Information on Service 'APFS TGAS Simple Query'

On Positions in APFS

APFS stands for "Apparent Places of Fundamental Stars", a term which we maintain due to its traditional use; introductorily, it might be appropriate to address the following issues:

1. "apparent places" in the traditional sense are positions with right ascension referred to the equinox ("first point of Aries"). It was recommended by IAU 2000 Resolution B1.8 to use the "non-rotating origin" as new reference point for right ascensions on the celestial equator. The "non-rotating origin" – now called "Celestial Intermediate Origin (CIO)" – constitutes "intermediate places" instead of the former "apparent places". Declinations are not affected by this change of the zero point for right ascensions. For an undetermined transition period we shall provide equinox-based "apparent places" besides the CIO-based "intermediate places". However, the user should be aware that equinox-based right ascensions refer to a concept of the past, rather than to a forward-looking one. The notation "apparent places" will sometimes be used in text components to denote both kinds of ephemeris.
2. 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/, (http://www.aanda.org/articles/aa/pdf/2015/02/aa25310-14.pdf). The accuracy of positions in TGAS may 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 consequently also for derived apparent places. Hence TGAS based apparent places are a major advantage in comparison with Hipparcos based apparent places. The accuracy of the latter may currently (2016) be estimated in the order of 20-40 mas for single stars. Moreover, TGAS data are available for more than 2 million stars but radial velocities have still not been determined by the GAIA satellite. The user should be aware, however, that 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.
3. From a star catalogues given in the "International Celestial Reference System (ICRS)", apparent places are obtained via the procedure explained e.g. in the Explanatory Supplement of the Astronomical Almanac. That means in principle for stellar ephemeris: transformation from J2000.0 to the desired epoch and from barycentric ICRS ephemeris to geocentric coordinates by considering space motion and parallax of a star as well as light deflection and annual aberration; finally, frame bias, precession and nutation have to be considered in order to obtain apparent places referred to the equator of date (and to the origin of date for right ascension).
4. The IAU 2000/2006 precession-nutation is used for intermediate and apparent positions. This precession-nutation model is recommended by IAU 2006 Resolution B1 and shall be valid from 1 January 2009 onwards. It includes a new precession component (P03 precession theory) which replaces the P00 precession of the IAU 2000A precession-nutation model. Right ascensions and declinations determined by using this new or the prior precession-nutation model are almost identical at milliarcsec level, and differences will hardly be noticed by APFS users. We use JPL DE430 ephemeris where required.

On times in APFS

The dates and times in this service are taken as and given in Terrestrial Time (TT).

TT is related to Coordinated Universal Time (UTC) by:

TT = UTC + Δ(AT) + 32.184 sec

where Δ(AT) is an integer term depending on irregularities of the Earth's rotation. In 2006: Δ(AT) = 33 sec. The current value for Δ(AT) can be found in IERS Bulletin C.

That means that TT and UTC differ by roughly one minute. The computed apparent places are usually not affected by this difference in time-scale, except the last given digit of right ascensions for stars near the poles (as an effect of the underlying spherical coordinate system). In any case, if apparent places are used for conventional geodetic applications there is no need to consider this intricateness, and UTC may be set instead of TT. UTC is the civil time as disseminated by radio or TV, corrected for your time zone.

References

• Hilton et al., Celest. Mech., 94, 351, (2006)
• Hilton, Hohenkerk, A&A 413, 765 (2004)
• Capitaine and Wallace, A&A 450, 855 (2006)
• Capitaine, Wallace, Chapront, A&A 412, 567 (2003)
• Capitaine, Wallace, McCarthy, A&A 406, 1135 (2003)
• Capitaine, et al., Latest proposal of the IAU WG on Nomenclature for Fundamental Astronomy (NFA)
• Seidelmann, Kovalevsky, A&A 392, 341 (2002)
• Capitaine, Guinot, McCarthy, A&A 355, 398 (2000)
• Feissel, Mignard, A&A, 331, L33 (1998)
• Gaia Collaboration, Gaia Data Release 2. Summary of the contents and survey properties, A&A 161, A1 (2016)
• Lindegren, L., et al, Gaia Data Release 2. The astrometric solution, A&A 616 A2 (2018)
• Soubiran, C., et al, Gaia Data Release 2. The catalogue of radial velocity standard stars, A&A 616, , A7 (2018)
• Wielen et al., Veroeff. Astron. Rechen-Institut., Heidelberg, No. 35, FK6 Part I, (1999)
• Wielen et al., Veroeff. Astron. Rechen-Institut. Heidelberg, No. 37, FK6 Part III, (2000)
• Wielen et al., A&A 360, 399 (2000)

Software Routines from the IAU SOFA Collection were used. Copyright © International Astronomical Union Standards of Fundamental Astronomy (http://www.iausofa.org)

Parts of this work make use of data from the European Space Agency (ESA) mission Gaia (http://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, http://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.