Oblateness, radius, and mean stratospheric temperature of Neptune from the 1985 August 20 occultation

W. B. Hubbard, Philip D. Nicholson, Emmanuel Lellouch, Bruno Sicardy, André Brahic, Faith Vilas, Patrice Bouchet, Robert A. McLaren, Robert L. Millis, Lawrence H. Wasserman, J. H. Elias, K. Matthews, J. D. McGill, C. Perrier

Research output: Contribution to journalArticlepeer-review

36 Scopus citations


The occultation of a bright (K∼6) infrared star by Neptune revealed a central flash at two stations and provided accurate measurements of the limb position at these and several additional stations. We have fitted this data ensemble with a general model of an oblate atmosphere to deduce the oblateness e and equatorial radius a0 of Neptune at the 1-μbar pressure level, and the position angle pn of the projected spin axis. The results are e=0.0209±0.0014, a0=25269±10 km, pn=20.1°±1°. Parameters derived from fitting to the limb data alone are in excellent agreement with parameters derived from fitting to central flash data alone (E. Lellouch, W.B. Hubbard, B. Sicardy, F. Vilas, and P. Bouchet, 1986, Nature 324, 227-231), and the principal remaining source of uncertainty appears to be the Neptune-centered declination of the Earth at the time of occultation. As an alternative to the methane absorption model proposed by Lellouch et al., we explain an observed reduction in the central flash intensity by a decrease in temperature from 150 to 135°K as the pressure rises from 1 to 400 μbar. Implications of the oblateness results for Neptune interior models are briefly discussed.

Original languageEnglish (US)
Pages (from-to)635-646
Number of pages12
Issue number3
StatePublished - Dec 1987

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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