170 
planet is at V and U, of course it will in those 
positions seem to be stationary. 
When Venus or Mercury is in its superior 
conjunction, or at N, the whole enlightened 
hemisphere is towards the earth, and its 
entire disc is ' visible : as it passes towards 
its inferior conjunction, its enlightened he- 
misphere turns by degrees, from the Earth, 
till at the inferior conjunction M, its enlight- 
ened part is wholly r turned from the Earth, 
and the planet becomes invisible, unless it 
appears on the Sun’s disc ; it is then called a 
transit. 
Of the Earth. The Earth which we in- 
habit is a globular body, as may be proved 
from a variety of circumstances, the chief of 
which we shall here enumerate. It is always 
©bserved by mariners, that as they sail from 
any high objects, such as mountains, steeples, 
&c. they first begin to lose sight of the lower 
part of those objects, and then gradually of 
the higher parts ; also, persons on shore iirst 
discover the upper parts of the masts of ap- 
proaching vessels. This could not be the 
case, if the Earth was a plane ; but is very 
easily accounted for, on the supposition of 
its being a sphere, as will be easily under- 
stood by examining tig. 5. Various navi- 
gators have also sailed completely round the 
earth, by continuing in the same direction, 
at last coming to the same place from which 
they set out. 
The Earth, however, is not a perfect 
sphere, but a spheroid, having its equatorial 
diameter longer than the polar diameter, or 
axis. It is consequently flattest at the poles, 
and more protuberant at the equator. The 
diameter at the equator is 7977 English 
miles ; that at the poles is 7940 miles. The 
surface of the Earth is much diversified with 
mountains and valleys, land and water. The 
highest mountains in it, are the Andes in 
South America, some of which are about 
four miles in perpendicular altitude. About 
two-thirds of the globe are covered with 
water. 
In consequence of the Earth’s being a 
globe, people standing upon opposite sides 
of it, must have their feet towards each 
other. When in this situation they are 
called antipodes to each other. Hence it 
appears that there is no real up or dozen’, 
for what is up to one country', is down 
to another. It must seem strange to those 
who are ignorant of the shape of the earth, 
to suppose that, if we could bore a hole 
downwards, deep enough, we should come 
to the other side of the world, where we 
should find a surface and sky like our 
own ; yet if we reflect a moment, we shall 
perceive that this is perfectly true. As we 
are preserved in our situations by the power 
of attraction, which draws us towards the 
centre of the earth, we call that direction 
down, which tends to the centre, and the 
contrary. 
We mentioned before, that the Earth has 
two motions ; the one a diurnal motion, 
round its own axis, in twenty-four hours ; the 
other an annual motion, round the Sun, in 
365 days, 6 hours, 56 minutes, 4 seconds. 
It is the former which causes light and dark- 
ness, day and night; for when one side of 
tire Earth is turned towards the Sun, it re- 
ceives his rays, and is illuminated, causing 
day ; on the contrary, when one side of the 
Earth is turned from the Sun, we are in dark- 
ASTRONOMY. 
ness, and then we have night. By the di- 
urnal revolution of the Earth round its own 
axis from west to east, the heavenly bodies’ 
appear to us to revolve in the same time 
from east to west. And since the Sun and 
fixed stars are bodies immensely larger than 
the Earth, and at almost inconceivably great 
distances from it, we see upon how much 
more simple principles the alternate succes- 
sion of day and night is effected by the revo- 
lution of the earth about its axis, than by 
supposing the Earth fixed, and the Sun and 
stars whirled round it, with an indefinite 
velocity. 
The natural days are not equal to one 
another ; for a natural day is the time in 
which the Earth performs one revolution 
round its axis, and such a portion of the 
second revolution, as is equal to the space 
which the Sun has apparently travelled 
that day r ; but these spaces are unequal, 
therefore, the additional portion of the 
second revolution, will be sometimes greater 
and sometimes less, and consequently the 
times in which the natural days are com- 
pleted will be unequal. Hence arises the 
difference between a sun-dial and a well regu- 
lated clock, as measures of time ; the former 
measuring the length of the natural day, the 
latter dividing time into equal portions of 
twelve hours each : the clock will be before 
the dial, when the natural day is more than 
twenty-four hours ; and after it, when the 
natural day' is less than twenty-four hours ; 
and they will be together, only when the 
natural day is exactly twenty-four hours. 
The equation of time is the difference 
between the mean length of the natural day, 
or twenty-four hours, and the length of any 
single day measured by' the sun’s apparent 
motion, or between mean and apparent time. 
