NO. 9 BRIGHTNESS OF LUNAR ECLIPSES — FISHER 3 



Grade o. — When apertures of 6 inches or more are needed. This 

 covers the larger part of fixed observatory instruments. 



Table i shows the almanac date of the beginning of totality, the 

 magnitude, the grade on the above scale, and the position of the 

 terrestrial terminator (sunrise-sunset line) at mid-eclipse, for each 

 lunar eclipse i860- 1922 for which data suitable to grade on the above 

 scale have been found — 68 eclipses in all. These facts for each eclipse 

 are followed by the name of the observer, the place, a number in 

 parenthesis referring to the list of references placed later in the 

 article, and an abstract of the observations reported, beginning with 

 the condition of the sky and the aperture of the telescope, or some 

 statement about the means employed. Strength of twilight is as 

 stated by the observer, or based on moon's altitude. Under each 

 echpse the observers are mostly arranged in an order indicated by 

 the column headed Phase — Air Mass. The phases of an eclipse are 

 indicated by 



a, e, first and last external contacts. 



b, d, first and last internal contacts (lacking in partial eclipses). 



c, middle of the eclipse. 



The relative air mass along a ray from the moon's center to the 

 observer's eye is given to one decimal ; thus 5.3 means an air mass 

 5.3 times the mass along the zenithal ray of an observer at 45° and 

 sea-level. These air masses were found by three-place computing of 

 the moon's geocentric altitude, corrected for parallax and refraction 

 to about 0.1° ; values were then taken from Wolff's table.'' For a 

 station considerably above sea-level, the tabular number for the 

 refracted zenith distance is multiplied by the ratio of the pressure 

 there to 760 mm., pressures being taken from Humphreys' table.* 



The code used for brevity in this column may be thus illustrated : 



c=i.4, relative air mass at mid-eclipse = 1.4. 



a = 2.3, relative air mass at first external contact = 2.3. 



b 1.7 c, relative air mass at a moment between first external contact 

 and mid-eclipse = 1.7. 



The grade of the eclipse, 2, i, or o, is derived from a comparison 

 of the observations. The tendency in grading has been to favor 

 brightness, i. e., positive statements of visible details have been pre- 

 ferred in general to statements of the contrary. So that the means 

 of brightness grades are not likely to be too low. 



It was expected that the arrangement according to air mass would 

 tend to explain some of the discrepancies among the different reports. 



^ H. Wolff, Beitrage zur Geophysik, 11, table, pp. 412-413, 1912.. 

 ^W. J. Humphreys, Physics of the Air, 1920, p. '/2. 



