Epoch J2000 Research Articles

The DENIS Point Source Catalogue towards the Magellanic Clouds

We have compiled the near infrared Point Source Catalogue (PSC) towards the Magellanic Clouds (MCs) extracted from the data obtained with the Deep Near Infrared Survey of the Southern Sky - DENIS (Epchtein et al. 1997). The catalogue covers an area of of 19.87* 16 square degrees centered on (RA, DEC)=(5h27m20s, -69o00\'00\'\') for the Large Magellanic Cloud (LMC) and 14.7* 10 square degrees centered on (RA, DEC)=(h02m40s, -73o00\'00\'\' for the Small Magellanic Cloud (SMC) at the epoch J2000. It contains about 1300000 sources towards the LMC and 300000 sources towards the SMC each detected in at least 2 of the 3 photometric bands involved in the survey (I, J, Ks). 70% of the detected sources are true members of the Magellanic Clouds, respectively and consist mainly of red giants, asymptotic giant branch stars and super-giants. The observations have all been made with the same instrument and the data have been calibrated and reduced uniformly. The catalogue provides a homogeneous set of photometric data.

Report on Astronomical Constants

AbstractRecent progress in the determinations of astronomical constants is reviewed. First is the latest estimation of the general relativistic scale constants, LC, LG, and LB (Irwin and Fukushima, 1999). By reestimating the uncertainty, the value of the first constant is given as LC = 1.480 826 867 4 × 10–8 ± 1.4 × 10–17. Also noted is the rigorous relation among these three, LB = LC + LG – LCLG. Based on the latest determination of the geoidal potential W0 in the IAG 1999 Best Estimate of Geodetic Parmeters (Groten, 1999), LG and LB were reevaluated as LG = 6.969 290 09 × 10–10 ± 6 × 10–18 and LB = 1.550 519 767 3 × 10–8 ± 2.0 × 10–17. Since LG is roughly related to W0, a proposal to fix its numerical value is presented in order to remove the geophysical ambiguity in its evaluation in the future. In that case, LG becomes a defining constant for the scale difference between the geocentric and terrestrial coordinate systems. While LC and LB remain as a primary and derived constant, respectively. Next is the correction to the current precession constant, Δp. The recent estimates of Δp based on Very Long Baseline Interferometry (VLBI) observation seem to converge to a value close to –0.30″/cy (Mathews et al., 2000; Petrov, 2000; Shirai and Fukushima, 2000; Vondrák and Ron, 2000). Unfortunately this is significantly different from –0.34″/cy, the latest value determined from the Lunar Laser Ranging (LLR) data (Chapront et al., 1999). The difference is roughly ten times larger than the sum of their formal uncertainties. Since the cause of this difference is not clear, we first arranged the best estimates based on VLBI and LLR techniques, respectively, then took a simple mean of these two best estimates, and recommend it as the current best estimate. The value derived is p = 5 028.78 ± 0.03 ″/cy. Similar estimates were given for some other quantities related to the precession formula; namely the correction to the obliquity rate of the IAU 1976 precession formula (Lieske et al., 1977), Δε1 = (–0.024 5 ± 0.002 5) ″/cy, and the offsets of the Celestial Ephemeris Pole of the International Celestial Reference System, Δψ0 sin ε0 = (–17.5 ± 0.8) mas and Δε0 = (–5.2 ± 0.4) mas. As a result, the obliquity of the ecliptic at the epoch J2000.0 was estimated as ε0 = 23°26′21.″405 6 ± 0.″000 5. The draft IAU 2000 File of Current Best Estimates of astronomical constants, that is to replace the 1994 version (Standish, 1995) or maybe even the formal IAU 1976 System of Astronomical Constants (Duncombe et al., 1977), after discussion at the 24th General Assembly of the IAU is presented.

