2.7 O eclipse solar de 15 Mar¸co de 1858
3.1.2 Observa¸c˜ao sistem´atica do Sol
Nos anos 40 do s´eculo XIX, v´arios autores procuraram melhorar a determina¸c˜ao do valor do per´ıodo de rota¸c˜ao solar (tabela 3.1). A incerteza do valor do per´ıodo de rota¸c˜ao do Sol e, essencialmente, as abordagens utilizadas no estudo do mesmo come¸caram a ser questionadas por diversos astr´onomos. Peters, em 1855, afirma que
Certainly it is not enough to tell the number of spots for every day, nor to determine occa- sionally from a larger one the elements of rotation; it seems necessary to follow the spots or faculae in their relation to the sun’s body and to each other, by computing their heliographic positions from exact measurements, made, if possible, from day to day. This way is, indeed, a little troublesome, but nevertheless promises very interesting results.40
33
Humboldt, Friedrich Wilhelm Heinrich Alexander Freiherr von: Cosmos. Volume 3, Paris: Gide et J. Baudry. 1856; Bosler, Jean: Les Th´eories Modernes du Soleil. Paris: O. Doin et Fils. 1910.
34
John Frederick William Herschel (1792–1871).
35
Herschel, John F. W.: Outlines of Astronomy. Philadelphia: Blanchard and Lea. 1849.
36
Christian Heinrich Friedrich Peters (1813–1890).
37
Peters, Christian Heinrich Friedrich: Contributions to the Atmosherology of the Sun. Proceedings of the American Association for the Advancement of Science, Ninth meeting, August 1855 1856.
38
Lardner, Dionysious: Hand-Books of Natural Philosophy and Astronomy, third course Meteorology and Astro- nomy. Philadelphia: Blanchard and Lea. 1858, p. 248.
39
Pinto, Rodrigo Ribeiro de Sousa: Elementos de Astronomia - Tomo 2o. Imprensa da Universidade de
Coimbra. 1873b, p. 117.
40
Peters, Christian Heinrich Friedrich: Contributions to the Atmosherology of the Sun. Proceedings of the American Association for the Advancement of Science, Ninth meeting, August 1855 1856.
Data Per´ıodo de rota¸c˜ao Autor 1841 25d 04h 30m Petersen 1842 25d 08h 09,6m Laugier 1846 25d 02h 10m Kysaeus 1847 25d 12h 59m 26s Biot 1852 24d 12h 30m B¨ohm
Tabela 3.1: Per´ıodo de rota¸c˜ao do Sol determinado por v´arios autores
E Richard Carrington confessa-se surpreendido com
the capricious manner in which the observations of the solar phenomena had commonly been taken up and laid aside again, the entire neglect of the subject by the public establishments, grave defects in the methods of observation commonly employed, and as might be expected, large discrepancies in the results of previous observers in respect to the Elements of Position of the Pole and Period of Rotation.41
Como consequˆencia, quer Peters quer Carrington iniciam longos programas de monitoriza¸c˜ao solar. Entre Setembro de 1845 e Outubro do ano seguinte, Peters observou, continuamente, a superf´ıcie do Sol. Na an´alise dos dados apresentada, em 1855, na nona reuni˜ao da American
Association for the Advancement of Science, conclui a existˆencia de movimentos pr´oprios
das manchas em latitude e em longitude.42 Estes movimentos explicariam as discrepˆancias existentes entre os diferentes valores obtidos do per´ıodo da rota¸c˜ao.
Carrington, por seu lado, inicia, a 9 de Novembro de 1853, um programa de monitoriza¸c˜ao solar no seu observat´orio de Redhill com o objectivo muito espec´ıfico de
[...] tracing regularity in the distribution of the maculae, detecting the true period of Rotation of the Body of the Sun, and the determination of the systematic movements or currents of the surface, if such existed in any definable manner.43
Em Novembro de 1858, Carrington confirma a varia¸c˜ao da latitude heliocˆentrica das manchas com o tempo mas, ao contr´ario de Peters, conclui que o movimento n˜ao ´e apenas uma migra¸c˜ao para a zona equatorial e posterior desaparecimento, mas que a este se segue o aparecimento de novas manchas a latitudes elevadas.44 Em Janeiro de 1859, Carrington relata que as varia¸c˜oes
da velocidade em longitude dependem da latitude solar e, em Abril de 1860, apresenta a tabela 3.2 que resume os resultados obtidos at´e ent˜ao. Carrington tinha descoberto a rota¸c˜ao diferencial do Sol.45
Note-se que as vantagens da observa¸c˜ao di´aria e cont´ınua do Sol s˜ao mencionadas por outros cientistas contemporˆaneos essencialmente quando esta ´e associada `a utiliza¸c˜ao, como meio de registo, da nova t´ecnica fotogr´afica. Em 1847, John Herschel considera que uma s´erie interrupta de imagens
41
Carrington, Richard Christopher: Observations of Spots on the Sun from November 9, 1853 to March 24, 1861 made at Redhill. Williams and Norgate. 1863, p. 1.
