ASTRONOMY.
303
Sequel of Dr. Bradley's mean. Astronomical Refractions in Altitude, corrected by the late celebrated M. de La lande.
Appar.
Altitude.
Refraction.
Distance
from
Zenith.
Appar.
! Distance JRefraction. 1 from |j Zenith. |
Appar.
Altitude.
Refraction.
D istancefromZenith.
Appar.
Altitude.
:
Refraction.
Distanct
from
Zenith.
O /
/
II
O
/
O
/
/
«
O
/ j
O
/
"
O
■t
1 0
/
<
u
O
1
39 0
1
10,2
51
0
51
0
0
7
39
0 '
65
0
0
26,4
25
0
i 78
0
0
12,0
12
0
30
1
8,9
50
30
52
0
0
44,1
38
0 ,
66
0
0
25,2
24
0
1 79
0
0
11,0
11
0
43
o
o
4‘> 5
37
0 -
()
o
94 0
93
o
j 80
0
0
10,0
10
0
40 0
1
7,7
50
0 .
;> i
0
0
41,0
36
0 i
08
0-
0
22'8
oo
0
! 81
0
0
9,0
9
0
41 0
1
5,3
49
0
55
0
0
39,5
35
0 !
69
0
0
21,6
21
0
; 82
0
0
8,0
8
0
42 0
1
3,1
48
0 :
50
0
0
38,1
34
o :
70
0
0
20,5
20
0
j 83
0
0
7,0
7
0
4.3 0
1
0,9
47
0 i
5 7
0
0
36,7
33
0 I
71
0
0
19,4
19
0
I 84
0
0
6,0
6
0
44 0
0
58,8
46
0 1
58
0
0
35,4
32
o !
72
0
0
18,3
IS
0
j 85
0
0
5,0
5
0
45 0
0
56,8
45
0 i
59
0
0
34,2
31
0
73
0
0
17,2
17
0
; 86
0
0
4,0
4
0
46 0
0
54,8
44
0
60
0
0
32,9
30
o :
74
0
0
16,1
16
0
1 87
0
0
3,0
3
0
47 0
0
52,9
43
0
61
0
0
31,6
29
o >
75
0
0
15,1
15
0
1 88
0
0
2,0
2
0
48 0
0
51,0
42
0
62
0
0
30,3
28
0 :
76
0
0
14,0
14
0
i 89
0
0
1,0
i
0
49 0
0
49,2
41
0
63
0
0
29,0
27
0 !
77
0
0
13,0
13
0
; 90
0
0
0
0
0
50 0
■
0
47,4
40
0
04
0
0
27,7
26
o :
i
1
i
(146.) The rays of light falling on the atmosphere are refracted andvariously rellected by its particles, which occasions the twilight.When the Sun is more than 18 p below the horizon the etl'ect isnot in general sufficient to render it sensible, but from about thatpoint till the Sun rises, the effect is more and more visible. Thetwilight is longest in a parallel sphere, and shortest in a rightsphere: and in an oblique sphere, the nearer the sphere ap-proaches to parallel, the longer is the twilight. In a parallel sphere,the twilight will continue till tiie Sun’s declination towards the de-pressed pole is 18° : but in this sphere his declination is nevermore than 23-/j-degrees; whence the twilight will only cease,whiist the Sun’s declination is increasing from 18° to 0.3 degrees,and decreasing again till it once more becomes 18 degrees.—Fig. 8. Plate XXV. illustrates this subject. The arcs I?/, I N,shews the ratio of the length of twilight in a right sphere, and inthe latitude of London , where it may be observed that the Sun’sgreatest depression O 1), about midsummer, is less than 18°, andtherefore there is then no dark night. Dr. Keill, in his Lectureson Astronomy , observes, that it is entirely owing to the atmospherethat the heavens appear bright in the day-time. For w ithout it,only that part of the heavens would be luminous in which the Sunis placed ; and if w e could live without air, and should turn ourbacks to the Sun, the whole heavens would appear as dark as inthe night. In this case also we should have no twilight, betasudden transition from tin* brightest sun-shine to dark night imme-diately upon the setting of the Sun ; which wotdd be extremelyinconvenient, if not fatal, to the eyes of mortals. See Keill’sAstron. Lett. XX. 'The light, whether coining from the Sun,Moon , or stars, is equally effected by the refraction, but thequantityof the refraction, and consequently the duration of twilight, are in-fluenced by the perpetual changes of heat and cold, moisture anddryness, &rc. of the atmosphere.
