O. G. Franz and L. H. Wasserman, Lowell Observatory, write that measurement of plates obtained with the 155-cm astrometric reflector at the U.S. Naval Observatory's Flagstaff Station leads to the differences dR.A_o cos Decl. and dDecl._o (in the sense Uranus minus SAO 158687) shown below; the results were corrected for differential refraction. The computed differences dR.A._c cos Decl. and dDecl._c were derived from the A.E. positions of Uranus (corrected for parallax) and the SAO position of the star (corrected to the epoch and equinox of observation). 1977 UT dR.A._o dR.A._c O-C dDecl._o dDecl._c O-C Jan. 25.54587 +242"06 +242"23 -0"17 -124"21 -122"78 -1"43 25.55277 +242.59 +242.66 -0.07 -124.37 -122.92 -1.45 25.56008 +243.21 +243.24 -0.03 -124.45 -123.06 -1.39 25.56649 +243.46 +243.39 +0.07 -124.67 -123.19 -1.48 Although the mean O-C difference of -0s.003 in R.A. is negligible, that of -1".44 in Decl. shows that the occultation will not in fact be visible from wherever Uranus is above the horizon (Taylor 1973, J. Br. Astron. Assoc. 83, 352). With an assumed radius of 26 000 km for Uranus the nominal northern limit of the occultation will be from equatorial Africa, across the Indian Ocean, somewhat north of Perth, Western Australia, and then into daylight. P. K. Seidelmann, U.S. Nautical Almanac Office, indicates that the correction to the A.E. ephemeris of Uranus is -0s.023 in R.A. and -0".21 in Decl., suggesting that the correction to the star's declination is about +1".2, a value that is consistent with preliminary reductions of recent transit-circle observations at the Naval Observatory. He suggests that the uncertainty in the relative declination of Uranus and the star is still +/- 0".2, corresponding to an uncertainty of about +/- 2500 km in the northern limit of the occultation track. There is also an uncertainty of 1000-2000 km due to the uncertainty in the radius of Uranus. Any occultation will occur near 21h00m UT, the duration greater for the more southerly observers. With further reference to the Miranda appulse (cf. IAUC 3005), W. B. Hubbard, Lunar and Planetary Laboratory, remarks that an occultation of the star by Miranda is now somewhat more likely, the full suggested Decl. correction of -1".44 putting the track near the south pole about 2.5 hours after the Uranus occultation.
International Astronomical Union Circular
- Pub Date:
- February 1977