a question about Mercury

Until 1962 it was thought that Mercury's "day" was the same length as
its "year" so as to keep that same face to the Sun much as the Moon
does to the Earth. But this was shown to be false in 1965 by doppler
radar observations.

One thing I'd love to know: just *why* did people believe this? I'd
have thought there'd be no telescopes at that time that could show
surface markings on Mercury, so how would anyone be able to even guess
its rotation period?

"brian" wrote in message
ups.com...
Until 1962 it was thought that Mercury's "day" was the same length as
its "year" so as to keep that same face to the Sun much as the Moon
does to the Earth. But this was shown to be false in 1965 by doppler
radar observations.

One thing I'd love to know: just *why* did people believe this? I'd
have thought there'd be no telescopes at that time that could show
surface markings on Mercury, so how would anyone be able to even guess
its rotation period?

It was thought that, due to Mercury's close proximity
to the Sun, tidal braking would have damped out the
axial rotation and caused Mercury to face one side
always towards the Sun. It's the same thing that
caused the Moon's condition with respect to the Earth.

"brian" wrote in message ups.com...
Until 1962 it was thought that Mercury's "day" was the same length as
its "year" so as to keep that same face to the Sun much as the Moon
does to the Earth. But this was shown to be false in 1965 by doppler
radar observations.

One thing I'd love to know: just *why* did people believe this?

Same reason they still believe in virgin births and only-one-speed-of-light for all.
They are brainwashed, brainbleached and braindried-out-in-the-sun so that they
repeat what they hear like sheep, all bleating the same "baa".
http://www.androcles01.pwp.blueyonder.co.uk/SR.GIF
Nature works by democratic vote.



I'd
have thought there'd be no telescopes at that time that could show
surface markings on Mercury, so how would anyone be able to even guess
its rotation period?

Anyone can guess, it's far easier than discovery.
Mars had canals once instead airline routes.
http://www.nasa.gov/centers/langley/...ain_Mesaba.jpg
http://ssed.gsfc.nasa.gov/tharsis/canals.html

Most of the world is populated by knuckle-draggers.
Neanderthals see what they expect to see and if they have no way of checking
then the best guess wins, and *that* is why.

As to the tidal braking theory, Mercury is in an eccentric (0.2) elliptical orbit.
It should oscillate back and forth like the balance wheel of a clock.
http://www.androcles01.pwp.blueyonde.../Analemmae.htm
Conservation of angular momentum be damned.

Greg Neill wrote:
"brian" wrote in message
ups.com...
Until 1962 it was thought that Mercury's "day" was the same length as
its "year" so as to keep that same face to the Sun much as the Moon
does to the Earth. But this was shown to be false in 1965 by doppler
radar observations.

One thing I'd love to know: just *why* did people believe this? I'd
have thought there'd be no telescopes at that time that could show
surface markings on Mercury, so how would anyone be able to even guess
its rotation period?

It was thought that, due to Mercury's close proximity
to the Sun, tidal braking would have damped out the
axial rotation and caused Mercury to face one side
always towards the Sun. It's the same thing that
caused the Moon's condition with respect to the Earth.

Actually there's more to it.

Mercury is best viewed in the evening sky in the spring and in the
morning sky in the fall, owing to the angle which the ecliptic makes
with the horizon. So suppose you're an avid Mercury observer in the
Northern Hemisphere. You'll see it for a week or two in March, then
about six months later you'll see it for another week or two. In those
intervening six months, Mercury will have completed two revolutions
(176 days is not quite half a year) and three rotations. It will
be showing more or less the same face to the sun! This geometry
will persist for virtually *all* the favorable apparitions in the
Northern Hemisphere.

Now it's difficult to discern markings on Mercury visually -- it's low
in the sky, usually in twilight, and tough to see in daylight. Is it
surprising that people interpreted the available observations in favor
of synchronous rotation?

-- Bill Owen