CALCULATING THE OCCULTATION OF STARS BY THE MOON. 



^Q 



«3 



this T, the time of immersion is (Greenwich conjunction + t, + li 

 hours). 



Emersion. — Proceed as above, only add 150 to H (instead 

 of 75) and take out a new value of - say t^. The time of emersion 

 is (Greenwich conjunction + to + 2| hours). 



For example, take the occultation of // Piscium on 15th 

 February, 1918. From the Almanac H = - 1, a;' = -57, Greenwich 

 conjunction 3 hours 47 minutes, tj for + 74 is + 48 and t., for 

 + 149 is + 80. Calculated immersion, 6.5 p.m. ; calculated 

 emersion, 7.37 p.m. The observed times were about 6.16 and 

 7.33. The advantage of this rough method is that it generally 

 leaves a five or ten minutes margin in which the observer can 

 pick up the star before occultation. It may be noted that at 

 first glance this was not a promising occultation, as its " average 

 of limits " was —41 (see page 782)» and I will now proceed to describe 

 the proper criteria for an occultation. 



It is first of all obvious that an occultation will not take 

 place unless the star's apparent angular distance from the moon's 

 centre becomes less than the latter's radius, and it remains to 

 find out what the moon's radius is in terms of the other quantities 

 given in the Almanac. Now all these quantities in the Almanac 

 (stated as units and decimals) are founded on the earth's radius 

 as unity. This convention avoids the effects of the varying 

 distance of the moon from the earth. On this scale the moon's 

 radius is ^^\ or -273. Now the nearest approach of star and 

 moon is naturally near conjunction but is not coincident with 

 conjunction unless the quantity y' is zero (moon in (iemini or 

 Scorpio, with its decHnation invariable). Usually the moon is 

 elsewhere and its declination is changing, consequently the star 

 does not cross the moon straight east and west, but obHquely. 

 The angle of slope is given approximately by y'/x' = tan (y' and 

 x' from Almanac). 



West^ 



b-asi 



In the diagram BMCA is a line representing the apparent 

 motion of a star which disappears at B and re-appears at A. We 

 see from the diagram that another star, a Uttle further south, 

 which just misses the moon at S (its nearest point of approach) 



o 



