132 _ ASTRONOMY. 
straight lines; on all other days of the year as ellipses, whose convex sides 
are turned for half a year towards the north, and for the same length of time 
towards the south, and whose greatest curvature takes place shortly before 
March 10th and September 10th. Observations on the sun’s spots have 
enabled us to ascertain a rotation of the sun on its axis in a period of 253 
lays. 
Herschel’s hypothesis with respect to the nature of the spots on the sun, 
appears to be the most probable. He assumes a threefold concentric envelope 
of the obscure body of the sun proper. This first envelope .is the photo- 
sphere, or atmosphere of light; beneath it the second, a transparent and very 
elastic medium ; and beneath this layer the third, a cloudy obscure envelope, 
illuminated on its outer side, and reflecting the light to our eyes. In this 
manner it forms an ash grey border, which is seen sometimes on the sun 
without a central spot, whenever an opening may exist in the first, or first 
and second layers. Whenever this fissure or opening extends, as is gene- 
rally the case, through the third layer, the dark nucleus of the sun is then 
perceived, and about it the above mentioned grey border, which is, accord-. 
ingly, nothing else than the light passing into the opening from the outermost 
layer, and reflected back from the inner atmosphere to our eye. 
The group of spots represented in fig. 9, was discovered May, 1799, by 
Fritsch of Quedlinburgh. The western spot, very near the border of the 
sun, appeared as a black nucleus of oval shape, with an equally oval nebu- 
lous inclosure. Eastward of the oval spot, Fritsch observed another circular 
one, both united by a so called valley or mountain way, having the 
appearance of a ring mountain of our moon. From this mountain way run 
lateral branches, and both appear to the eye whiter and fainter than the 
rest of the solar surface. The spots, pl. 9, figs. 10, 11, 12, 18, were disco- 
vered by Pastorff, in Frankfurt on the Oder, May 24th, 1828. The largest 
(fig. 10), abcd, had at ab a diameter of 100 seconds, and at cd one of 60. It 
now appears, however, as a straight line of 392 geographical miles on the 
surface of the sun, under an angle of almost one second, seen from the earth ; 
consequently, the true diameter of this spot amounted to 39,200 geographical 
miles at ab, and to 23,520 at cd. The greatest diameter amounted to more 
than five diameters of the earth (= to 6880 geographical miles), consequently 
the surface of this spot contained nearly 928,000,000 square geographical 
miles. Furthermore, ef (fig. 10) had an apparent diameter = 110 seconds ; 
gh=60; no=68; pq=30; tk (fig. 11)=38; Im (fig. 12) = 6G; 
rs = 24; tu (fig.13) =46; and wr=12. All these numbers multiplied 
by 392 give the dimensions in geographical miles, altogether equal to an area 
of 2,496 millions of square geographical miles, or about 17 times as great as 
the whole surface of the earth. 
The honor of the first discovery of the sun’s spots appears due to the 
English astronomer, Harriot, who saw them Dec. 8, 1610. The eminent 
physician Averrhoes (in the twelfth century) was perhaps the first who 
saw aspot with the naked eye, erroneously supposed by him to be the planet 
Mercury. Phrystus of Wittemberg published the first treatise on these spots 
in 1611; the Jesuit, Father Scheiner, however, sought to appropriate the 
132 
