Geology Reference
In-Depth Information
about 3 s, with the Sun, Moon and stars wheeling continuously
around, parallel to the equator. It would be obvious at once that
the Moon was moving from west to east, against the starry
background, and swinging north of the equator, then south, over
the minute or so which it took to go around the sky and complete
its cycle of phases. Even on the second time around, it should be
obvious that the Moon's path against the stars had slipped against
the stars. It might even be obvious that the Sun's pull was literally
dragging the Moon back, forcing the orbital plane to rotate.
The other factor at work, the pull of Earth's equatorial bulge,
would be harder to deduce. Meanwhile the Sun's movement would
become apparent, spiraling up the heavens to the Tropic of Cancer
at northern-hemisphere midsummer, then south again to reach the
Tropic of Capricorn at the midwinter solstice. It might take quite
some time to realize that the Sun, too, was moving west to east
against the stars, and the apparent movement up and down the sky
was a result of the tilt of Earth's axis. Since the major clues would
be lacking at the center of a transparent Earth, the observer might
never realize that Earth was going around the Sun, not vice versa.
Over a year - subjectively, about a quarter of an hour - the
slippage of the Moon's orbit would be very apparent. It takes only
18.61 years to precess right around the sky. The corresponding
effect, of the pull of Sun and Moon on Earth's equatorial bulge,
would take longer to show, but eventually it would become clear
that, while the Sun's path against the stars remained unchanged,
the whole reference frame of equator, tropics and poles was trac-
ing a very slow circle against the stars. That effect, the precession
of the equinoxes, takes 26,000 years for a single cycle; by the time
he recognized it our imaginary speeded-up observer might real-
ize that the axial tilt itself was diminishing, bringing the tropics
very slowly closer together. Over 5,000 years the midsummer Sun
would have moved south by about its apparent diameter, and the
midwinter Sun northward by the same amount.
Suppose, finally, that our observer moves up to Earth's
surface, that Earth becomes opaque, and that the atmosphere
arrives to complicate matters - displacing objects from their true
position in the sky by the effect of refraction. And suppose that our
observer's time sense is slowed by more than 30,000 times, to the
normal rate of human perception. Now it takes 4 min for Earth's
Search WWH ::




Custom Search