ARI Database for the Apparent Places of Fundamental Stars


Introduction

General Information:

Apparent places have been published by the Astronomisches Rechen-Institut (ARI) for a long time. Since 1998 more and more data have been made accessible for the world wide web, and as a consequence the printed volumes have become secondary. Various data sources referred to apparent places will be explained below.

Apparent places are coordinates of a celestial object referred to the true equator and equinox of date. The celestial object can be visualized as it would be seen from the center of a transparent and massless Earth. Consequently space motion, parallax, light deflection, annual aberration, precession, and nutation have been considered for the computation of intermediate and apparent places. On the other hand, those effects depending on the topocentric location of an observer cannot be taken into account. These are for example refraction, diurnal aberration, and geocentric parallax, the latter generally being negligible for stellar objects (except the Sun). Polar motion may be a concern if latitude and longitude referred to the International Terrestrial Reference System (ITRS) are processed. It remains in the hands of the observer to handle these effects.

The database ARIAPFS provides for the years 1998 and 1999 the apparent places of the "classical" fundamental stars. For these years, the basic stellar data are those published in the Fifth Fundamental Catalogue, Part 1 (FK5) . Apparent places are given in tables for 1535 FK5 stars, also Sidereal Time at 0 h UT is available in an additional table. Two other auxiliary tables (Short period terms of nutation and Besselian day numbers) had been continued only until 2005.

From 2000 onwards the apparent places are based on the data given in the Sixth Catalogue of Fundamental Stars (FK6) and in the HIPPARCOS Catalogue. Apparent and intermediate places are available in tables for 878 stars of the FK6 Part I plus Polaris, either the FK6 Part I or the Hipparcos Catalogue has been used as basis. The tables for apparent places are completed by a table providing Sidereal Time at 0 h UT [Query Form] . The IAU 2000A Precession-Nutation Model had been adopted from 2006 to 2009 and has been replaced by IAU 2000/2006 Precession-Nutation from 2010 onwards. The latter precession-nutation model includes a new precession component (P03 Precession Theory, see IAU 2006 Resolution B1). Right ascensions and declinations determined by using the one or the other precession-nutation model are almost identical at milliarcsec level, and differences will hardly be noticed by users.

The global standard error of apparent places can be estimated straightforwardly from the data provided by Tables 2 and 6 in the printed FK6 Part I. This error estimation leads to mean errors of about 7 milliarcseconds (mas) at epoch 2010 both in right ascension and declination, and 10 mas at epoch 2020. For the same group of FK6 stars, but based on Hipparcos positions and proper motions, the mean error reaches about 14 mas in right ascension and 11 mas in declination at epoch 2010, or 22 mas and 17 mas at epoch 2020. The mean errors in right ascension have to be divided by (cos δ) for a star with declination δ. The reduction from catalogue values to apparent places has been neglected with respect to error propagation (among other things parallax errors would enter), and systematic errors have been ignored.

Intermediate and apparent places derived by means of the IAU 2000/2006 Precession-Nutation are accessible via the link VO compliant interface (App. Places FK6), leading to a query form which is related to the German Astrophysical Virtual Observatory (GAVO). Intermediate and apparent places for 3272 stars of the FK6 Part III are also available there, besides those from 878 stars of the FK6 Part I. Further information and some references can be found in the Service info of the corresponding web page. This GAVO-related service has gone on line since 2008.

From 2009 onwards intermediate and apparent places for more than 100000 Hipparcos stars are available via the link VO compliant interface (App. Places HIP). The user should be aware, however, that many Hipparcos stars are double or multiple stars; their instantaneous proper motions as observed by the Hipparcos satellite may not reflect the long-term space motion of the star, and hence the accuracy of intermediate and apparent places can be severely degraded. For instance, 'Δμ-binaries' represent such stars of questionable accuracy. Also FK5 stars which had not been accepted for the FK6 fall into this category. Further information on double and multiple stars can be found in the HIPPARCOS Catalogue. Hipparcos star parameters enter the computational procedure as they are published in the HIPPARCOS Catalogue, consequently radial velocities are not considered. From this follows that intermediate and apparent places derived from the FK6 are preferable for various reasons.

