Journal of Astronomical History and Heritage, 10(1), 31- 48 (2007). E.E. BARNARD AND THE ECLIPSE OF IAPETUS IN 1889 James Bryan McDonald Observatory, 1 University Station, C1402, Austin, Texas 78712-0259, USA E-mail: [email protected] Abstract: On 1-2 November 1889 E.E. Barnard observed Iapetus as it passed through the shadows of Saturn and its ring system. Over 2.6 hours he produced 75 differential visual magnitude estimates of Iapetus relative to Tethys and Enceladus. The resulting light curve demonstrated the C ring's already known translucence, but it also showed something unexpected. As Iapetus 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 con- forms 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 Iapetus in eclipse. Keywords: Barnard, Saturn, Iapetus, spokes, Lick Observatory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igure 1: Shadows of Saturn’s rings projected upon the ;6(+ $'<0.+ .#-')2+ %60+ %-&)*'%'()&$+ 40&-*+ (,+ %60+ $&%0+ planet's northern hemisphere as observed by the Cassini spacecraft from orbit on 10 June 2005. The shadow of the A )')0%00)%6+ &).+ 0&-$4+ %;0)%'0%6+ "0)%#-'0*+ ;60)+ ring with its narrow, sunlit Encke gap is northernmost, followed /6(%(2-&/64+-0/$&"0.+040*'26%+&*+%60+"6'0,+80%6(.+')+ southward by the wide, sunlit Cassini division, the very dark B (3*0-<&%'()&$+ &*%-()(84+ K*00+ 56006&)=+ @DDAM9+ + P%+ &+ ring shadow, and the tenuous C ring shadow, respectively. %'80+ ;60)+ )(+ ')*%-#80)%+ ;&*+ *#/0-'(-+ %(+ %60+ 6#8&)+ Barnard's result for the eclipse of Iapetus implies that normal optical depths in Saturn's inner ring system were different than 040+ &).+ 2((.+ U#.280)%+ %(+ 80&*#-0+ 8&2)'%#.0*+ ,(-+ they are today. What does his observation mean? Tethys, one *%&-*+ &).+ (3U0"%*+ %6&%+ $((F0.+ $'F0+ *%&-*=+ O&-)&-.+ of the satellites that Barnard used to visually estimate Iapetus' /-(.#"0.+-0*/0"%&3$0+<'*#&$+/6(%(80%-4+,(-+?&/0%#*+')+ changing brightness in eclipse, appears in the foreground 0"$'/*09++?)+0<&$#&%')2+6'*+;(-F=+60+"(),')0.+6'8*0$,+%(+ (after NASA/JPL/SSI, 2005b, PIA 07545 with permission from &)+ #)-08&-F&3$0+ "()"$#*'()+ %6&%+ '2)(-0.+ *%-&)20+ the Cassini Imaging Team, NASA/JPL/SSI). 0<'.0)"09++P//&-0)%$4=+60+*&;+()+?&/0%#*+&+*6&.(;+(,+ *(80%6')2+#)F)(;)+%6&%+(-'2')&%0.+30%;00)+%60+/$&)0%+ 2 BACKGROUND &).+W+-')29++L#-%60-=+&$%6(#26+60+;&*+#)&;&-0+(,+'%=+6'*+ X60+ 3-'26%+ *#/0-)(<&=+ 5+ P).-(80.&0+ K5+ P).M=+ %6&%+ .&%&+'8/$4+.0)*'%4+2-0&%0-+%6&)+8(.0-)+(/%'"&$+.0/%6*+ &//0&-0.+ ')+ %60+ P).-(80.&+ I03#$&+ ')+ P#2#*%+ @JJA=+ 31 James Bryan E.E. Barnard and the 1889 Eclipse of Iapetus !"#$%!&'(")*$+,,"-$.!/0&'*-$!(0$10(2$#3440(0"&$,5607&*-$ &9!&$;,,M$;3M0$*&!(*<$ 5'&$4,($8!("!(#$&902$:0(0$(0;!&0#<$$."$=>>?-$!&$!@0$A>$ BC3@'(0$ AD-$ 90$ 9!#$ 500"$ 4,($ &:,$ 20!(*$ !$ *&'#0"&$ !&$ E!"#0(53;&$ F"310(*3&2<$ $ G90$ H"0:$ *&!($ 3"$ &90$ I"#(,J +0#!$K05';!)$7(0!&0#$*0(3,'*$&90,(0&37!;$/(,5;0+*$4,($ !*&(,",+0(*<$$L,:010(-$8!("!(#)*$",&05,,M$!"#$/'5J ;3*90#$ /!/0(*$ +!M0$ 7;0!($ &9!&$ 4,($ 93+$ &90$ *&!($ :!*$ *&(37&;2$!"$,5*0(1!&3,"!;$+!&&0(<$$N,"*/37','*;2$!5*0"&$ 4(,+$93*$!77,'"&$:!*$!$*0(30*$,4$13*'!;$+!@"3&'#0*$4,($ &90$ *'/0(",1!<$ $ ."*&0!#$ ,4$ +!@"3&'#0*-$ ;3M0$ +!"2$ ,4$$ 93*$ 7,"&0+/,(!(30*-$ 8!("!(#$ /(,13#0#$ /37&'(0*O'0$ #0*7(3/&3,"*$ ,4$ 3&*$ 79!"@3"@$ 5(3@9&"0**<$ $ G93*$ :!*$ !$ *3@"$,4$&90$&3+0*-$5'&$3&$!;*,$3"#37!&0#$93*$*M3;;$;010;<$$."$ =>>?$90$#3#$",&$M",:$9,:$&,$/(,#'70$0*&3+!&0#$*&0;;!($ +!@"3&'#0*$!77,(#3"@$&,$&90$50*&$+0&9,#*$!1!3;!5;0<$ Figure 3: James Clerk Maxwell (1831–1879) as photographed in 1855 at age 24 while a Fellow of Trinity College Cambridge. In his Adams Prize Essay for 1856 he demonstrated mathematically that Saturn's rings would remain stable as a system of particles. However, when it came to the collisional environment of a vast number of particles with unknown sizes and shapes, Clerk Maxwell had no mathematics to explain particle motions. He privately speculated that the equatorial region of Saturn might be constantly bombarded by particles that were displaced from their orbits by collisions. By 1889 many, including Barnard, had accepted Clerk Maxwell's hypothesis of particulate rings, but they apparently did not know his equatorial bombardment hypothesis. Barnard disbelieved and ignored his own evidence for activity between the C ring and the planet that he obtained during the eclipse of Iapetus. If he had known Clerk Maxwell's concept of equatorial bombardment, would this have influenced his interpretation? Figure 2: Edward E. Barnard (1857–1923) as photographed in (Courtesy: Master and Fellows of Trinity College, Cambridge). 1885 at age 28. The eclipse of Iapetus was one of several unique observations that Barnard made at Lick Observatory from 1888 to 1895. These secured his reputation as a pre- ."$&90$;!*&$9!;4$,4$&90$"3"0&00"&9$70"&'(2$3"&0(0*&$3"$ eminent visual observer (courtesy: Mary Lea Shane Archives %!&'("$ :!*$ 93@9<$ $ R5*0(10(*$ *!:$ &:,$ ,513,'*$ 5(3@9&$ of the Lick Observatory, University of California, Santa Cruz). 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