Now You See It, Now You Don’t - The Stones and the Stars: Building Scotland's Newest Megalith - page 15

change in a way that showed that most of its stars were traveling
together. The observer might even realize that some of the other
bright stars spread around the sky shared the same motion, so that
the Sun was passing through a widely scattered cluster. (That was
big news, early in the twentieth century [ 2 ] .) But most of the star
background would seem to be motionless, over thousands upon
thousands of years.
Slow down by another factor of a thousand, however, so that
single years go by in subjective seconds, and the planets would be
visible - although Mercury and Venus would be moving so fast
that it would be hard to see them as moving points rather than
flickering bands. But it would be obvious, even for the more distant
planets, that each one moves around the Sun in a fixed plane, relative
to the starry background.
Earth, trailing across the field of vision, would not be very
conspicuous - in the model Gavin Roberts planned, with the
Sighthill circle representing the Sun and the orbit of Pluto set at
Glasgow's city boundary (Chap. 10 ), Earth was less than an inch
across and 200 yards out! The Moon, smaller and less reflective,
would be still harder to see, but its flitting around Earth would
be obvious - above, ahead, below, behind, and not repeating on a
year-to-year basis. On this timescale, and assuming no abnormal
powers of vision, the Moon would be the only visible object with
a continuously changing path.
Earth and Moon form a twin-planet system. The Moon is so
far out that the perturbing pull of the Sun would wrench it away
from us, but the combined gravitational attractions of the Earth
and Moon are enough to keep them together. Strictly speaking,
however, the Moon does not orbit around Earth. It orbits the Sun
in a path that is continuously modified by Earth into a series of
scallop-shaped curves [ 3 ]. Seen from the Sun, the Moon would
move continually forward against the starry background. If it were
close enough to Earth, and therefore moving fast enough to move
“back,” seen from the Sun, it would be moving too fast for the
human eye to follow, on this timescale.
Supposing that Earth, too, were a transparent airless sphere,
and that the viewpoint moved to its center; then the timescale
would have to be slowed by another factor of a thousand to make
events comprehensible. Even then it would seem that the day lasted