Construction of the Tycho Reference Catalogue

The Tycho Reference Catalogue (TRC) project, as proposed in 1992 by the present authors, aimed to extend the Hipparcos reference system to fainter magnitudes. To assure rigidity of the reference system realization, the project relied on the Tycho Cataloguethat provides highly-accurate positions of more than one million stars on the Hipparcos system. To maintain the precision of the reference system over longer periods of time, high-precision proper motions must be derived from Tycho and earlier ground-based measurements, most notably the century-old Astrographic Catalogue(AC). The recently published first version of the Tycho Reference Catalogue provides highly-accurate positions and proper motions for 990 182 stars of the Tycho Catalogue. It realizes the ICRS/Hipparcos system in the optical domain with a median accuracy of 45 mas at epoch J2000.0. The realization is based on stars of median magnitude mag, providing an average of 24 reference stars per square degree. The median accuracy of the TRC proper motions, estimated from external comparisons as 2.4 mas/yr, ensures a reference system realization with an accuracy better than 100 mas over the time span of a few decades. Highly-accurate astrometry combined with homogeneous two-colour Tycho photometry makes TRC an ideal choice for a wide range of astronomical applications. The present paper covers the major steps of the TRC construction, including the Astrographic Cataloguereduction onto the Hipparcos system, identification of the Tycho Catalogue stars in the AC, derivation of proper motions, a posteriori systematic corrections of the proper motions to account for residual AC systematic errors, and the TRC quality assessment. The relation of the TRC with the recently published ACT catalogue is outlined. A future version of the TRC, to be based on the new Tycho data processing and a global block adjustment of the Astrographic Catalogue, is briefly discussed.

The ACT Reference Catalog

The US Naval Observatory has completed the compilation of the ACT Reference Catalog, containing 988,758 stars covering the entire sky. The motivation behind the ACT was to provide accurate proper motions for the majority of the stars in the Tycho Catalogue. To do this, positions from new reductions of the Astrographic Catalogue (AC 2000) were combined with those of the Tycho Catalogue. The large epoch span between the two catalogs yields proper motions about an order of magnitude more accurate than those found in the Tycho Catalogue. The astrometric data contained in the ACT Reference Catalog include positions and proper motions and their corresponding errors. These are on the Hipparcos system (ICRS) for epoch J2000.0. Photometric and variability data from Tycho are included. In addition, cross-references to the Tycho, AC 2000, Bonner Durchmusterung (BD), Cordoba Durchmusterung (CD), Cape Durchmusterung (CPD), Henry Draper (HD), and Hipparcos catalogs are given. The ACT is now available at the international data centers and by contacting the authors via electronic mail.

The New Catalogue of Positions and Proper Motions of 5115 Bright Stars Compiled as the Result of the International “Bright Stars” Program

At Kyiv University Observatory, in accordance with IAU resolution, the Combined Catalogue of Positions and Proper Motions of 5115 Bright Stars in the FK5 system for Epoch and Equinox J2000.0 (BSC, see reference) has been compiled using 20 source catalogues obtained within the framework of ‘Bright Stars’ International Program. The source catalogs are Bordeaux-66, Bucharest-65, Bucharest-68, Belgrade-79, Cape-68, Kharkiv-70n, Kharkiv-70s, Kyiv-66, Kyiv-73, El Leoncito-70, Moscow-76, Mykolaiv-65, Perth-70, Perth-75, Santiago-65, Santiago-67, Strasbourg-65, Tashkent-64, Tokyo-68, Washington-66. The BSC accumulates around 30,000 source catalogue positions observed within 1960-1980. The standard errors of BSC do not exceed 0.1 arcsec and 0.25 arcsec/cy for positions and proper motions, respectively. The systematic differences between BSC and FK5 in positions for the mean BSC epoch 1970 as a rule do not exceed several hundredths of arcsec. A comparison of the BSC with the PPM catalogue indicates that in the latter the accuracy of positions and proper motions of the ordinary bright stars is slightly worse than that of average values given by the authors of the PPM. It is important to note that the comparison of the BSC with PPM for various star subsets shows deviations of both the HPS bright star system and the rest of the PPM bright stars, on the one hand, from the stars of the FK5 Extension contained in the PPM on the other hand. To examine the BSC proper motion system the stellar astronomy constants and correction to precession have been determined using the BSC proper motions. The parameters obtained agree with the standard ones. But the Oort constants are slightly smaller than those of standard.The BSC catalogue as a whole is acceptable for astrometry usage with respect to both accidental and systematic accuracy.

Explanatory supplement to the Astronomical Almanac

original Explanatory to the and the and Nautical Almanac was published in 1961 and revised in 1974. In 1981 the series of publications titled The Ephemeris and the American and Nautical Almanac were continued under the single title The Almanac. Thus, the title of the new edition of the Explanatory Supplement. In the 1970s it was recognized that there was a need to revise the planetary and lunar ephemerides, the FK4 star catalog, the time scales, the System of Constants and the Theory of Nutation. In 1984 all these revisions were introduced using the new standard epoch J2000.0, thus making the original Explanatory Supplement out of date. Hence, the new version was prepared based on the FK5 star catalog, the IAU (1976) System of Constants, the new time scales, including those adopted in 199 1, the IAU (1980) Theory, of Nutation, and the DE200/LE200 ephemerides. Improvements since 1984 have also been incorporated. Current values of astrometric and planetary data are included. Explanatory to the Almanac provides a detailed description of the means of performing practical astronomy or spherical astronomy. It shows how computations for the Astronomical Almanac are performed and the underlying information required by someone using that publication or wanting to perform related computations. It is intended to be a reference volume for anybody working with accurate positional astronomical data and anyone interested in investigations of the solar system.

