282 Rotation and Fission of Stars [ch. x 
where e 2 stands for (a 2 - b°)ja 2 , so that e is the eccentricity of the surface 
of the core. 
Solar Rotation. 
252. Observations on the rates at which faculae, sunspots and calcium 
flocculi travel round the sun shew that the periods of rotation are different in 
different latitudes, and formulae of the general type of (251/2) and (2513) are 
found to fit the observations best. In fig. 48* the dots shew the period of 
Fig. 48. Solar rotation periods in different latitudes derived from measures of faculae 
(Greenwich Observatory). 
Formula ¿ = 14°-49 - l°-78sin 2 \ - 8°-16 sin«X, thus 
„ = 14 0, 54 - 2 0, 81 sin 2 X „ 
rotation of various faculae in different latitudes which had been observed at 
Greenwich from 1888 to 1923. The two curves across the diagram shew the 
best fits that can be obtained from formulae of the type of (25T2) and (25T3) 
respectively. Other determinations of the rotations in different latitudes, as 
fitted to formulae of this type, are shewn in fig. 49. 
* M.N. lxxxiv. (1924), Plate 10. I am indebted to Sir F. W. Dyson, Astronomer Royal, for 
permission to reproduce this figure and also fig. 49.