The Motion and Distribution of the Sun spots 



79 



In the present case the arithmetical mean is thus approximately double the size of 

 the geometrical, the following two terms in the expression for only amounting to 

 about — 0.02 and -f- 0.04. 



Although the arithmetical mean cannot be so accurately determined, I have, 

 however, computed it, especially as it will be attended by approximately the same 

 error in the different latitude-zones. In table XXXII the daily spot-density p is also 

 given. This density is defined as the quotient between the daily spot-area and the 

 area of the corresponding zone between — 60" and 60° from the central meridian. 



Such spots as have not existed for at least two consecutive days having been 

 omitted in our catalogue, this will evidently cause an error in the computed spot- 

 densities- The omitted spots being, however, very small, this error will undoubtedly 

 be less than that arising in computing the arithmetical mean from the geometrical. 



B. The Distribution of Spots on both Halves of the Sun. 



The strange fact that, upon an average, more spots are observed on the eastern 

 half of the sun than on the western, has, it is true, been previously mentioned, 

 but the fact in question does not seem to have been the subject of the attention 

 it possibly deserves. 



In table XXXIII I give a summary of all the spots and groups of spots which 

 have been observed on the photographs taken daily at Greennnch and other English 

 observatories. The observations are divided according to years and the distance 

 from the central meridian. From the table it is evident that this preponderance 

 of spots on the eastern half caimot be regarded as merely accidental. 



That the earth might exercise an influence on the spot-activity of the sun 

 sufficiently large to explain this difference in the frequency of spots, seems scarcely 

 probable, nor can this fact be explained by assuming a certain distribution of the 

 life-duration of the spots. Tims we may suppose that this preponderance of spots 

 is merely apparent, owing to a smaller spot being more easily observed on the 

 eastern half than on the western. This may possibly be due to the conditions of 

 absorption and refraction being different on the two sides of the central meridian 

 on account of the rotation of the sun. 



A greater absorption in the solar atmosphere would evidently be the cause of 

 a number of small spots being invisible. Whether such a difference between the 

 atmospheric conditions on the two halves on the sun does actually exist we shall 

 leave undecided. As we shall see later on, there is, however, a circumstance that 

 seems to support the idea of a somewhat different refraction on the two halves 

 of the sun. 



Another possibility, suggested by Dr. Block, is that a spot owing to its struc- 

 ture, perhaps caused by different angular velocities of different layers of the sun, 

 may appear unequal on the two sides of the central meridiai]. The supposition 