Positions, parallaxes (distances) and proper motions for more than 2 million stars using the Tycho-Gaia Astrometric Solution (TGAS) are part of Gaia's first data release published on 14 September 2016 ( http://sci.esa.int/gaia/58275-data-release-1/ , The Tycho-Gaia astrometric solution (PDF File)). The accuracy of positions in TGAS may now be assumed to be better than 1 milli-arcsecond (mas) for most regions of the sky, but nearly everywhere better than 3 mas. This holds approximately also for current apparent places. Hence apparent places derived from TGAS data are a major advantage in comparison with apparent places derived from Hipparcos data. Moreover, TGAS data are available for more than 2 million stars but radial velocities have still not been determined by the GAIA satellite. However, we cannot validate these stars. Among them there are multiple stars, spurious detections, and non-stars. It remains the full responsibility of the user to verify that a selected star meets a specified accuracy. Apparent and intermediate places can be retrieved via the link VO compliant interface (App. Places TGAS) .

Software Routines from the IAU SOFA Collection were used. Copyright © International Astronomical Union Standards of Fundamental Astronomy (http://www.iausofa.org/). For GAVO we use additionally JPL DE430/LE430 ephemerides. A facility to compute Greenwich Apparent Sidereal Time (GAST), Greenwich Mean Sidereal Time (GMST), and Earth Rotation Angle (ERA) [VO compliant interface (GAST, GMST, ERA)] has also been implemented. Further information can be found in the Service info of the web page.

A brief remark concerning notation might be appropriate: "apparent places" in the narrower sense are only positions with right ascensions referred to the equinox, as in the past; right ascensions referred to the celestial intermediate origin (CIO) constitute "intermediate places" [IAU 2006 Resolution B2]. Declinations are the same in both cases. Nonetheless, the term "apparent places" may embrace both meanings depending on the context.

Either FK6-, Hipparcos-, or TGAS star numbers have to be inserted in the query forms for intermediate and apparent places, depending on the specific service: therefore a list of star names and corresponding FK5/FK6- and Hipparcos numbers is provided for the 1535 stars of the Basic FK5 [FK5/FK6-Stars: names and numbers]. For abbreviations (e.g. Tuc, Oct, Psc etc.) see IAU Constellations where also constellation charts can be found. A cross-identification between FK6 Part III numbers and Hipparcos numbers can be found on the corresponding ARI-webpage for the FK6. A tentative list with corresponding Hipparcos-FK6-TGAS star numbers for some 2000 presumably single stars may also be useful: [HIP-FK6-TGAS].

All information and data on APFS web pages are provided without warranty [Publishing Information, Legal Matters].



The Printed Version:

Starting with the APFS volume for the year 2000, the printed version of the 'Apparent Places of Fundamental Stars' (published annually by the Astronomisches Rechen-Institut since 1960) is reduced from a bound volume of over 500 pages to a small booklet containing all information how to use the data and giving the apparent places merely for a few fundamental stars as examples. The separation between the so-called '10-Day-Stars' and the 'Circumpolar Stars' has been retained in the booklet as well as the conventions concerning the dates of transit. For a detailed explanation, reference is made to the introductory part of one of the volumes of the APFS for the years prior to 2000 or to one of the booklets from 2000 onwards. The booklet can be purchased from "Der Kleine Buch Verlag", Karlsruhe.



The Internet Version:

Apparent and intermediate places of each fundamental star can be obtained individually for a selected year by using the Query Form. The APFS are tabulated for all stars daily and they hold for moment of the upper transit at Greenwich. The short period terms of nutation are included for all stars. In order to have also consistency with the data provided in the printed version we have for the so-called "10-Day Stars" (stars with declinations from -81 to +81 degrees) marked with an asterisk those transits which are given in the printed version.

The following three auxiliary tables can also be obtained by using the Query Form.

1) the terms of short period of nutation (to 2005)
2) the Besselian day numbers (to 2005)
3) the Greenwich Sidereal Time at 0h UT

Apparent and intermediate places, Sidereal Time and Earth Rotation Angle provided by GAVO: VO compliant interface (App. Places FK6), VO compliant interface (App. Places HIP), VO compliant interface (App. Places TGAS), and VO compliant interface (GAST,GMST, ERA).


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