TYC 
TYC 
eighteen degrees below it; after which it 
ceases to be ilhiminated thereby, till it has 
got within as many degrees of the eastern 
side of the horizon ; at which time it begins 
to illuminate the atmosphere again, and in 
appearance to diffuse its light throughout 
the heavens, which continues to increase 
till the sun be up. Hence it is, that during 
that part of the year in which the sun is 
never eighteen degrees below our horizon, 
there is a continued twilight from sun- 
setting to sun-rising. Now that part of the 
year in the latitude of London is, w'hile the 
sun is passing from about the fifth degree 
of Gemini to the twelfth of Cancer; that is, 
from the middfe of May to the middle of 
July. 
As the twilight depends on the quantity 
of matter hi the atmosphere fit to reflect 
the sun’s rays, and also on the height of it 
(for the higher the atmosphere is, the longer 
will it be before the upper parts of it will 
cease to be illuminated), the duration of it 
will be various. For instance, in winter, 
when the air is condensed with cold, and 
the atmosphere upon that account lower, 
the twilight will be shorter; and in summer, 
when the limits of the atmosphere are ex- 
tended by the rarefaction and dilatation of 
the air of which it consists, the duration of 
the twilight will be greater. And for the 
like reason, the morning twilight, the air 
being at that time condensed and con- 
tracted by the cold of the preceding night, 
will be shorter than the evening one, when 
the air is more dilated and expanded. 
The beginning and end of twilight has 
been variously stated, by different ob- 
servers ; but, in our latitude, it may be said 
to begin and end when the sun is about 
eighteen degrees below the horizon : henc'e, 
when refraction is allowed for, the atmos- 
phere must be capable of reflecting sensible 
light at the height of about forty miles. 
The duration of twilight is greater or less, 
as the sun moves more or less obliquely 
with respect to the horizon : hence it is 
shortest near the time of the equinoxes, 
because the equinoxial intersects the hori- 
zon less obliquely than any lesser circle 
parallel to it. Dr. Long has calculated the 
duration of twilight, in different latitudes, 
and for the several different declinations of 
the sun : the result he laid before the public 
in the following table, where the letters c d 
signify that it is then continual day ; c n 
continual night ; and w n, that the twilight 
lasts the whole night : 
TABLE. 
Latitude 
0 
10 
20 
30 
40 
45 
50 
52§ 
55 
GO 
65 
70 
75 
80 
85 
'90 
o 
En 
h. 
m. 
h. 
m. 
h 
m 
b. 
m 
h. 
m. 
h. 
m. 
h. 
m. 
h. 
IT) 
h. 
m 
h. 
m. 
b. 
m. 
h. 
no 
li. 
m 
h. 
m 
b. 
in. 
b. 
in 
lers 
35 
1 
18 
1 
21 
1 
28 
1 
41 
2 
8 
2 
3s; 
w 
u 
w 
1] 
w 
)] 
w 
H 
w 
Q 
c 
d 
c 
d 
C 
d 
C 
d 
(i 
n 
A 
1 
16 
1 
19 
1 
25 
1 
36 
1 
58 
2 
10 
3 
3 
w 
11 
w 
n 
w 
n 
w 
I) 
c 
d 
c 
d 
c 
c 
d 
d 
b 
D! 
1 
13 
1 
15 
1 
20 
1 
28 
43 
1 
55 
2 
12 
2 
25 
2 
41 
3 
55 
w 
n 
w 
u 
w 
1) 
c 
ci 
c 
d 
d 
T 
-rt. 
1 
12 
1 
13 
1 
17 
1 
24 
i 
35 
1 
4^ 
1 
55 
2 
2 
2 
10 
2 
33 
3 
8 
4 
18 
w 
n 
w 
)j 
w 
n 
w 
n 
K 
n 
1 
]3 
14 
1 
18 
24 
1 
35 
1 
43 
51 
o 
0 
2 
8 
2 
27 
2 
56 
8 
41 
5 
17 
32 
w 
n 
vv 
11 
Jvw 
t 
1 
16 
n 
1 
2] 
1 
28 
40 
1 
40 
2 
1 
2 
8 
2 
18 
2 
43 
3 
26 
11 
38 
11 
14 
10 
•32 
8 
38 
n 
W 
1 
18 
10 
I 
23 
30 
1 
43 
1 
53 
2 
6 
2 
15 
2 
26 
2 
5-, 
4 
4 
10 
24 
9 
sej 
7 
4f 
c 
I) 
c 
11 
TYCHONIC system, or hypothesis, an 
order or arrangement of tlie heavenly ho- 
llies, of an intermediate nature between 
the Copernican and Ptolemaic, or partici- 
pating alike of them both. This system 
had its name and original from Tycho Brahe, 
a nobleman of Denmark, who lived in the 
latter part of the last century. The philo- 
sopher, though he approved of the Coper- 
nican system, yet could not reconcile him- 
self to the motion of the earth ; and being, 
on the other hand, convinced the Ptolemaic 
scheme could not be true, he contrived one 
different from either. In this the earth has 
no motion allowed it, but the annual and 
diurnal phenomena are solved by the mo- 
tion of the sun about the earth, as in the 
Ptolemaic scheme ; and those of Mercury 
and Venus are solved by this contrivance, 
though not in the same manner, nor so 
simply and naturally as in the Copernican 
system. The Tychonic system then sup- 
posed the earth in the centre of the world, 
that is, of the firmament of star.*, and also 
of the orbits of the sun and moon ; but at 
the same time it made the sun the centie 
of the planetary motions, viz. of the orbits 
of Mercury, Venus, Mars, Jupiter, and Sa- 
turn. Thus the sun, with all its planets, 
was made to revolve about the earth once 
