SOLAR ECLIPSE EXPEDITIONS— MITCHELL 149 



of totality, can be determined with greater accuracy. By means of 

 piiotographs taken throughout the partial ecliiDse, both before and 

 after totality, the relative positions of the sun and moon can be 

 determined with a much greater accuracy than can be obtained 

 visually, and in addition many more observations are available than 

 the four times of contact. 



At the eclipse of 1905, totality came ahead of the predicted time 

 by 20 seconds, in 1918 by 12 seconds, and in 1922 by 16 seconds. 

 For guiding the work of the eclipse astronomer, it is now deemed 

 desirable to know the times of beginning and ending of a total solar 

 eclipse within an error of a few seconds. Accordingly, it has become 

 customary in recent years to secure from Washington or Greenwich, 

 a month or more before the eclipse takes place, corrections to the 

 almanac positions of the moon. 



On account of the difficulty of seeing the moon when near the time 

 of new moon, there is ordinarily a gap in the regular lunar observa- 

 tions, whether these are made by occultations or by meridian circle. 

 An intense interest was aroused in the United States by the total 

 eclipse of January 24, 1925, which was visible to almost one- tenth of 

 the total population of the country. From large numbers of careful 

 observations it was hoped that it might be possible to obtain a very 

 high degree of accuracy and thus permit a comparison of different 

 methods of observation in order to detect any sj^^stematic errors 

 peculiar to any one method. For predicting the eclipse it was neces- 

 sary to add a correction of 7."0 to the mean longitude of the moon 

 as given in the American Ephemeris. The times of second contact 

 by skilled observers with every facility for obtaining accurate posi- 

 tions of theu' stations and accurate time signals on eclipse day differed 

 from locality to locality by over 4 seconds, and the duration of totality 

 had a range of 6 seconds, variations in fact much larger than were 

 expected. However, it must not be forgotten that the outline of the 

 moon departs greatly from a perfect circle and that the time of the 

 beginning and ending of a total solar eclipse depends very largely on 

 the character of the lunar surface at the point of contact, the begin- 

 ning of the total eclipse not taking place until the last Baily bead has 

 disappeared. The comparison of the different methods shows that 

 the meridian observations of the moon differed widely among them- 

 selves. Occultations and eclipse results agreed with each other 

 within their probable errors, but there were systematic differences 

 from the meridian observations. To obtain greater accuracy, obser- 

 vations must be continued at each succeeding eclipse. Times fur- 

 nished by observations of eclipses give information not only about 

 the motion of the moon but also about the time of diurnal rotation 

 of the earth on its axis, with the result that an observation of an 

 eclipse of the sun made 3,000 years ago has an important bearing on 

 recent refined researches on the motion of the moon. 