And the hour of the day by apparent time 
being known, in order to determine what is 
then true time, the equation is to be added 
to apparent time, if the day by the clock is 
shorter than the day by the dial : and the 
equation is to be subtracted from the ap- 
parent time when the day by the clock is 
longer than the day by the sun-dial. If the 
natural day is twenty-four hours three mi- 
nutes long, the day by the clock being 
twenty-four hours in length, it will be 12 by 
a good clock 3 minutes before it is 12 by 
the dial ; in this case mean time precedes 
apparent. 
The difference between mean and ap- 
parent time, depends upon two causes: (1) 
the obliquity of the ecliptic with respect to 
the equator : and (2) the unequal motion of 
the earth in an elliptical orbit. The ob- 
liquity of the ecliptic to the equator would 
make the sun and clocks agree on four days 
of the year, viz. when the sun enters Aries, 
Cancer, Libra, and Capricorn. But the 
other cause which arises from his unequal 
motion in his orbit would make the sun and 
clocks agree only twice a year, that is, when 
he is in perigee, and apogee ; consequently', 
when these two points fall in the beginnings 
of Cancer and Capricorn, or of Aries and 
Libra, they will concur in making the sun 
and clocks agree in those points. But the 
apogee, at present, is in the ninth degree of 
Cancer, and the perigee in the ninth degree 
of Capricorn ; and, therefore, the sun and 
clocks cannot be equal at the beginnings of 
these signs, nor, indeed, at any time of the 
year, except wh?n the swiftness and slow- 
ness of the equation resulting from one of 
the causes, just balances the slowness or swift- 
ness arising from the other ; which happens 
about the 15th of April, the 15th of June, 
the 31st of August, and the 24th of De- 
cember; at all other times the sun is too 
fast or too slow for equal time by' a certain 
number of minutes and seconds, which at 
the greatest is 16 minutes, 14 seconds, and 
happens about the first of November : every 
other day' throughout the year having a cer- 
tain quantity of this difference belonging to 
it, which, however, is not exactly the same 
every year, but only' every fourth year, for j 
which reason, it is necessary, where great 
accuracy is required, to have four tables of ; 
this equation, viz. one for each of the four 
years in the period of leap year. . The fol- 1 
lowing concise table, adapted to the second I 
year after leap year, will always be found 
within about a minute of the truth, and there- j 
fore sufficiently accurate for common clocks 
and watches. 
. TABLE, 
FOR THE EQUATION OF TIME. 
Days. 
Months. 
Equation J 
in | 
Minutes. 
Days. 
Months. 
Equation 
in 
Minutes. 
Days. 
Months. 
Equation 1 
in 
Minutes. 
Jan. 1 
4 + 
Apr. 24 
2— 
Sept.27 
9— 
3 
5 
30 
3 
SO 
10 
5 
6 
May 13 
4 
Oct. 3 
11 
7 
7 
29 
3 
6 
12 
9 
8 
June 5 
2 
10 
13 
12 
9 
10 
1 
14 
14 
15 
10 
15 
0 
19 
15 
18 
11 
* 
— 
27 
16 
21 
12 
20 
1+ 
Nov.15 
15 
25 
13 
25 
2 
20 
14 
SI 
14 
39 
3 
24 
13 
Feb. 10 
15 
July 5 
4 
27 
12 
21 
14 
11 
5 
30 
11 
27 
13 
28 
6 
Dec. 2 
10 
Mar. 4 
12 
Aug. 9 
5 
5 
9 
8 
11 
15 
4 
7 
8 
12 
10 
20 
3 
9 
7 
15 
9 
24 
2 
11 
6 
19 
8 
28 
1 
13 
5 ; 
22 
7 
31 
0 
16 
4 
25 
6 
* 
— 
18 
3 
28 
5 
Sept. 3 
1 — 
20 
2 
Apr. 1 
4 
6 
2 
22 
1 
4 
3 
9 
3 
24 
0 
7 
2 
12 
4 
* 
— 
11 
1 
15 
5 
26 
1+- 
15 
0 
18 
6 
28 
2 
* 
— 
21 
7 
30 
3 ; 
19 
1 — 
24 
8 
• 4 
Those columns that are marked -j-> shew that 
the clock or watch is, or ought to be, before 
the sun ; and those marked — , that it is slower* 
To regulate, a clock or watch on the 1st of 
January, the moment the sun-dial is 12, the. 
clock or watch must be put 4 minutes after. 
On the 13th of May, when the dial is 12, the 
clock to be right must want 4. minutes of that- 
hour. See Ferguson’s- Astronomy, ch. 13. Phil 
Trans, vol. 54. 
Twilight is owing to the refraction of the 
rays of light by our atmosphere, through 
which they pass ; and which, by bending 
them, occasions some to arrive at a part of 
the earth that could not receive any direct 
rays from the sun, or so as to bring him into 
sight, every clear day, before he rises in. the 
I 
