So, if for time, observe in doubles, viz., a star
E. and a star W. Also, if for lunars, let your sets be in doubles--one
set of distances to a star E. of moon, and one to a star W. of moon.
Whenever you begin on lunars, give three hours at least to them, and
bring away a reliable series; you will be thus possessed of a certainty
to work upon, instead of the miserably unsatisfactory results obtained
from a single set of lunars taken here and another set there, scattered
all over the country, and impossible to correlate. A series should
consist of six sets, each set including three simple distances. Three of
these sets should be to a star or stars E. of moon, and three to a star
or stars W. of moon. Lunars not taken on the E. and W. plan are almost
worthless, no matter how numerous they may be, for the sextant, etc.,
might be inaccurate to any amount, and yet no error be manifest in their
results. But the E. and W. plan exposes errors mercilessly, and also
eliminates them. One of the best authorities on the requirements of
sextant observations in rude land travel, the Astronomer Royal of Cape
Town, says to this effect:--"Do not observe the altitude of the star in
taking lunars, but compute it. The labour requisite for that observation
is better bestowed in taking a large number of distances." So much
delicacy of hand and of eyesight is requisite in taking lunars that shall
give results reliable to seven or eight miles, and so small an exertion
or flurry spoils that delicacy, that economy of labour and fidget is a
matter to be carefully studied.
Pages:
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61