OBSERVATIONS FOR TIME AND LONGITUDE. 
169 
within the parallels given in the f Nautical Almanac/ that the occupa¬ 
tion will therefore be visible to him. The first point for him to consider 
is whether the moon will be above the horizon, at the time of conjunction. 
This can easily be determined by applying the assumed longitude in time 
to the G.M.T. of conjunction in E.A. of the moon and star, which he will 
find among the elements of occupations in the f Nautical Almanac/ 
adding the longitude in time if it be East , and subtracting if it be West', 
and then by reference to the time of the moon’s meridian passage 
(p. iv. N.A.), and her semi-duration above the horizon (Table VIII.), he can 
ascertain whether that time will include the period of occultation, and 
whether the occupation will take place in daylight, in which case it cannot 
be observed, if the star, as is most frequently the case, is one of small 
magnitude. The general effects of parallax must be taken into considera¬ 
tion, as parallax will accelerate the occurrence of the occultation when 
the moon is east of the meridian, and retard it when west; and under 
certain conditions this acceleration or retardation may amount to more 
than an hour and a half, or it may so affect the apparent relative 
positions of the moon and star thP the occultation may not take place 
at all at that station. To prevent loss of time and disappointment, the 
circumstances of the occultation should be computed beforehand by the 
simple method given, p. 171 et seg. The traveller will then know whether 
the occultation will take place at his station, the approximate local mean 
time of immersion and emersion, and the position on the moon’s limb 
where the star will disappear and reappear. 
If a traveller neglects to compute the circumstances of an occultation 
he wishes to observe, he must compute the local time of the phenomenon 
by applying the assumed longitude in time to the G.M.T. of conjunction 
in R.A. of the moon and star, which he will find among the elements of 
occultations in the c Nautical Almanac/ adding the longitude in time if 
it be East, and subtracting if it be West. An hour before the time so 
found, he should point his telescope to that limb of the moon by which 
the star will be occulted; it is necessary to take this precaution as his 
time may be in error, and the effects of parallax may accelerate or retard 
the occultation at his station according as the moon is east or west of 
the meridian. The moon will be seen to approach the star from west to 
east, until its eastern limb will reach the star and occult it; note the 
jnstqnt when this ta^es place. After a certain interval the sta,r will 
