SDNSPOTS a;nd some of theik peculiarities. 63 



A third peculiarity of siinspots is that a different rotation 

 period for the sun is found according to the latitude of spots 

 under observation. Here on the earth, the same rotation period 

 is obtained whatever be the latitude of the observing station ; 

 it is 23h. 56m. of time from the transit of a star across the 

 meridian one night to its transit on the next night, whether the 

 observation be made at the equator or at the polar circle or 

 anywhere in between. In other words, the earth rotates on its 

 axis like a solid body. The sun, on the other hand, so far as we 

 can observe its rotation by means of its spots, does not rotate 

 as a solid body, the time of rotation being shortest for the equator 

 and lengthening out as we pass from the equator towards the 

 poles. Thus, the mean siderial rotation period of the sun's 

 equator is 24' 65 days, but for latitude 35° it is two days longer. 



But these values for the rotation period are averages only, 

 obtained by taking the means from an immense number of 

 independent groups. If we take groups in any one particular 

 latitude, and treat them separately, we find that they differ 

 very widely amongst themselves, so that there is nothing unusual 

 in a high latitude group giving a shorter period than an equatorial 

 group at the same time, or in the same group, whatever its 

 latitude, giving quite different values for the rotation period 

 at different times of its life history. 



It has been already ]Dointed out that some groups of spots 

 attain amazing dimensions. Thus, the great sunspot group of 

 1892 February, had a total length of 166,000 miles, the principal 

 spot being 93,000 miles in length. The breadth of the group was 

 65,000 miles, and its surface was 18 times as great as the super- 

 ficial area of the entire earth. The sunspot group of 1917 August 

 was even larger. These dimensions are enormous, but when we 

 compare them with those of the sun itself, they appear quite 

 small. The area of the group of 1892 February was only three 

 parts in one thousand of the entire surface of the visible hemi- 

 sphere of the sun. If the group had been absolutely black, 

 radiating to us no light or heat at all, the sun's light and heat 

 would have been diminished by only three parts in one thousand, 

 but as the penumbra may be taken as being about two-thirds as 

 bright as the disc, and the umbra as one-fourth as bright, the 

 loss of light would be little more than one-part in a thousand. 

 It must further be remembered that not only was the loss of 

 light due to the spot small, but the spot itself only attained its 

 greatest dimensions on one particular day ; it was only visible 



