60
magnitude can be observed without difficulty. A delicate level and a filar micrometer constitute the zenith-telescope attachment. The level is affixed to one side of the tube, with its length at right angles to the horizontal axis, and revolves on a centre, its indications serving to give a measure of any small change of inclination of the telescope consequent on reversal. The value in arc of one division of this level, as determined at the Washington Naval Observatory, is 1.067. The micro- meter-screw carries three parallel threads perpendicular to the ruled lines of the transit reticule, the middle one of the three being that which is commonly used for measuring differences of zenith- distance. The value in are of one revolution of the screw, as ascertained from numerous careful observations at the Washington Observatory, is 68."698; and, as the graduated screw-head is divided into 100 parts, each of which can be read to tenths by estimation, it affords a measure of .069 of a second of arc. The whole number of turns is read off on a comb at the side of the field of view, and these, for convenience, are numbered consecutively from left to right as viewed, the central tooth being Number 20. Parallactic motion is given to the eye-piece by a long-pitched screw, so that stars can be accurately observed either on the north or the south of the middle of the field.
Observing-hut-For shelter, I had a roomy and well-designed portable wooden observatory, with the requisite openings, which was given to me by Lieutenant-Commander GREEN.
Pir. The instrument-pier is of Canton blue bricks set in Portland cement, and rests on a shallow footing of brickwork, but not isolated below the ground-level, though it has no contact with any part of the hut. This was found to answer very well, no unsteadiness having been perceptible during ordinary movements in the hut.
Zenith-telescope method of determining the Latitude. In determining the latitude or declination of the zenith by the zenith-telescope method, the general procedure is as follows. Pairs of stars are chosen for observation, such that their meridianal zenith-distances shall be nearly equal, but on op- posite sides, north and south, of the zenith, and that they shall pass the meridian within a few minutes of one another. There is no fixed limit to the zenith-distance suitable for this method, but it is desirable not to pass much beyond 25°, and in this determination I have limited myself to within 20°, while the mean for all the pairs observed is just 8°. For difference of zenith-distance of any pair of stars the usual limit is 20' of are, in order that the stars may not be too near the edge of the field, and also that the resulting latitude may not be too largely affected by any error in the adopted value of the micrometer-screw. I have practically conformed to this limit on the present occasion, having in the case of two pairs only passed a little beyond it. The difference of Right Ascension should be not less than one minute, in order to allow time for reading and reversal, and it should not exceed twenty minutes, on account of the risk of instrumental changes in the interval, resulting from changes of temperature. About sixteen minutes is the greatest interval for any pair used in this determination.. Method of observation. For observing, the instrument, having been previously adjusted carefully to the meridian, and its cross-axis levelled, is clamped at the mean of the zenith-distances of the two stars, and then directed at the proper time to the star which will pass the meridian first of the pair, the bubble of the telescope-level being brought to the middle of its run by the screw for that This star is bisected with the micrometer thread at or near the moment of its passage over the middle purpose. transit thread, the time being noted and, immediately afterwards, the level and micrometer-screw readings. The instrument (telescope and level remaining clamped) is then reversed, and it will evidently now be inclined to the zenith at the same angle as before, but on the opposite side of it, so that the second star will at the proper time appear in the field. This star is bisected at or near its transit, and the chronometer, level and micrometer read as before. The above constitutes one complete observation for latitude.
**
over
Advantages and disadvantages of the method.--The chief advantages of this beautiful method all others are--that it substitutes for measurements of large ares small micrometric measurements of difference only of zenith-distance; that the time needs not be accurately known, an error of two or three seconds being of no moment; that possible errors arising from incorrect estimates of refraction are reduced to a minimum, inasmuch as the only term involving refraction which affects the result is the difference of refraction for stars at small and nearly equal zenith-distances; and that the effects of errors of flexure are similarly minimised. Its "weak point," at the present day, is the scarcity, for any station, of stars fulfilling the necessary conditions whose declinations have been determined with high precision. The stars available are mostly of small magnitudes, a circumstance which, though favourable to accurate bisection, presents the drawback that, as compared with the Clock and other Standard stars, they are seldom observed at the fixed observatories, except under special arrangements for this particular purpose.
Selection of Stars, and Computation of Mean Places-It follows that, when possible, stars from the best catalogues only should be chosen; and, keeping this in view, I have limited myself to using none but stars that have been observed at the Royal Observatory, and whose places are given in the Greenwich Nine-Year Catalogue for 1872. In that catalogue, the Mean Places of the stars are given for the epoch 1872, January 1. Those places have been reduced to the epoch 1882, January 1, by the formula,
n
X
100
Mean R.A., 1882, January 1
Ꭶ A ÷ (p + q + where A is the star's Mean Right Ascension for 1872, January 1, p its Annual Precession in
2) n + c
* The method was invented in 1834, by the late Captain A. TALCOTT, U. S. Engineers.
سم
Right Ascension, s the variation of p in 100 years, 7 the star's Annual Proper Motion in Right Ascension, n the number of years elapsed, and c the small correction from the Table at page 5 of the Introduction to the Catalogue.
