ASTRONOMY. 
&e. whose content's are equal to one another 
from the properties of the ellipsis. We have 
now given a general idea of the solar system ; 
we shall next describe the bodies that com- 
pose it. 
Of the sun. The sun, as the most conspicuous 
and most important of all the heavenly bodies, 
would naturally claim the first place in the 
attention of astronomers. Accordingly its 
motions were first studied, and they have 
had considerable influence on all the other 
branches of the science. That the sun has 
a motion ofits own, independent of the appa- 
rent dinrnal motion common to all the hea- 
venly bodies, and in a direction contrary to 
that motion, is easily ascertained, by ob- 
serving with care the changes which take 
place in the starry hemisphere during a com- 
plete year. If we note the time at which 
any particular star rises, we shall find that it 
rises somewhat sooner every successive day, 
till at last we lose it altogether in the west. 
But if we note it afterthe interval ofa year, 
we shall find it rising precisely at the same 
hour as at first. Those stars which are 
situated nearly in the track of the sun, and 
which set soon after him, in a few evenings 
lose themselves altogether in his rays, and 
afterwards make their appearance in the 
east before sunrise. The sun then moves 
towards them in a direction contrary to his 
diurnal motion. It was by observations of 
this kind that the ancients ascertained his 
orbit. But at present this is done with 
greater precision, by observing every day 
the height of the sun when it reaches the 
meridian, and the interval of time which 
elapses between his passing the meridian 
and that of the stars. The first of these ob- 
servations gives us the sun’s daily motion 
northward or southward, in the direction of 
the meridian ; and the second gives us his 
motion eastward in the direction of the pa- 
rallels ; and by combining the two together 
we obtain his orbit. The height of the sun 
from the horizon, when it passes the meri- 
dian, on the arch of the meridian between 
the sun and the horizon, is called the sun’s 
altitude. The ancients ascertained the sun’s 
altitude in the following manner : — They 
erected an upright pillar at the south end of 
a meridian line, and when the shadow of it 
exactly coincided with that line, they accu- 
rately measured the shadow’s length, and 
then, knowing the height of the pillar, they 
found by an easy operation in plane trigo- 
nometry the altitude of the sun’g upper 
limb, whence, after allowing for the appa- 
rent semi-diameter, the altitude of the sun’s 
centre was known. But the methods now 
adopted are much more accurate. In a 
known latitude, a large astronomical qua- 
drant, of six, eight, or ten feet radius, is 
fixed truly upon the meridian ; the limb of this 
quadrant is divided into minutes and smal- 
ler subdivisions by means of a vernier, and 
it is furnished with a telescope, having cross 
hairs, &c. turning properly upon the centre. 
By this instrument the altitude of the sun's 
centre is very carefully measured, and the 
proper deductions made. The orbit in 
which the sun appears to move is called the 
ecliptic. It does not coincide with the 
equator, but cuts it, forming with it an angle, 
which in the year 1769 was determined by 
Dr. Maskelyne at 23° 28' 10", or 23°.46944. 
This angle is called the obliquity of the ec- 
liptic. 
It is known that the apparent motion of the 
sun in its orbit is notunifonn. Observations, 
made with precision, have ascertained, that 
the sun moves fastest in a point of his orbit 
situated near the winter solstice, and slowest 
in the opposite point of his orbit near the 
summer solstice. When in the first point, 
the sun moves in 24 hours 1°.01943 ; in the 
second point, he moves only 0°.95319. The 
daily motion of the sun is constantly vary- 
ing in every place ofits orbit between these 
two points. The medium of the two is 
0°.98632, or 59' 11", which is the daily mo- 
tion of the sun about the beginning of Oc- 
tober and April. It has been ascertained, 
that the variation in the angular velocity of 
the sun is very nearly proportional to the 
mean angular distance of it from the point 
of its orbit where its velocity is greatest. 
It is natural to think, that the distance of 
the sun from the earth varies as well as its 
angular velocity. This is demonstrated by 
measuring the apparent diameter of the 
sun. Its diameter increases and diminishes 
in the same manner and at the same time 
with its angular velocity, but in a ratio 
twice as small. In the beginning of Ja- 
nuary his apparent diameter is about 32' 
39", and at the beginning of July it is about 
3T 34 ", or more exactly, according to De 
la Place, 32' 35" = 1955" in the first case, 
and 31' 18"= 1878" in the second. Opti- 
cians have demonstrated, that the distance 
of any body is always reciprocally as its ap- 
rent diameter. The sun must follow the 
same law ; therefore its distance from the 
earth increases in the same proportion that 
its apparent diameter diminishes. In thqt 
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