Sidereal Time

I need to caluculate the RA and Dec in J2000 for the zenith at a
specfic latitude and longitude on a specific date, which could be far
in the future or past. If anyone can point me to code or psuedo
code/formulae that would assist me in this, I would be appreciative.

At first blush it seems that I could calculate Local Sidereal Time to
get the RA and then calculate the DEC based on the latitude. How would
I take precession of the Earth into account though?

Ideally I need to to be quite accurate millions or billions of years in
the past/future.

I appreciate any suggestions.

Thanks!
Michael Koppelman

Michael Koppelman wrote:
I need to caluculate the RA and Dec in J2000 for the zenith at a
specfic latitude and longitude on a specific date, which could be far
in the future or past. If anyone can point me to code or psuedo
code/formulae that would assist me in this, I would be appreciative.

At first blush it seems that I could calculate Local Sidereal Time to
get the RA and then calculate the DEC based on the latitude. How would
I take precession of the Earth into account though?

Ideally I need to to be quite accurate millions or billions of years in
the past/future.

This last requirement implies knowing the Earth's angular position
(time integral of angular velocity) to high accuracy over long time
scales. This is _very_ hard: A major source of the slowing of the
Earth's spin rate is friction from tides flowing over shallow seabeds,
and the amount of this dissipation depends on the details of the
ocean-bottom topography.

I have read that calculating this dissipation over time scales of a
few _thousands_ of years (to analyze ancient solar-eclipse observations)
is already a difficult problem, due to lack of knowledge of the
ocean-bottom topography,
[or at least the lack of such knowledge that isn't a
military secret: the world's navies know a lot about
ocean topolgraphy that they don't make public]
especially the meter-scale roughness. If I remember correctly, the
cumulative tidal-friction dissipation over a few thousand years is
already enough to change the Earth's angular position (from what it
would have been with a constant angular velocity) by something on
the order of 90 degrees.

On geological time scales ocean depths, topography, roughness, etc,
are all going to change due to continental drift. I don't think
there's any practical way of calculating this.

ciao,

--
-- "Jonathan Thornburg (remove -animal to reply)"
Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam

Michael Koppelman skrev:

I need to caluculate the RA and Dec in J2000 for the zenith at a
specfic latitude and longitude on a specific date, which could be far
in the future or past. If anyone can point me to code or psuedo
code/formulae that would assist me in this, I would be appreciative.

At first blush it seems that I could calculate Local Sidereal Time to
get the RA and then calculate the DEC based on the latitude. How would
I take precession of the Earth into account though?

Have a look at

http://www.iers.org/iers/publications/tn/tn32/

Chapter 5.10 shows you how to transform from the Terrestial Reference System
to the Celestial Reference System using the given fortran routines.

Ideally I need to to be quite accurate millions or billions of years in
the past/future.

I doubt even millions of years are possible, although this depends on
your definition of quite accurate.

[Mod. note: MIME-damage fixed up -- it's hard on people whose names
contain non-ASCII characters, but please post in plain ASCII if you
can -- mjh]

--
Oeystein Olsen, oystein.olsen_at_astro.uio.no, http://folk.uio.no/oeysteio
Institute of Theoretical Astrophysics, http://www.astro.uio.no
University of Oslo, Norway

Michael Koppelman wrote:

Ideally I need to to be quite accurate millions or billions of years in
the past/future.


Impossible. This would mean that we can know exactly all forces
and all masses in the solar system.

The solar system is chaotic. The orbit of the earth is chaotic.
You can't go forward/backward billions of years.

See
http://members.fortunecity.com/templarser/solarsys.html
http://www.imcce.fr/Equipes/ASD/prep...002_laskar.pdf
http://www.physics.uoguelph.ca/summe...es/scor208.htm

and many others

Thanks for the replies, everyone! I am learning a lot as I research
this. I have code that calculates precession, nutation and annular
aberration but it it based on knowing certain values at certain epochs.
It seems my accuracy is going to be limited by the oldest date that we
know some of those values.

Cheers,
Michael