180 
HINTS TO TRAVELLERS, 
appearance, measured from the north point of the limb towards the east, 
is therefore P D S = 139°. 
Similarly the angle of reappearance is 360° - P' E S = 280°. 
From the Vertex of the Moon's limb * :—Since the parallax of a heavenly 
body lies in the plane passing through that body, the earth's centre and 
the vertex of the observer, it follows that if, on the figure, are plotted 
the positions of any point of the body as affected by parallax, and as 
unaffected by the same, the line joining these two positions, and all lines 
parallel to it, represent portions of celestial great circles passing through 
the vertex of the observer, and one of the points at which such a line 
passing through the centre of the moon cuts its limbs will be a vertex 
of the moon, according as the observer is north or south of the same. 
In the figure, C is the position of the moon’s centre at 16 hrs. plotted as 
affected by parallax, and F is its real position, that is unaffected by 
parallax. H D is drawn parallel to F C, then H is the vertex of the moon’s 
limb, and the angle of disappearance measured towards the east is H D S 
= 89°. Similarly the angle of disappearance is 360° - K E S = 223°. 
The most convenient way of drawing the figures is on what is known 
as logarithm paper, ruled with blue or red lines into squares. If these 
lines are drawn about a quarter of an inch apart, and each division is 
taken to represent one minute of arc, a figure can conveniently be drawn 
on half a sheet foolscap size. 
After a very little practice, the calculations of hour angles, scaling off 
the parallaxes, and drawing the diagram can all be done in from a 
quarter of an hour to twenty minutes, and if done with only a moderate 
amount of care, the error of the time either of disappearance or re¬ 
appearance arrived at should not exceed ten minutes. The mean error of 
a large number worked out was 4* 5m. The angles, however, should 
differ only a degree or two from the correct angles of disappearance or 
reappearance respectively, 
* The substance of this paragraph is taken from a paper by E, A. Reeves, 
f.r.a.s., printed in the Geographical Journal for Feb., 1898, 
