Eambaut — On the Lunar Eclipse of January 28, 1888. 45 



assumed value of PQ will have a very small effect on those of 

 ab and ac. 



Suppose s h s 2 , and s 3 (Fig. 2) to be three stars occulted in the order 

 named, the first being a re-appearance, and the two others disap- 

 pearances, and suppose the hour angle of the moon when these 

 occultations take place to be 4 h , 3| h , and 3 h respectively ; then if 

 we suppose the moon to become stationary just when it occults s x , 

 and if we move the star s 2 through a space equal and parallel to 

 the motion of the moon's centre in the interval which has elapsed 

 between the occultations of s L and s 2 , but in the opposite direc- 

 tion, it is obvious that the moon and s 2 will be in the same relative 

 position as at the moment when s 2 was occulted. That is to say, 

 * 2 will lie on the circumference of the moon's disc. If we treat s 3 

 in the same way, we will get all three on the circle representing 

 the circumference of the moon at the moment when it occulted s u 

 Now, the apparent motion of the moon is composed of two 

 parts — its own motion and the displacement due to parallax. Let 

 the hourly motion of the centre be represented by MN\ then we 

 must bring s 2 back to s 2 through a distance equal to half ifcfiV, and 

 parallel to it ; and, again, to s" where s 2 s 2 f is parallel and equal 

 to ba. Similarly s s is to be brought to s 3 ' through a distance equal 

 and parallel to il/iVand to s 3 " where s 3 ' s" is parallel and equal to 

 ca. Then the three points 8 U s 2 \ s", all lie on the circumference 

 of the circle, which represents the position of the moon at the moment 

 when Si. was occulted. This gives the direction in which the moon 

 appeared at the observatory under consideration, and if similar 

 observations are made at another observatory, their distances apart, 

 and these two positions of the moon would furnish data from 

 which the moon's parallax or distance might be found. 



Under ordinary circumstances the occultations of only the brighter 

 stars can be observed, owing to the intensity of the moon's light, 

 and a further disadvantage is, that observations at the bright and 

 dark limbs are not quite comparable, being made under very diffe- 

 rent circumstances. A bright star, such as Aldebaran or Regulus, for 

 instance, when passing behind the bright limb, is very often seen as 

 if it were interposed between the moon and the earth, or as if the 

 moon were partly transparent ; and when it has appeared for a few 

 seconds projected on the disc, it is extinguished. This effect is due 

 to irradiation, which causes the disc to appear of larger dimen- 



