OBSERVATIONS FOR TIME AND LONGITUDE. 
183 
Longitude by Lunar Distance. 
In this observation the observed distance is not only liable to errors 
caused by a defect of parallelism in the telescope, which always makes 
the observed distance too great, but to all other instrumental errors, 
some of which may very possibly be unknown to the observer, and as an 
error in the distance, of whatever kind, produces about thirty times its 
amount in longitude, it will be readily understood that but little value 
can be attached to the results obtained from a single set of lunar 
distances, even when the observation has been taken by a competent 
person, as making the contact slightly above or below the centre of the 
field, combined with the effects of irradiation, may very well cause an error 
of 20" in the observed distance, the effect of which would be, in average 
cases, 600" or 10' error of longitude. For these reasons lunar observa¬ 
tions cannot be recommended to any person who has not acquired a perfect 
knowledge of the use of the sextant, its errors and adjustments; or who 
is unable to remain at one place long enough to take a series of distances 
east and west of the moon. 
To Measure the Angular Distance between the Moon and Sun.— As the 
enlightened limb of the moon is always nearest to the sun, the angular 
distance measured is always that of the near limbs; but since, on 
account of her comparatively feeble light, it is necessary to observe the 
moon by direct vision, and since the sun at the time of observation may 
be either to the east or the west of the moon, the sextant has to be held 
with its face up or down as the case may require. In north latitude, 
when the sun is to the west of the moon, the instrument is held with its 
face upwards ; but when the sun is to the east of the moon, it must be 
held with its face downwards. In south latitude the opposite of this rule 
must be followed. This is often much easier if the observer can hold the 
sextant in his left hand; the position of the hand and wrist may otherwise 
be cramped and almost painful. Before taking an observation, look at 
the sun through the dark shades, and select those which reduce its 
brightness in the greatest degree compatible with good definition; put 
these down before the index glass; see that the inverting telescope is 
adjusted to focus; set the index to zero (0°); and hold the instrument 
with its plane parallel to a line joining the sun and moon; look at the 
moon through the telescope collar and horizon glass, and move the index 