Sect. IX. Of the Phenomena of the Moon .
(147.) On the Moon ’s disk many darkish spots appear tothe nakedeye: and through a telescope their number is prodigiously increased:she also appears very plainly to be more protuberant in the middlethan at the edges, orlo have the figure of a globe, and nota flat circle.When the Moon is gibbous or horned, the one side appears very-ragged and uneven, but the other always exactly defined and cir-cular. The spots in the Moon always keep their places exactly ;never vanishing, or going from one side to the other, as those ofthe Sun do. We sometimes see more or less of the northern andsouthern, and eastern and western part of the disk or face ; whichis owing to what is called her libration.
(148.) Plate XXV. fig. 1. gives a representation of the F nil Moon inher mean libration, with the numbers to the principal spots accord-ing to Iticcioli, Cassini, and Mayer; underneath which the figureof part of the Moon ’s surface is more accurately delineated, ihenames are as follow :
1 Grimaldus.
2 Galileus.
3 Aristarchus.
4 Keplerus.
5 Gassendus.
6 Slrikardus.
7 Ilarpalus.
8 Ileraclides.
9 Lansbergius.
10 Reinoldus.
12 Helicon.
13 Capuanus.
14 Bullialdus.
15 Fratosthenes.
16 Timocharis.
17 Plato.
18 Archimedes.
19 Insula Sinus Medii.
20 Pitatus.
21 Tycho.
22 Eudoxus.
26 Hermes.
27 Possidonius.
28 Dionysius.
29 Piinius.
( Catharina Cvrillus.j Theophilus.
31 Fracastorius.
, 9 ( Promontorium aculum,' ) Censoriims.
33 Messaia.
34 Promontorium Somnii.
35 Proclus,
36 Cleomedes.
37 Sneilius et Furnerius,
38 Petavius.
39 Langrenus.
40 Taruntius.
A Mare Humorum.
B Mare N uhium.
C Mare Imbrium.
D Mare Nectaris.
23 Aristoteles . E Mare Tranquillitatis.
24 Manilius. F Mare Serenitatis.
25 Menelaus. G Mare Fcecunditatis.
II Mare Crisium.
(149.) It is allowed on all hands, that there are great inequali-ties on the Moon ’s surface. This is proved by looking at herthrough a telescope at any other time than when she is full; forthen there is no regular line bounding light and darkness ; but theconfines of these parts appear as it w ere toothed and cut with in-numerable notches and breaks; and even in the dark part, nearthe borders of tire lucid surface, there are seen some small spacesenlightened by tire Sun’s beams. Upon the fourth day after NewMoon , there may be perceived some shining points like rocks orsmall islands within the dark body of the Moon ; and not far fromtiie confines of light and darkness there are observed other littlespaces which join to the enlightened surface, but run out into thedark side, which by degrees change their figure, till at last theycome wholly within the illuminated face, and have no dark partsround them at all. Afterwards many more shining spaces are ob-served to arise by degrees, and to appear within the dark side ofthe Moon , which, before they drew near to the confines of lightand darkness, were invisible, being without any light, and totallyimmersed in the shadow. The contrary is observed in the de-creasing phases, where the lucid spaces which joined the illumi-nated surface by degrees recede from it; and, after they are quiteseparated from the confines of light and darkness, remain for sometime visible, till at last they also disappear. Now- it is impossible,that this should be the case, unless these shining points were high -r
th.ni.