sidereal reference frames - Ed.

[Guest guest]

Ed wrote:

 

 

<<Then why, when I go to Astrolog, does the " sidereal " value for Spica

change every year?>>

 

 

 

 

There are some clarifications needed.

 

 

 

 

Astronomers work with " inertial " or " quasi-inertial " reference frames,

i.e., fiducials points that [almost, or practically] are fixed in

space, i.e., that are independent of the relative motions of the Earth

and the Sun. These fiducials are necessarily sidereal by definition.

Astronomers always work with these sidereal positions, both in the

development of planetary theory (theories of motion of the bodies of

the solar system) and in the " reduction " of observations.

 

 

 

 

Observed tropical astronomical positions are always reduced to some

arbitrary " epoch " (e.g., 2000 or 1900.0) which represents the position

of the equinox and equator at some specific point in time used as

fiducial. These " epochs " are all sidereal.

 

 

 

 

Earth's orientation in space varies according to (mostly) precession,

nutation, and aberration. These displacements must always be removed

from the observations and the resulting positions compared with

catalogs of reference positions of stars that by definition are

sidereal. This is why you see the right ascension positions in

astronomical references with respect to the J2000 or some other epoch,

i.e., they are always sidereal right ascensions.

 

 

 

 

The so-called " apparent " (tropical) geocentric positions given by

astronomical almanacs are a convenience necessary in the process of

removing the Earth's motion from the observed positions. They are used

because they are closer to the observed positions, but these position

are only the first step in the process of reduction of the observed

coordinates.

 

 

 

 

Historically, it is known that the Babylonians never used tropical

positions even though they were aware of at least the difference

between the duration of the tropical and the sidereal year (and

therefore most probably of precession). They were never interested in

the tropical reference frame, based on the Earth-Sun relationship, and

referenced everything with respect to relative positions in the sky.

The use of tropical positions in astronomy was a late development, and

came parallel with the development of (abstract) geometric cinematic

models by the Greeks.

 

 

 

 

The " synetic " (Fagan/Bradley) vernal point is not based on the

position of Spica. It happened to be empirically very close (about

0,06'05 " ) to the position that Spica would have in a sidereal zodiac

defined with Spica roughly at 29 degrees Virgo and Aldebaran-Antares

at 15 degrees Taurus-Scorpio (as originally assumed by Fagan from

Babylonian material). The position of the vernal point that exactly

coincides with the sideral position of Spica at 0 Libra by definition

is the so-called " Lahiri " zodiac.

 

 

 

 

Astronomically, the apparent position of a star is never used as

reference point, but its so called " catalog " or " epoch " position,

which is sidereal. Since by definition the catalog position is fixed

in space, the " proper motion " of a star is not considered --once its

catalog or epoch position has been established. In other words, proper

motion may be needed to know where a star is at a time different than

the time of its observation, but once this second, calculated position

has been established and is being used as reference, proper motion is

never used.

 

 

 

 

Juan