ASTRONOMY. 
point of the orbit in which the sun is nearest 
the earth, his apparent diameter is greatest, 
and his motion swiftest ; but when lie is in 
the opposite point, both his diameter and 
the rapidity of his motion are the smallest 
possible. 
To determine the distance of the sun 
from the earth, has always been an interest- 
ing problem to astronomers, and they have 
tried every method which astronomy or 
geometry possesses in order to resolve it. 
The amplest and most natural is that which 
mathematicians employ to measure distant 
terrestrial objects. From the two extre- 
mities of a base whose length is known, the 
angles which the visual rays from the object, 
whose distance is to be measured, make 
with the base, are measured by means of a 
quadrant ; their sum subtracted from 180° 
gives the angle which these rays form at the 
object where they intersect. This angle is 
called the parallax, and when it is once 
known it is easy, by means of trigonometry, 
to ascertain the distance of the object. Let 
A B, in fig. 4, be the given base, and C the 
object whose distance we wish to ascertain. 
The angles CAB and C B A, formed by the 
rays CA and CB with the base, may be 
ascertained by observation ; and their sum 
subtracted from 180° leaves the angle A C B, 
which is the parallax of the object C. It 
gives us the apparent size of the base A B 
as seen from C. When this method is ap- 
plied to the sun, it is necessary to have the 
largest possible base. Let us suppose two 
observers on the same meridian, observing 
at the same instant the meridian altitude of 
the centre of the sun, and his distance from 
the same pole. The difference of the two 
distances observed will be the angle under 
which the line which separates the observers 
will be seen from the centre of the sun. 
The position of the observers gives this line 
in parts of the earth's radius. Hence, it is 
easy to determine, by observation, the an- 
gle at which the semidiameter of the earth 
would be seen from the centre of the sun. 
This angle is the the sun’s parallax. But it 
is too small to be determined with precision 
by that method. We can only conclude 
from it, that the sun’s distance from the 
earth is at least equal to 10,000 diameters 
of the earth. Other methods have been 
discovered for finding the parallax with 
much greater precision. It amounts very 
neai-ly to 8". 8 : hence it follows that the 
distance of the sun from the earth amounts 
to at least 23.405 semidiameters of the earth. 
The sun was long considered, from its 
constant emanation of heat and light, a? an 
immense globe of tire. When viewed 
through a telescope several dark spots are 
visible on its surface, which are of various 
sizes and durations. From the motion of 
these spots the sun has been found to move 
round its axis, and its axis is found to be in- 
clined to the ecliptic. Various opinions have 
been formed respecting these spots; they 
have been considered as opaque islands in 
the liquid igneous matter, and by some as 
pits or cavities in the body of the sun. In 
1788, Mr. King published a Dissertation oil 
the Sun, in which he advanced tiiat. the 
real body of the sun is less than its appa- 
rent diameter ; that we never discern the 
real body of the sun itself, except when we 
behold its spots ; that the sun is inhabited 
as well as our earth, and is not necessarily 
subject to burning heat, and that there is in 
reality no violent elementary heat existing 
in the rays of the sun themselves essentially, 
but that they produce heat only when they 
come into contact with the planetary bodies. 
Several years after this Mr. Hersclie! pub- 
lished his theory of the nature of the sun, 
which is briefly as follows : he considers the 
sun as a most magnificient habitable globe, 
surrounded by a double set of clouds. Those 
which are nearest its opaque body are less 
bright, and more closely connected together 
than those of the upper stratum, which 
form the luminous apparent globe we be- 
hold. This luminous external matter is of 
a phosphoric nature, having several acciden- 
tal openings in it, through which we see 
the sun’s body, or the more opaque clouds 
beneatii. Those openings form the spots 
that we see. 
Mercury. This planet being the nearest 
to the sun, and the least in magnitude, is 
very seldom visible. It never appeal’s 
more than a few degrees from the sun’s 
disc, and is generally lost in the splendor 
of the solar beams. On this account astro- 
nomers have had few opportunities of mak- 
ing accurate observations upon it ; no spots 
have been observed upon it, consequently 
the time of its rotation on its axis is not 
known. Being an inferior planet it con- 
sequently must shew phases like the moon ; 
and it never appears quite full to ns. It is 
seen sometimes passing over the sun’s disc, 
which is called its transit. 
Venus is the brightest and largest to ap- 
pearance of all the planets, and is distin- 
guished from the rest by her superiority of 
lustre. It is generally called the Morning 
or Evening Star, according as it precedes or 