Issues of Time for Reference Systems

At the International Astronomical Union General Assembly in 1991 the terminologies Geocentric Coordinate Time (TCG) and Barycentric Coordinate Time (TCB) were introduced as the time coordinates of four dimensional geocentric and barycentric coordinate systems, respectively. The reasons for these time-like arguments and their applications will be discussed. The relationships between the different time-like arguments will be discussed along with their applications and relationships to the astronomical constants.Since these time-like arguments have secular differences, there are new issues to be resolved. These new issues include the definition of the epoch J2000.0, relationship between mean and apparent sidereal time, the time-like argument for specifying constants, the definition and future use of ephemeris time, delta T values and the transition to, or future use of, TCG, TCB and TDB.

Astronomical algorithms

Astronomical algorithms

Precise positions of optical counterparts of some radio objects

We present optical positions in the improved IAU System at the standard epoch J2000.0 of some extragalactic radio sources obtained at Torino Astronomical Observatory. The primary reference stars are taken from the Carlsberg Automatic Meridian Catalogues 1 and 2 (CAMC).

Effect of the New Equinox Definition on the Zero-Point of Longitude of the Indian Calendar

Indian calendars follow a sidereal system of astronomy taking a fixed initial point on the ecliptic as the origin from which the longitudes are measured. Its position for the official Indian Calendar has been defined by the Calendar Reform Committee (1955) as the point on the ecliptic whose true tropical longitude was 23°15′00″ as on 21 March 1956, 0h UT. Its position was determined upto the year 1984 in accordance with Newcomb's value for general precession using the relation where T is in centuries of 36525 ephemeris days from 1900 January 0.5 ET. Recent changes in the location and the motion of the equinox with reference to the epoch J2000.0 have necessitated corresponding changes to be included in the determination of the mean and true positions of the above initial point. The new algorithm worked out is where T is in Julian centuries of 36525 days from J2000.0.

Mean and Apparent Place Computations in the New IAU System. II. Transformation of Mean Star Places from FK4 B1950.0 to FK5 J2000.0 Using Matrices in 6-Space

view Abstract Citations (17) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Mean and Apparent Place Computations in the New IAU System. II. Transformation of Mean Star Places from FK4 B1950.0 to FK5 J2000.0 Using Matrices in 6-Space Yallop, B. D. ; Hohenkerk, C. Y. ; Smith, C. A. ; Kaplan, G. H. ; Hughes, J. A. ; Seidelmann, P. K. Abstract A 6 x 6 matrix method for transforming a catalog mean place from epoch and equinox B 1950.0 on the FK4 system to epoch and equinox J2000.0 on the FK5 system is described. A step-by-step comparison is made between the matrix method and the classical spherical formulas. Publication: The Astronomical Journal Pub Date: January 1989 DOI: 10.1086/114980 Bibcode: 1989AJ.....97..274Y Keywords: ASTROMETRY full text sources ADS | Related Materials (2) Part 1: 1989AJ.....97..265S Part 3: 1989AJ.....97.1197K

VLBI Positions of Eight Stellar Systems

Radio positions of eight stellar systems have been derived with accuracies of 3 to 300 milliarcseconds from MkIII VLBI observations conducted with multi-station arrays. The best accuracy was obtained during a relatively strong outburst of the RS CVn system HR 5110. The epoch J2000.0 positions are obtained in the Jet Propulsion Laboratory VLBI reference frame of extragalactic radio sources.

Meridian & Astrolabe Observations of Radio Stars: A Contribution to the Connection of Radio and Optical Reference Systems

Optical positions of eight radio stars observed with the Photoelectric Transit Instrument (PTI) at Torino and with the Danjon Astrolabe (AST) at Cagliari are presented here. The values of observed right ascensions have been computed at the Epoch J2000.0.

4. Ephemerides (Ephemerides)

This report covers the period from 1 July 1984 to 30 June 1987. The ephemerides that have been published during this period have made use of the new system of astronomical constants adopted at the XVIth General Assembly of the lAU in Grenoble. Yet some difficulties have arisen because of the lack of catalogues or maps of the heavens established for the epoch J2000.0. This is awkward for observers of comets and minor planets and, for that reason, Commission 20 decided, at the New Delhi meeting of lAU that there would be a gradual introduction of the J2000.0 system as far as those bodies are concerned.