On 9/21/12 1:35 PM, oriel36 wrote:
> On Sep 21, 5:23 pm, Sam Wormley <
sworml...@gmail.com> wrote:
>>
>> Eratosthenes of Cyrene first to calculate the tilt of the Earth's
>> axis with respect to the ecliptic, and with remarkable accuracy.
>> Ref:
http://en.wikipedia.org/wiki/Eratosthenes
>>
>
> How sweet !,empiricists tend to manufacture history on an industrial
> scale and what you and the rest are trying to discuss is rotational
> orientation which does not change in terms of axial precession and to
> the orbital points of solstices and equinoxes.The ancients in remote
> antiquity,and I mean over 5000 years ago would have seen the
> circumpolar stars still turn around Polaris as we see the same thing
> today and that time scale is 20% into the great orbital cycle of the
> precession of the equinoxes and not an axial trait as previously
> believed.
>
Adapted from Carl Sagan's COSMOS book as part of a presentation
I gave more than a year ago:
__________________
There was once a time when our little planet seemed immense, when
it was the only world we could explore, its true size was first
worked out in a simple and ingenious way by a man who lived in
Egypt in the third century B.C.
In Alexandria, at that time, there lived a man named Eratosthenes.
One of his envious contemporaries called him Beta, the second
letter of the Greek alphabet, because, he said, Eratosthenes was
second best in the world in everything, but it seems clear that in
many fields Eratosthenes was Alpha: he was an astronomer,
historian, geographer, philosopher, poet, theater critic and
mathematician.
He was also the chief librarian of the great library of Alexandria
and one day while reading a papyrus book in the library he came
upon a curious account: far to the south, he read, at the frontier
outpost of Syene, something notable could be seen on the longest
day of the year. On June 21 the shadows of a temple column or a
vertical stick would grow shorter as noon approached and as the
hours crept towards midday the sun's rays would slither down the
sides of a deep well which, on other days would remain in shadow
and then precisely at noon columns would cast no shadows and the
sun would shine directly down into the water of the well. At that
moment the sun was exactly overhead.
It was an observation that someone else might easily have ignored:
sticks, shadows, reflections in wells, the position of the sun,
simple everyday matters, so what possible importance might they
be.
But Eratosthenes was a scientist, and his contemplations of these
homely matters changed the world, in a way, made the world.
Because Eratosthenes had the presence of mind to experiment to
actually ask whether back here near Alexandria a stick cast a
shadow near noon on June the twenty first, and it turns out sticks
do. An overly skeptical person might have said that the report
from Syene was an error but an absolutely straightforward
observation why would anyone lie on such a trivial matter?
Eratosthenes asked himself how it could be that at the same
moment a stick in Syene would cast no shadow and a stick in
Alexandria, eight hundred kilometers to the north, would cast a
very definite shadow.
If the shadow at Syene is of a certain length and the shadow at
Alexandria is the same length that also makes sense on a flat
Earth, but how could it be, Eratosthenes asked, that at the same
instant there was no shadow at Syene and a very substantial shadow
at Alexandria.
The only answer was that the surface of the earth is curved, not
only that, but the greater the curvature the bigger the difference
in the length of the shadows. The sun is so far away that its rays
are parallel when they reach the Earth. Sticks at different angles
to the sun's rays will cast shadows of different lengths for the
observed difference in the shadow length the distance between
Alexandria and Syene had to be about 7 degrees along the surface
of Earth, by that I mean if you would imagine these sticks
extending all the way down to the center of the Earth they would
there intersect at an angle of about 7 degrees, well seven degrees
is something like a fiftieth of the full circumference of the
Earth 360 degrees.
Eratosthenes knew the distance between Alexandria and Syene, he
knew it was eight hundred kilometers, why? because he hired a man
to pace out the entire distance so that he could perform the
calculation. I'm talking about now 800 kilometers times fifty is
forty thousand kilometers so that must be the circumference of the
Earth, that's how far it is to go once around the earth, that's
the right answer.
Eratosthenes's only tools were sticks, eyes, feet, and brains
plus a zest for experiment. With those tools he correctly deduced
the circumference of the Earth to high precision with an error of
only a few percent, that's pretty good figuring for twenty-two
hundred years ago.
__________________
USENET News is a discussion protocol, part of the internet, that
predates the World Wide Web, browsers and graphical interfaces by
at least ten years. I have been a participant in Physics and
Astronomy News groups for twenty years.
There is a fellow who lives somewhere in Scotland, named Gerald
Kelleher, who has the annoying habit of interrupting threads on
various topics of astronomy with his rants that scientists, such
as Isaac Newton, ruined observational astronomy in the 17th
century.
Eventually I engaged, Gerald, trying to understand why he rants
and what might be the basis of his misunderstanding. His trouble
is rooted in the Anglican interpretation of the rotation of the
earth. Gerald regularly expresses his utter contempt and disgust
that we professors and teachers of astronomy note that the earth
rotates once, four minutes shy of 24 hours. The effect can be
observed by anyone, that the stars rise in the east 4 minutes
earlier each night. School kids, using two sticks can sight any
prominent star in the nighttime sky two nights in a row and time
that the star lines up with the two sticks every 23h 56m and 4s.
We notice that the sun appears to travel south in the winter and
back north in the summer. From a fixed perspective one can see
that the sun rises and sets at a different place along the horizon
everyday, changing most rapidly near equinoxes and coming to what
seems like a standstill at the solstices. And yet it moves!
Science is all about observation and experiment. We enhance our
understand of nature all around us, by taking the time to observe
and think, often needing little more than sticks, eyes, feet, and
brains plus a zest for learning and understanding.