174 
HINTS TO TKAVELLEMS. 
range of the moon’s declination, are divided each into one hundred parts 
in the south-west quadrant of the figure. The proportion of the perpen¬ 
dicular to the radius of the particular declination circle, multiplied by 
the moon’s horizontal parallax, is the parallax in right ascension. 
Both parallaxes will be in terms of arc or time, according as the 
horizontal parallax is stated in arc or time. 
Let us take, as an example, the same values as those in the first 
example of parallax in declination. The point G represents the place of 
the observer plotted at latitude 10° 30'; whether north or south is 
immaterial, and lh. 40m., the hour angles being counted, as before 
explained, from the centre outwards. GH, the perpendicular let fall from 
G on to the centre meridian, is a measure of the parallax. The moon’s 
declination is practically 21°, and so GH is measured on the scale JK, 
and equals forty-five parts, so that— 
Parallax = x horizontal parallax 
= x 59' 16" 
= 26' 36" (arc) 
= 1m. 46s. (time) 
Sign of the Parallax in Bight Ascension .—If the sidereal time at place 
exceeds the moon’s right ascension, that is, if the moon is to the west of 
the meridian, the effect of parallax is to decrease the moon’s right 
ascension. The converse is also true. 
The most convenient way of using the diagram is to cover it with a 
piece of tracing-paper, and to draw a line on the tracing-paper across the 
diagram at the latitude of observer's station. Place a ruler to represent 
the lin£ joining the centres of the Earth and moon. Then with one leg of 
a pair of compasses on the point at which the hour circle cuts the latitude 
line, adjust the other leg so that, when swept round, it touches the edge 
of the ruler in one case, or the central meridian in the other; the 
compasses are then open to the length of the perpendicular, and the 
proportion to the particular radius can can be scaled off at once. These 
proportions can be conveniently multiplied by the horizontal parallax by 
means of a slide rule. 
