178 
HINTS TO TRAVELLERS. 
In the above example the G-.M.T. of geocentric conjunction is 
14h. 27m. 40secs., ancl the calculation is commenced with the view of 
finding the parallaxes at 14h., 15h., 16h., so as to plot the position of 
the moon at those three times, and from those positions as plotted, to 
draw the path of the moon’s centre. Before we can plot the parallaxes 
off the diagram, the hour angles must be determined, and the first portion 
of Ihe calculation is for this purpose. The hour angle at 14h. is found 
to be lh. 11m. 54*6 secs., and since the sign is + the moon is on the west 
of the meridian. This, according to the rule before stated for the sign 
of the parallax in right ascension, throws back the moon in right 
ascension, and as far as the effect of that only is concerned, delays the 
time of conjunction; so that we may infer that this time, instead of being 
between 14h. and 15h., may be between 15h. and 16h., and it will 
consequently be better to plot the position of the mcon at three hours, 
and the hour angles for those times are noted down. It is not necessary 
lo recalculate the hour angles, but for each difference of one hour of 
G-.M.T. add algebraically about 18m. to the hour angle. That is to say, 
when the moon is on the west of the meridian the hour angle may be 
considered positive and is increasing, and when the moon is on the east 
of the meridian the hour angle may be considered negative and is 
decreasing. 
The moon’s horizontal parallax and semi-diameter are next taken 
from the N.A.; they should be corrected approximately to time of 
occupation. 
The remainder of the calculation consists simply in applying the 
parallaxes, scaled from the diagram, to the right ascensions and declina¬ 
tions of the mcon taken from the N.A., and in taking the differences of 
the right ascensions and declinations as well as those of one of the posi¬ 
tions of the moon and of the star. These differences are taken out only 
to facilitate plotting the relative positions on a figure or drawing. See 
that the right ascensions and their parallaxes are stated both either in 
time or in arc. 
Construction of the Figure . 
The point A (see diagram facing p. 180) is taken as the position 
of the moon’s centre at 14h. G.M.T., and relatively to this B represents 
