by 0.6 inches every year. At the end of
12,000 years, it should be 600 feet farther away from the
Earth, and action and reaction are always equal and op-
Was it possible to measure? Garrard doubted it, but he
got out his ephemeris and his dividers anyhow, and took
pictures. While he worked, the Earth grew nearer. By the
time he had finished his first calculationwhich was indeci-
sive, because it allowed a margin for error greater than the
distances he was trying to checkEarth and Moon were
close enough in the telescope to permit much more accurate
measurements.
Which were, he realized wryly, quite unnecessary. The
computer had brought the DFC-3 back, not to an observed
sun or planet, but simply to a calculated point. That Earth
and Moon would not be near that point when the DFC-3 re-
turned was not an assumption that the computer could make.
That the Earth was visible from here was already good and
sufficient proof that no more time had elapsed than had been
calculated for from the beginning.
This was hardly new to Garrard; it had simply been
retired to the back of his mind. Actually he had been doing