POOR, THE FIGURE OF THE SUN 387 



the following month by Bessel/ who showed that the observations could 

 be represented equally well by periodic changes in the transit instrument 

 itself. 



BiANCHi. — Piazzi^ and afterwards Bianchi^ investigated the subject, and 

 reached conclusions diametrically opposed to those of Lindenau. While 

 they found the sun to be an ellipsoid, they made the equatorial radius the 

 greater. Bianchi based his work on some 440 measures of the vertical and 

 439 of the horizontal diameter made during the years 1827, 1828 and 1829. 

 From the measures of the vertical diameter alone, he deduced a polar com- 

 pression of Ya^' ^^^^ tl^6 horizontal measures, a compression of jj^', 

 while from a combination of all the observations, he found the value ^^. 

 This latter value would make the equatorial exceed the polar radius by 

 some 3".87. 



Secchi. — In spite of these papers, the generally accepted conclusion 

 was that the solar disk was circular and its diameter constant. Not until 

 after a lapse of forty years was the subject reopened. In 1871 Secchi 

 became interested in the question of the sun's shape, and, with his assist- 

 ant Rosa, undertook a systematic series of observations with the meridian 

 circle of the CoUegio Romano. As these observations apparently showed 

 great variations in the sun's diameter, Secchi induced the Palermo Obser- 

 vatory to make an independent series of measures. These confirmed the 

 results obtained at Rome. According to these investigations,* the diam- 

 eter of the sun varied with the number of sun-spots, being greatest when 

 the number of spots was least, and least when the number of spots was 

 greatest. In conformity with this, he also found that those diameters 

 which pass through regions of 'ntense spot-activity are less than those 

 passing through other regions. He found the diameter which passes 

 through latitude 21° to 23° to be some V.5Q smaller than the diameter 

 which passes through latitude 6°. 



HiLFiKER. — Hilfiker, assistant at the Observatory of Neuchatel, pub- 

 lished the results of twenty-two years' observations. In all he made use 

 of 3468 homogeneous transits of the sun, all observed with the same instru- 

 ment and in precisely the same manner. This instrument had an objective 

 of 115 mm. aperture and 2 m. focal length. In making the observations, 

 a uniform magnification of 200 was used, and each limb of the sun was 

 observed on thirteen threads of the reticle. From this great mass of obser- 

 vations, Hilfiker drew the definite conclusion that the sun's diameter is 



» Zach, Monatliche Correspondenz, July, 1809. 



^ Specola Astronomica di Palermo, Liv. VI. 



3 Astronomische Nachrichten, Vol. IX, No. 213, August, 1831. 



* Atti deirAccadeinia del Lincel, January, 1872. 



