Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007jahh...10...31b&link_type=abstract
Journal of Astronomical History and Heritage (ISSN 1440-2807), Vol. 10, No. 1, p. 31 - 48 (2007).
Astronomy and Astrophysics
Astronomy
Barnard, Saturn, Lapetus, Spokes, Lick Observatory
Scientific paper
On 1-2 November 1889 E.E. Barnard observed lapetus as it passed through the shadows of Saturn and its ring system. Over 2.6 hours he produced 75 differential visual magnitude estimates of lapetus relative to Tethys and Enceladus. The resulting light curve demonstrated the C ring's already known translucence, but it also showed something unexpected. As lapetus crossed the sunlit gap between Saturn's upper atmosphere and the C ring's inner boundary, instead of remaining constant in brightness, the satellite steadily faded. Apparently, it passed through a shadow, but in 1889 nothing was known to exist in this space. Barnard dismissed the effect as unreal. Although he could not have known, his light curve also implied greater density in the C ring than exists today near the B-C ring boundary. What is the significance of his observation? Were Barnard's visual magnitude estimates wrong? Was the inner ring system significantly different in 1889? Did Barnard observe an event that temporarily affected the ring's density in the line of sight? There are no conclusive answers because he observed the eclipse alone and visually. Yet his method of observation and light curve are thought-provoking. What he recorded conforms in certain ways to the presence of spokes on Saturn's rings. Spacecraft have observed spokes only on the B ring, but visual observers as early as 1873 have seen spokes and spoke-like objects in the A, B and C rings. I speculate on the possibility that Barnard observed spoke shadows intermingled with ring shadows on lapetus in eclipse.
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