Transit circles can produce observations of the positions of celestial objects that are said to be absolute in the sense that the positions do not explicitly rely on any previous observations of these objects. The absolute positions are defined with respect to an internally consistent frame that is unique to the particular instrument. The uniqueness of this instrumental frame is a function, not only of the instrument itself, but also of the methods used to form the catalog. To form a compiled catalog of absolute observations from several instrumental catalogs these frames must be brought into coincidence. This is usually done by aligning the instrumental frames, by use of observations of solar system objects, to a dynamical one as defined by a standard ephemeris.
It has been the practice at the U.S. Naval Observatory and other institutions
involved in the production of compiled catalogs to align the
instrumental frame of absolute transit circle catalogs with a
dynamically defined system through corrections to the equinox point
and equatorial plane, referred to as the equinox correction and
equator correction,
derived from observations of the Sun and major
planets. The equinox and equator corrections would be solved for
simultaneously with corrections to the orbital elements of each
planet. The five to ten year duration of a transit circle program
necessarily restricted this treatment to observations of the Sun,
Jupiter and the planets interior to Jupiter. Only small fractions
of the orbits of the outer planets could be covered during a
typical program making unsuitable their inclusion in the
solutions. With the release of JPL's DE200 ephemerides (Standish
1990), it was realized that the orbits of the planets contained
therein, and therefore the dynamical system defined by them, were so
well established that it was no longer necessary or desirable to
use the observations from a single transit circle catalog to
correct the orbital elements.
The W1
(Holdenried, in press),
an absolute catalog of star, Sun and planet
positions observed by the Six-inch transit circle at the U.S. Naval
Observatory in Washington DC during the years 1977-1982,
is the first catalog in the
series to follow this suggestion and eliminate from the alignment solutions
the corrections to orbital elements.
This change in methodology offered an opportunity to reconsider on a more fundamental level the entire philosophy of the alignment process as well as the formation of the instrumental frame. As a result we have proposed a new approach. To explain this new approach and to give an appreciation for its divergence from the old the following historical digression is necessary.