NO. 9 



BRIGHTNESS OF LUNAR ECLIPSES FISHER 



47 



of the exposure; a and fi' are the same, displaced by parallax. On 

 the ecliptic, a and b are the geocentric positions of the center of the 

 earth's shadow at beginning and end of the exposure ; a' and b' are 

 the points where the axis of the shadow pierces a plane at the moon's 

 distance ; they are displaced from a and b by parallax. As, at Taunton, 

 the parallax of the moon increased during the exposure, nearly as 

 the sine of the zenith distance, and so not in proportion to the time, 

 the parallactic displacements of corresponding points are greater and 

 differently directed at the end of the exposure. This partly accounts 

 for the departure of the upper and lower limbs of the moon trail from 

 parallelism with the orbit, and for the curvature of these limbs, slight 

 but real in the dim northern limb, easily seen in the southern limb. 



Mr. Metcalf also points out another cause — enlargement of the 

 moon's image by irradiation. 



The half-tone shows gradations of brightness in the umbra, which 

 are more easily seen in the original. In the diagram these are dis- 

 tinguished as in three zones ; A, an outer bright zone ; B, an inter- 

 mediate dimmer zone, single hatched ; C, an inner dark zone, double 

 hatched. The boundary of A is, of course, the outer edge of the 

 umbra, not shown ; the boundary of B is roughly concentric with the 

 center of the shadow, which lies from a' to b' ; C is markedly elliptical, 

 or flattened on its south side. These peculiarities cannot be due to 

 lunar surface features, which are hard to make out anywhere on 

 the negative ; the Mare Frigoris made a faint streak, and faint stripes 

 due to one or two other seas, etc., can barely be traced. 



