NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Eqn. of time
From: Frank Reed CT
Date: 2007 Jan 9, 01:41 EST
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From: Frank Reed CT
Date: 2007 Jan 9, 01:41 EST
Bill, you wrote:
"That caused me to wonder if at some time in the past there was
a conscious effort to link the equation of time to some celestial event."
a conscious effort to link the equation of time to some celestial event."
er... how could there be? Its numerical value follows directly
from its definition. The values on specific dates do change over the centuries
and millennia. Is that what you were asking about?
And also:
"Looking at it from a purely (modern) calendar event, the central zero
point
occurs close to the midpoint of a calendar year in June, but is past that
calendar midpoint."
occurs close to the midpoint of a calendar year in June, but is past that
calendar midpoint."
Well, you could re-define the calendar in such a way that the midpoint
falls on a specific date, but that has never been done. And there wouldn't be
any reason to do so. In fact, two of the zeroes in the equation time presently
fall on December 25 --Christmas in Western Christianity-- and April 15 --Fed Tax
day in the US. That's how I remember the zero points when I'm trying to estimate
the approximate time of apparent noon during the year, but there's no real
"physical" significance to those dates, unless the IRS knows something that we
don't!
You asked:
"When was the equation of time first understood and mapped?"
"When was the equation of time first understood and mapped?"
Thousands of years ago. Ptolemy knew about it and wrote a chapter on it in
the Almagest. But for astronomers back then, it was merely a curiosity. It was
the difference between time, as everyone understood it, and sidereal time, which
was known and significant only to astronomers.
And:
"At that point, was there an attempt to link it to a celestial event that
may
now be lost in change?"
now be lost in change?"
As best as I can understand what you're asking, no, there couldn't be.
There's no additional "freedom" in the definition of the equation of time that
would allow that sort of linkage to a celestial event.
And:
"On a related issue: If I understand it the pole star will--within a
generation or two--come as close to earth's rotational axis as it will come
for 26,000 years. All things being equal, will the winter solstice occur
during June in approx. 13,000 years?
If yes to the above, will the northern hemisphere (like Australia now) pick
up a bit more heat by virtue of proximity in the summer, and a bit less in
the winter?"
"On a related issue: If I understand it the pole star will--within a
generation or two--come as close to earth's rotational axis as it will come
for 26,000 years. All things being equal, will the winter solstice occur
during June in approx. 13,000 years?
If yes to the above, will the northern hemisphere (like Australia now) pick
up a bit more heat by virtue of proximity in the summer, and a bit less in
the winter?"
Basically, yes. All other things being equal, that would be the case. In
fact, there is a rather involved dance involving the precession of the
equinoxes, the oscillating tilt of the Earth's axis, and the oscillating
variation of the Earth's orbital eccentricity. These three cycles combined
determine the net "insolation", the solar energy received by each patch of the
Earth's surface, over thousands of years (under the assumption that the Sun's
luminosity is relatively constant on that time scale). Almost a century ago,
Serbian geophysicist Milutin Milankovitch proposed that these cycles of
insolation are responsible for the cycles of the ice ages. There is very good,
but not conclusive evidence that the long-term climate is controlled by these
cycles in the Earth's precession, tilt, and eccentricity just as Milankovitch
suggested. The problem is not simple because large ice sheets make the climate
non-linear. Once a great sheet of ice forms, it tends to remain in place because
the high albedo of the ice reflects sunlight and keeps the climate cold. Best
guess right now is that the Milankovitch cycles control the ice ages, but strong
non-linear dynamics are almost as important.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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To post to this group, send email to NavList@fer3.com
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