Information on Service 'APFS Gaia DR2 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. Gaia Data Release 2 (Gaia DR2) was released on 25 April 2018. A five-parameter astrometric solution – positions on the sky, parallaxes, and proper motions – for more than 1.8 billion sources, with a limiting magnitude of G = 21 and a bright limit of G ≈ 3, has been provided by Gaia DR2. Median radial velocities (i.e. the median value over the epochs) for a minor part (7.2 million stars) are also available. Details can be found on ESA's website and corresponding references (see below). From the published uncertainties it follows that apparent places derived from Gaia DR2 may be assumed to have uncertainties of one milliarcsecond or less for the present epoch (FK6 ~ 10-20 milliarcseconds, Hipparcos ~ twice FK6). On the other hand, we cannot verify all these sources. It remains the responsibility of the user to verify that a selected source is identical with a specified star, and conforms to 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.