— - J- -—
2<i8
(132.) Tilt transits ol'Vcnu-, like those of Mercury, return in stated jperiods. They are determined by means of her synodic revolu-tion, 583 days, 22h. dm. 52, lo^.’.v ti. v. hich we find the timer of allher mean ini' tier conjunctions. Y> hen we have calculated theseconjunctions, we choose those that happen about her nodes, i. e.towards the beginning of June and December. We then deter-mine tne inferior conjunctions about her nodes more exactly, andhence iind the periods in which her transits return. ’1 he lir.-tand shortest period which can take place in the transits of Venus over the Sun's disk, at her ascending node, in the month of De-cember, is that of 8 sears; and the moment of the conjunctionis obtained by subtracting 2 days llh. 48m. from the time of thepreceding transit. At the end ot this period \ env.s moves in apath 24x minutes of a degree to the south of that ot the preced-ing transit. The second period is that of 235 Julian years, 2 days,13 hours, ora day more, if the year of the preceding transit be bis-sextile. The path of Venus at the end of this period is 24J mi-nutes of a degree more to the north. The third period is that ot243 years; and the time of the conjunction is had by only subtract-
ing owe hour from the first, -lint if the year of the first transitbe bissextile, the period is 243 years anil 23 hours. We must adda dav more for every omission cf an intercalation, according tothe Gregorian calendar. In the transits ot Venus , at the descenil-' ing node, the same periods take place, but with some difterence.
,The first period is that of S years; and the moment of the con-junction is had by subtracting 2 days, 7 hours, 30 minutes. Venus ,at the end of this period, is 19' 34" to the north of her precedingj transit. The second period is that of 235 years, 2 days, 1 1 y hours.
' Venus moves 21’ 46" to the south of her first path at the end ofi this period. The third period is that, of 243 years, 4 hours, 9 mi-nutes, Venus, at the end of this period, being only 2' 16" to thesouth of her tirst track. The following is a Table of the transitsof Venus for 14 centuries, calculated from M. de La Lande’sTables of Venus, and reduced to the meridian of Greenwich :One asterisk denotes a transit to be visible in part to England;but if there be two asterisks prefixed, then the Sim is expectedto be above the horizon of London , during the w hole of thetransit.
(133.) Table nf the Transits of Venus for Fourteen Centuries, from A. D. 1631 to A. D. 2984, {both inclusive.)
Y ear.
M-Time of Conjunction.
Geocentric Long.
Middle of Transit, | Semidur. for
True Time.
the cent, of J
Nearest Dist.Centres.
1631
7 Dec. at
H.
5
M.
19
s.
29 morn.
14
58
U
50
II.
4
M.
52
s.
23
morn.
n.
1
M.
35
s.
5
>
14
56
N.
*1639
4 Dec.
6
0
20 aft.
t
12
32
15
6
30
20
alt.
3
17
n
9
0
S.
*1761
6 J une
5
35
14
morn.
n
15
30
31
5
20
50
morn.
3
8
0
9
30
s.
N.
*1769
3 June
9
53
34
night
n
13
27
8
10
27
3
night
o
59
53
10
10
1874
9 Dec.
4
8
24
morn.
t
16
57
49
3
43
28
morn.
4
41
13
51
N.
*1882
6 Dec.
4
10
24
aft.
t
14
29
14
4
49
52
aft.
3
1
43
10
29
S.
**2004
8 June
8
51
24
morn.
n
17
54
23
S
26
59
morn.
2
44
50
11
19
s.
N.
*2012
6 June
1
18
40
morn.
n
15
45
22
1
37
26
morn.
3
20
45
8
20
2117
11 Dec.
o
57
17
morn.
t
18
56
52
a
34
1
morn.
2
22
50
13
0
N.
*2125
8 Dec.
3
9
20
aft.
t
16
28
33
3
44
31
aft.
2
48
20
11
28
s.
**2247
11 June
12
21
3
noon.
rr
20
13
16
11
51
14
rtlorn.
2
7
52
13
17
S.
N.
*2255
9 June
4
44
36
morn.
n
18
A
1
4
59
10
morn.
