MOUNT WILSON OBSERVATORY. 



187 



SOLAR RESEARCH. 

 The solar activity has been very low throughout the year and the large 

 number of days on which no spots have been observed indicates the approach 

 of the sun-spot minimum. This absence of spot groups and disturbed areas 

 on the sun's surface has led us to undertake an additional series of observa- 

 tions for the determination of the constants of the general magnetic field. 

 It is also favorable for such investigations as the solar rotation and the dis- 

 placements of the lines of the solar spectrum upon which the influence of 

 great solar disturbances may be very serious. 



SOLAR PHOTOGRAPHY. 

 Direct photography of the sun and observations with the spectroheliograph 

 have been continued regularly with the 60-foot tower telescope by Ellerman, 

 Nicholson, Pettit, Hickox, and L. H. Humason. During the year ending 

 August 31, 1923, the following photographs have been obtained: 



Photoheliograms of 6.5-inch image, 279 on 279 days. 



Spectroheliograms of 6.5-inch image with 5-foot spectroheliograph in Ha light, 153 on 153 days. 



Spectroheliograms of 2-inch image and prominences and portions of 6.5-inch image, with 13-foot 



spectroheliograph in K and Ha light and light from continuous spectrum, 747 on 262 days. 



The Snow telescope and the 150-foot tower telescope have been used 

 entirely for spectrographic observations. 



SUN-SPOT ACTIVITY. 

 The number of sun-spot groups observed during the calendar year 1922 

 was 78, as compared with 140 in 1921, 168 in 1920, and 295 in 1919. Of 

 these, 46 were in northern and 32 in southern latitudes. No spots were 

 visible at the time of observation on 111 days, including intervals of 13 con- 

 secutive days in April, 13 in May (with 3 days of no observations adjoining), 

 and 16 in August. The recorded number of spotless days would undoubtedly 

 be larger were it not for several cloudy days which occurred at times of solar 

 quiescence. In 1921 there were 31 spotless days and in 1920 only 8. In 

 1922 the sun was observed on 309 days. The average number of groups 

 observed each day was as follows: 



The average latitude for the year was 9.2*, as compared with 10° in 1921, 

 11° in 1920, and 12° in 1919. 



SUN-SPOT POLARITIES. 

 As in previous years, drawings of sun-spots have been made daily at the 

 150-foot tower telescope, showing their positions, polarities, and field- 

 strengths. The following table summarizes the results of these observations: 