In this case, 12 **** 10, and the formula becomes,
+ e
Mean R.A., 1882, January 1 = A + 10 (p + q) + Similarly, I have computed the Mean North Polar Distances by the formula,
Mean N.P.D., 1882, January 1 D + 10 (p' + q) "fu
ཉྩ་་ where is the small correction from the Table at page 27 of the Introduction to the Catalogue.
Forty-eight stars, formed into 28 pairs, were used in the determination, each pair having been observed from one to five times. The average is two times for each pair, making 56 determinations, on eleven nights in January and February. Of these it has only been necessary to reject two as imperfeet. The following Table exhibits, for each star, in Columu I its name as taken from the Greenwich Nine-Year Catalogue for 1872; in Column II its number in that Catalogue; in Column III its number in the British Association Catalogue; in Column IV its Mean Right Ascension for 1882, January 1, computed as above; and in Column VI its computed Mean North Polar Distance for the same date. Columns V and VII show the number of observations at Greenwich, of Right Ascension and North Polar Distance respectively, in the Nine-Year period 1868 to 1876, which were combined to form the Mean Places given in the Catalogue for 1872, January 1.
TABLE I.
COMPUTED MEAN PLACES OF STARS FOR 1882, JANUARY 1.
53 Andromeda Piazzi I. 142.
106 Piscium 111 Piscium 57 Andromeda 24 Arietis
15 Trianguli 40 Arietis 41 Arietis
45 Arietis
38 Persei. 52 Tauri..
Star's Name.
Number in Greenwich Nine-Year Catalogue
Number in
B. A. C.
Mean R. A., 1892, January 1.
No. of Obs. of R. A.
Mean N. P. D.,
1882, January 1.
No. of Obs. of N. P. D.
for 1872.
1. m. 8.
0
164
500
1 33 37.187
5
50 1 16.47
10
137
510
1 34 36.340
5
47 58 43.94
160
618
1 35 17.410
101
85 6 36.40
41
170
674
1 47 28.777
5
87 23 44.50
5
190
628
1 56 89,541
28
48 14 13.86
27
216
745
2 18 29.505
70 55 28.58
6
Bradley 341
201
755
2 20 26.017
4
70 58 1.05
4
230
786
228 87.272
55 40 41.01
3
258
867
2 41 55.189
72 13 81.10
3
261
872
2 43 2.338
83 13 87,28
14
268
901
249 10.732
8 50.83
8
58 Arietis
297
000
3 સે 7.102
23 38.09
14
63 Arietis
311
1045
8 15
57.815
69 40 50.04
3
64 Arietis
313
1052
3 17 20.433
41
42.30
11
f Tauri
325
1087
3 24 21.540
28
7.70
10
340
1188
3 36 65.159
58
5 12.10
14
398
1326
4 13
6.000
00 55 68.43
3
61 Tauri
403
1840
4 16 7.792
72 44 7.85
75 Tauri
418
1377
4 21 41.875
73 54 19.55
5
Bradley 819
420
1391
4 93 48.471
5
74
3 50.87
5
Piazzi IV. 111
427
1408
4 27 15.083
4
61
17 13.94
4
429
1430
4 29 8.995
100
73 43 45.22
99
433
1482
4 31 24.271
2
74
12 10.47
2
92 Tauri
439
1437
4 32 31.606
2
74 19
1.70
2
Piazzi IV. 148
440
1444
4 33 56.656
61 36 54.37
6
11 Aurige..
482
1602
5 21.264
51 39 24.90
17
507
1687
5
18 48.100
83
45 31.82
110 Tauri
519
1726
5 25 17.702
71 29 42,01
125 Tauri 1832 Tauri
534
1778
6 32 25.422
64 10 13.96
551
1837
5 41 46.444
65 28 24,69
54 Orionis
562
1870
47 29.721
69 44 59.25
58 Orionis
566
1883
5 48 46.976
82 36 58,51
37 Aurige
671
1960
6 51 40.402
59 47 50.85
67 Orionis
587
1958
6
0 50.079
23
75
70 Orionis
594
1990
6
5 14.020
13 75 45 58.55
7.22
44 Auriga..
601
2001
6 7 51.473
48 Aurige
620
2082
6 20 58.950
49 Aurige.
635
2138
6 37 46.158
59 26 61 53 18.65
24 Geminorum
645
2163
6 80 53.625
73 30 4.99
34
54 Geminorum
706
2398
7 11 18.701
73 14 53.45
60 Geminorum
724
2442
7 18 23.804
01
58
7.68
* Geminorum
*33
2469
Jay
22 28.822
01
50 31.02
4
68 Geminorum
737
2486
20 52.852
78 55 15.25
9
78 Geminorum
760
2550
7 38 5.616
115
61 41 24.62
107
89 Geminorum
774
2617
7
40 16.483
62 55 48.61
9
3 Cancri...
782
2059
7
64
1.598
79 00
10.12
5
9 Caucri....
790
2700
7
59 18.628
1
67 1
4854
+
10 Cancri
F92
2714
8
0
49.111
68
4 36.36
11
87 Tauri 80 Tauri
24 Orionis
60 27 36.25 8.75
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