DT wrote in message ...
In message . net,
edgrsprj writes
Sun transit time question May 5, 2004

Posted by E.D.G. May 5, 2004

I am working with a professional computer programmer to develop Basic and
Perl language programs which will generate subsolar and sublunar point
latitudes and longitudes. The first program is now running and is being
"fine tuned." In developing it I noticed something and am curious regarding
what causes it. Perhaps someone reading this note will know the answer.

In theory the sun should be directly above 0 longitude, the Greenwich
longitude, at 12 noon each day. But the longitude numbers for its actual
location at 12 noon that this and other programs generate show that there is
a variation of perhaps as much as 15 minutes in that transit time at
different times of the year. And those variations are reproducible from
year to year.

Q: What is the cause of those variations? I am assuming that I am
interpreting the data correctly.

Might this be due to the fact that the Earth orbit is an ellipse rather than
a true circle. And for that reason the 0 longitude line is directly beneath
the sun at slightly different times during the year because the Earth
rotation does not perfectly match its constantly changing orbital speed
around the sun?


You're almost exactly right. Civil time is based on the concept of the
'Mean Sun', an imaginary Sun moving at a uniform rate in the equatorial
plane.

Historically and observationally incorrect,the equable 24 hour day is
based on the actual motions of the Earth,both axial and orbital, using
the Sun as a reference.



A sundial will show True time.

A sundial registers the actual motions of the Earth by means of a
shadow,there are two observed variations occuring for each passage of
a shadow back to a fixed marker.

Over the course of a full annual cycle,the range of the lenght of
shadows cast changes as the equatorial orientation of the Earth
changes ,lower on the horizon and therefore longer shadows in the
winter and shorter shadows in the summer.This variation due to axial
tilt has no effect on the pace of the shadow across the dial which
changes over the annual orbit but remains common in both hemispheres.

http://www.cmi.k12.il.us/Urbana/proj.../techpage.html

The pace of the shadow across the sundial from one complete rotation
to the next is due to the combination of constant axial rotation
combined with variable orbital motion.The Equation of Time basically
equalises this variable pace to a constant hence the equable 24 hour
day and the pace of the equable hour,minute and second.


The difference between the two is
a combination of the elliptical orbit of the Earth, and the inclination
of its axis of rotation to the ecliptic, i.e. The plane in which the
earth moves around the Sun. This difference is known as the Equation of
Time.

Historically and observationally incorrect.The graphics of the 21st
century permits anyone to put the components of the Equation of Time
into proper context,the natural inequality of a day using the Sun as a
reference is conditioned by the behavior of the Earth in its orbital
motion insofar as this is what causes the pace of the shadow to change
across the sundial in both hemispheres simultaneously.

It is only a matter of applying the appropriate minutes and seconds to
the noon determination and recognising how sometimes clock noon is
ahead of the natural noon determination and sometimes behind in terms
of the variable orbital motion equalised to a constant motion,Kepler's
second law in other words.

http://ircamera.as.arizona.edu/NatSc...res/kepler.htm



Google 'Equation of Time' and 'Analemma' for lots of references and more
detail.


He can indeed,try NASA who are oblivious to the fact that the winter
solstice in the Northern hemisphere implies that it is the summer
solstice in the Southern,try convincing the Aussies that it is
mid-winter on the 21st of Dec and see how far you get !.

http://imagine.gsfc.nasa.gov/docs/as...s/980116c.html

If the participant who began this thread can live with that
explanation comfortably he can live with anything.



It's quite possible that you will get some wacky responses to your
question here, so please be sure to get other references. ;-)

Denis

If he wants wacky he can turn to the Royal Observatory Greenwich who
seem to have entirely lost the plot.

"Each solar day the Earth rotates 360º with respect to the Sun.
Similarly the Earth rotates 360º with respect to the background stars
in a sidereal day. During each solar day, the motion of the Earth
around the Sun means the Earth rotates 361º with respect to the
background stars."

http://www.nmm.ac.uk/site/navId/00500300l005001000

No offense but when the association between geometry astronomy and
clocks becomes that bad with nobody around to untangle the mess,men
are hardly fit to call themselves astronomers.