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IAUC 3038: Occn OF SAO 158687 BY URANUS ON 1977 Mar. 10

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                                                  Circular No. 3038
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Cable Address: SATELLITES, NEWYORK     Telex: 921428
Telephone: (617) 864-5758

     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

   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.

1977 February 9                (3038)              Brian G. Marsden

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