42
Peters, Christian Heinrich Friedrich: Contributions to the Atmosherology of the Sun. Proceedings of the American Association for the Advancement of Science, Ninth meeting, August 1855 1856.
43
Carrington, Richard Christopher: Observations of Spots on the Sun from November 9, 1853 to March 24, 1861 made at Redhill. Williams and Norgate. 1863, p. 2.
44
Carrington, Richard Christopher: On the Distribution of the Solar Spots in Latitudes since the Beginning of the Year 1854, with a Map. Monthly Notices of the Royal Astronomical Society, 19 Novembro 1858a.
45
Carrington, Richard Christopher: On certain Phenomena in the Motions of Solar Spots. Monthly Notices of the Royal Astronomical Society, 19 Janeiro 1859; Carrington, Richard Christopher: On two cases of Solar Spots in High Latitudes, and on the Surface Currents indicated by the Observations. Monthly Notices of the Royal Astronomical Society, 20 Abril 1860.
Varia¸c˜ao em Latitude Longitude (′) Latitude (′)
50 N −94 +2 40 N −64 +5 30 N −40 +5 20 N −22 +1 10 N −10 −4 0 0 0 10 S −10 −4 20 S −22 +1 30 S −40 +5 40 S −64 +5 50 S −94 +2
Tabela 3.2: Varia¸c˜ao da velocidade das manchas com a latitude segundo Carrington
exhibiting a continuous view of the changes in the sun’s surface for every day in every year in the future, and as near an approach to it in past years as can now be recovered. It seems high time that some attempt of the kind should be made on a systematic and regular plan, as the only probable effectual means of arriving at a knowledge of the laws which govern these mysterious phenomena, and the periods, if any, which they observe in their formation, and thence of elucidating the nature of the sun itself. [...] Moreover, the exceeding facility with which photographic processes are executed, and especially the short time which the Talbotype process occupies, makes their execution on a given scale, and with every requisite degree of precision, easily attainable.46
E, em 1849, Faye, numa comunica¸c˜ao `a Acad´emie des Sciences de Paris afirma que
Le m´eme proc´ed´e [photographique] s’appliquerait `a la d´etermination des coordon´ees h´eliocentriques des taches du Soleil et ajouterait des nouvelles donn´ees `a celles que l’on doit aux travaux de M. Laugier sur les ´el´ements de la rotation du Soleil.47
No entanto, Carrington, tal como Schawbe, regista as suas observa¸c˜oes atrav´es do desenho, mesmo apesar de John Herschel lhe ter aconselhado a utiliza¸c˜ao da fotografia. Carrington n˜ao adoptou este conselho por dois motivos. Primeiro porque n˜ao desejava modificar a sua metodologia e segundo, porque estimava que a implementa¸c˜ao da nova t´ecnica lhe iria requerer trˆes longos anos48
A carta que John Herschel dirigiu ao General Sabine, em 24 de Abril de 1854, e a sua posterior apresenta¸c˜ao na reuni˜ao de 1854 da British Association for the Advancement of
Science levou, finalmente, a que fosse implementado, no observat´orio de Kew, na altura sob
a al¸cada da British Association, um programa de fotografia di´aria do Sol.49 Da execu¸c˜ao da mesma ficou encarregado o amador Warren De la Rue, um dos primeiros especialistas de fotografia astron´omica. Em 1858, obtiveram-se as primeiras imagens que foram interrompidas
46
Herschel, John F. W.: Extract of letter respecting Mr. Griesbach’s communication on the solar spots. Monthly Notices of the Royal Astronomical Society, 8 Novembro 1847.
47
Faye, Herv´e: Sur les observations du Soleil. Comptes Rendus de l’Acad´emie des Sciences de Paris, 28 1849.
48
Clark, Stuart: The Sun Kings - The unexpected tragedy of Richard Carrington and the tale how modern astronomy began. Princeton & Oxford: Princeton University Press. 2007, p. 74.
49
Herschel, John F. W.: Daily Photographing Register of the Spots on the Sun’s Disc. Em Report of the twenty-fourth meeting of the British Association held at Liverpool in September 1854 . 24 1855a; Herschel, John F. W.: On the Application of Photography to Astronomical Observations. Monthly Notices of the Royal Astronomical Society, 15 1855b, Nr. V.
devido `a desloca¸c˜ao do fotoheli´ografo de Kew para a observa¸c˜ao do eclipse solar de 1860, em Espanha.50 Este t´opico ser´a desenvolvido na sec¸c˜ao 6.1.1.