3
36
-
6
23
2360
13 Dec.
1
49
49
mom.
t
20
56
9
i
29
32
morn.
2
42
47
11
49
N.
*2368
10 Dec.
o
O'
42
aft.
f
18
27
48
2
38
6
aft.
2
29
22
12
37
S.
s.
**2490
12 June
3
49
15
aft.
n
22
31
58
3
13
59
aft.
1
14
15
14
**2498
10 June
,s
11
42
morn.
n
20
22
37
8
20
59
morn.
3
46
24
4
29
N.
2603
15 Dec.
12
44
56
nicht
t
22
55
36
12
25
55
night
2
56
47
10
50
N.
**2611
13 Dec.
1
1
52
aft.
*
20
27
3S
1.
40
31
aft.
15
20
13
20
s.
N.
**2741
13 June
11
34
39
55
morn.
n
22
40
58
1!
38
39
morn.
3
53
23
2
3^
2846
f6 Dec.
11
43
night
t
24
55
22
11
26
35
night
3
7
24
9
5j
N.
**2854
14 Dec.
12
4
9
noon
f
22
27
45
12
44
21
noon
i
54
10
14
12
s.
N.
**2984
14 June
2
53
2
aft.
n
24
59
1
2
51
53
alt.
3
56
9
0
45
Sect. VIII. Of the Figure, Motion, Sec. of the Earth .
(134 ) The Earth was accounted by some of the ancients to bein the form of an oblong cylinder, by others in the form of adrum, and by others to be flat. The moderns demonstrate it to benearly spherical from the following, among other considerations:
1. All the appearances of the heavens, both at land and at sea, jwe the same as thev would be if the Earth were a globe . Marinerslirst begin to lose sight of the lower parts of objects, and thengradually of the higher parts ; also, persons on shore first discoverthe upper parts of the ma$ts of approaching vessels. This couldnot be the case, if the Earth was a plane; but is very easily ac-counted for, on the supposition of its being a sphere, as will heeasily understood by examining Fig. 6. Plate XXV. 2. In lunar ieclipses the shadow of the Earth falling upon the Moon is alwayscircular, anil a body can be no other than a globe, which in all situ-ations casts a circular shadow. 3. Navigators sailing round the globehave steered their course directly south and west till they came tothe Magellanic sea, andfrom thence to the north and west, till theyre 1 umed to their port from the east; and all the phxuomenawhich should naturally arise from the Earth ’s rotundity, happenedto them. Besides, their method of sailing was also founded upontliis hypothesis, w hich could not have succeeded so happily, if theEarth’had been of any other figure. It is true, the surface of theEarth is not an exact geometrical globe, but tire inequalities are so
inconsiderable, that the highest mountains bear no greater pro-portion to the bulk of the Earth , than a grain of dust does to acommon globe. The figure of the Earth then was reckoned bymathematicians and geographers as perfectly spherical, exceptingthe small inequalities in its surface, of mountains and vallies, tillan accident engaged the attention of Sir Isaac Newton, and Mr.Huygens , who demonstrated from the laws of hydrostatics, anclthe revolution of the Earth about its axis, that its figure was not atrue sphere, but an oblate spheroid, flattened towards the poles.Monsieur Richer, when at the island of Cayenne, about five de-grees distant from the equator, found that his clock, which atParis kept true time, now lost two minutes and twenty-eightseconds every day. Now, though heat will lengthen pendulums,and consequently retard their motion, it is certain the heats ofCayenne were not sufficient to solve this phanomenon, which can(low only from a diminution in the pressure of gravity. For, asthe Earth revolves about its axis, all its parts will endeavour torecede from the axis of motion, and thereby the equatoreal parts,where the motion is quickest, will tend less towards the centre thanthe rest ; their endeavour to tly olf from the axis about which theyrevolve, taking off part of their tendency that way ; so that thoseparts will become lighter than such as are nearer the poles. Thepolar parts, therefore, will press in towards the centre, and raisethe equatoreal parts, till the quantity of matter in the hitter is sofar increased as to compensate for its lightness, and an equilibriumbe restored. On which account, the form which the Earth assumeswill be that of an oblate spheroid, whose shorter axis passes
6 through.