SOLAR ECLIPSE EXPEDITIONS— MITCHELL 147 



between the dates 1208 B. C. and A. D. 2162, together with charts of 

 the earth showing the locations where total and annular solar eclipses 

 may be observed. Oppolzer's predictions became possible on account 

 of the fact that the moon repeatedly takes nearly the same position 

 in the sky with reference to the sun and earth as is shown by the 

 following values: 



Days 



19 eclipse years 6,585.7806 



223 synodic months 6,585.3211 



242 nodical months 6,585.3572 



239 anomalistic months 6,585.5374 



More than 2,000 years ago it was known to Hipparchus that the 

 moon's node was not fixed. Nineteen returns of the sun to the node 

 or 242 of the moon take place in 6,585 days, an interval of time known 

 to the Chaldeans as the Saros (meaning "repetition"), which is 

 equivalent to 18 years 11 days or 18 years 10 days depending on 

 whether 4 or 5 leap years intervene. The author observed his first 

 total solar eclipse in Georgia on May 28, 1900. In 1918 he had to 

 travel westward to Oregon to observe the eclipse of June 8. The 

 following eclipse in the Saros occurred on June 19, 1936, and was 

 observed in Soviet Russia and in Japan. The chart on plate 3 

 shows 44 total eclipses in the series which includes the eclipse of 

 August 31, 1932. This series in A. D. 1211 began near the earth's 

 South Pole and the eclipse tracks will pass off the earth near the 

 North Pole in the year 1985, so that for 775 years this eclipse has 

 been repeating itself again and again after the lapse of the Saros 

 interval. Three times this interval is 54 years and 1 month, and 

 it is seen from the chuH that the eclipse track is fairly parallel to the 

 one 54 years earlier but farther north. In such a series of solar eclipses 

 there are between 68 and 75 repetitions, depending on circumstances, 

 extending over some 1,200 years. In each series there are approxi- 

 mately 25 partial and 45 central eclipses. These numbers vary for 

 different series. Of the central eclipses, total eclipses follow total 

 eclipses with about the same duration of totality and annular eclipses 

 follow annular eclipses. If eclipses take place at the moon's descend- 

 ing node, the eclipse tracks come on the earth at the South Pole and 

 move north, whereas if eclipses occur at the ascending node the 

 eclipse tracks start at the North Pole and move southward. 



According to Oppolzer, there are on the average 237.5 solar eclipses 

 in a century, or more than 2 solar eclipses per year. There are 83.8 

 partial, 77.3 annular, 10.5 annular and total (the vertex of the moon's 

 shadow just reaching the earth), and 65.9 total solar eclipses. On 

 the average, therefore, two total eclipses are visible in 3 years. The 

 chart showing the series including the 1932 eclipse shows that the 

 succeeding eclipses are difficult of access and therefore will probably 

 pass unobserved, hence a total solar eclipse is actually observed about 



