PDF《SOFA software support for IAU 2000》

AAS

2004 Denver,

2004 May

31 SOFA software support for IAU

2000 Patrick Wallace Rutherford Appleton Laboratory, UK ptw@star.

rl.ac.uk AAS

2004 Denver,

2004 May

31 Presentation outline ? Introduction to SOFA ? IAU

2000 and SOFA ? Software design choices ? Examples AAS

2004 Denver,

2004 May

31 SOFA ? SOFA (Standards of Fundamental Astronomy) is an IAU initiative to provide authoritative implementations of standard algorithms. ? Controlled by the international SOFA Reviewing Board. ? Currently offers

121 Fortran subroutines: ? Vectors and matrices ? Calendars and time scales ? Precession, nutation ? Reference frames ? etc. AAS

2004 Denver,

2004 May

31 www.iau-sofa.rl.ac.uk AAS

2004 Denver,

2004 May

31 IAU

2000 inputs to SOFA software ? New, quasi-classical, model for precession- nutation of the "celestial intermediate pole": ? Starts with IAU

1976 precession and IAU

1980 obliquity ? New nutation series + bias and precession corrections ? Alternative abridged nutation model ? New way to express Earth rotation: ? "Earth rotation angle" proportional to UT1 ? Zero point for matching RA is a "non-rotating origin" AAS

2004 Denver,

2004 May

31 Why there needed to be changes Errors in 1976/1980 precession-nutation model (mas) from VLBI AAS

2004 Denver,

2004 May

31 Numbers ?

1 mas ~ aberration you get from walking pace ?

1 ?as ~

30 ?m at Earth's surface (~

1 thou) ? Earth orientation known to

300 ?as RMS, ~

1 cm ? Smallest terms in nutation model ~

1 ?as ? Number of coefficients in nutation model =

2730 ? Error in IAU

1976 precession ~

1 mas/y ? Uncertainty in IAU

2000 precession ~

30 ?as/y ? Nutation-model noise floor ~

500 ?as (from FCN) AAS

2004 Denver,

2004 May

31 Free core nutation: the noise floor X(2000A)-X(VLBI), FCN uncorrected -1500 -1000 -500

0 500

1000 1500

2000 -2.00E-01 -1.50E-01 -1.00E-01 -5.00E-02 0.00E+00 5.00E-02 t (Jcy) microarcseconds AAS

2004 Denver,

2004 May

31 Earth rotation, old and new The old (classical) way: GST1982 (0h) = 24110.54841 + 8640184.812866 t + 0.093104 t2 - 6.2e-6 t3 + ?ψ cos ε + small correction terms t is UT. The terms in red are "cross-talk" from precession-nutation. The new (IAU 2000) way: ERA (0h) = 24110.54841 + 8639877.317376 t Calculating Greenwich hour angles: For GHA = ERA-RA to work (cf. GHA = GST-RA), we simply use a different zero-point for RA, namely the CEO in place of the equinox. Note the clean separation between Earth rotation and precession-nutation in the new system. AAS

2004 Denver,

2004 May

31 IAU

2000 GST expression Former IAU 1982/94 expression: GST1982 (0h) = 24110.54841 + 8640184.812866 t + 0.093104 t2 - 6.2e-6 t3 + ?ψ cos ε + small correction terms (2) IAU

2000 compatible expression: GST2000 (0h) = 24110.5493771 + 8639877.3173760 tu + 307.4771600 te + 0.0931118 te

2 - 0.0000062 te

3 + 0.0000013 te

4 + ?ψ cos ε + small correction terms (34) cf. ERA (0h) = 24110.54841 + 8639877.317376 tu AAS

2004 Denver,

2004 May

31 Zero points of right ascension ? Classical: zero point defined geometrically, by intersection with ecliptic. Messy (intersection of two moving planes) but familiar. ? New: zero point defined kinematically. Tidy but unfamiliar. AAS

2004 Denver,

2004 May

31 Where is the CEO? ? The CEO is merely a point on the moving celestial equator that stays as still as it can. ? Obviously, it has to move north-south in the sky as the equator precesses… ? …but it doesn't move along the equator: from moment to moment it moves only at right-angles to the equator. ? In fact it does creep along a bit, but very slowly. ? The CEO's present αICRS is about 00h 00m 00s.0001;