\Q6 
wards the year 1530, Copernicus, with a view 
0i obviating the inconveniences of the imagi- 
nary systems that preceded him, commenced 
at first by admitting the diurnal motion of 
t.ie Earth, or her motion round her own axis ; 
w hich rendered useless that prodigious cele- 
! ity in the motions of the heavenly bodies, of 
V- hich we have just spoken, and by these 
means simplified the system. This motion 
being once admitted, it was no violent step 
to admit a second motion of the Earth in the 
cciiptic. Ihese. two motions explain, with 
the utmost facility, the phenomena of the 
stations and motion of the planets. Accord- 
ing to Copernicus, then, the Sun is the cen- 
tre of our planetary system, and the pla- 
nets turnabout him in the order following: 
Mercury, Venus, the Earth, Mars, Jupiter, 
Saturn (to which we may add the Herschel), 
at a distance from the .Sun nearly as the 
numbers 4, 7, 10, 15, 52, 95, 191. The 
Moon also he supposed to be carried round 
the Earth, in an orbit which goes along w ith 
the Earth in her annual motion round the 
Sun. In like manner about Jupiter, Saturn, 
and the Herschel, are the four satellites of 
the lirst, the seven satellites of the second, 
and the six satellites ot the third. Although 
the celestial phenomena explain themselves 
with the greatest facility according to the sys- 
tem of Copernicus, and though observation 
and reason are equally favourable to it, yet 
he found in his time an able astronomer who 
rejected the evidence of his discoveries. 
1 vcho-Brahe, from the experiment that a 
stone thrown from a high tower fell at its feet, 
argued that the Earth must be without motion; 
never reflecting that the Earth, in that case, 
is like a vessel in full sail ; where if a stone is 
thrown from the mast, it will fall at the foot 
ot that mast, provided the motion of the ves- 
sel was neither accelerated nor retarded. Ty- 
cho-Brahe, therefore, invented a system be- 
tween that of Ptolemy and that of Copernicus. 
He supposed that the Earth was at rest, and 
that the other planets turn round the Sun, 
turning also with him round the Earth in 
twenty four-hours. It was towards the end 
ot the sixteenth century that he proposed 
his system. He placed the Earth immoveable 
as the centre, and made the Moon turn round 
her, the Sun also, and the fixed stars : the 
planets, viz. Mercury, Venus, Mars, Jupiter, 
and Saturn, turning round the Sun, in orbits 
which are carried with him in his revolution 
round the Earth. 
Celestial Phenomena . — Upon examining 
the heavens, the first and most obvious phe- 
nomenon that presents itself tp observation, 
is the apparent diurnal motion of the sun, 
moon, and stars, or that by which they are 
seen to rise and set once in twenty -four 
hours. 
If, to consider more attentively the cir- 
cumstances of this diurnal motion, w r e place 
ourselves in an elevated situation, we shall 
perceive a circle terminating our view on all 
sides, by the apparent meeting of the earth 
and heavens. 'Phis circle is called the hori- 
zon : it divides the heavens into two parts ; 
that which is above the horizon only is vi- 
sible ; and this appears to us like a 'concave 
hemisphere, which we call the sky, in w hich 
we see the heavenly bodies move. The sky 
is not a real substance ; its blue colour is 
only owing to the refraction of the rays of 
light which pass through it 
ASTRONOMY. 
On considering with attention for one or 
more nights the motions of the stars, we find 
each star describing a circle in about twenty- 
four hours. 1 hose stars that appear north- 
ward describe smaller circles than those that 
are more to the south. If we look towards 
the south, we observe some stars just appear- 
ing abgve the horizon, grazing this circle, 
but not rising above it, and then vanishing ; 
others a little farther from the south, rise 
above the horizon, making a small arc, and 
then go down ; w hile some again describe a 
larger arc, and take a longer time in setting. 
It we now turn lo the north, we shall find 
that some just skim the horizon, mount to 
the top of the heavens, and then descend, 
and again touch the horizon, and mount 
without ever disappearing. Others, that are 
higher, describe complete circles in the sky, 
without coming to the horizon; and these 
circles diminish, till at last we arrive at a star 
that scarcely seems to move from the point 
where it is stationed, the rest wheeling round 
it. 
It may be easily conceived, that as there 
is a hemisphere above, there is also another 
beneath, though invisible ; and that, of course, | 
the horizon is a great circle of the sphere, 
dividing the concave heavens into two parts, 
the visible above, and the invisible below. 
The general appearance, therefore, of the 
starry heavens, is that of a vast concave 
sphere turning round two fixed points diametri- 
cally opposite to each other ; the one in the 
northern hemisphere visible to us ; and the 
other in the southern hemisphere. i 
r i he fixed points round which this sphere 1 
is supposed to turn, are the poles, and a line j 
drawn from one to the other is called the ' 
axis of the sphere ; and round this line the j 
heavens seem to turn every day. 
To understand this more clearly, we must 1 
have recourse to a figure, or diagram. Let 
H O (see Plate, Astronomy, fig. 1) represent j 
the circle of the horizon, seen edgeways, 
when it will appear as a straight line ; let 
HP I* O R Q be the complete sphere of the 
heavens, of which we shall suppose H P E O j 
to be the visible hemisphere, and II Q R O 
the invisible hemisphere : then P will be the 
pole, or fixed point, among the stars visible 
to us, round which they alt appear to turn, 
and R will be the opposite pole, or fixed 
point, in the sphere ; a line from P to R will be 
the axis of the sphere. If through the centre 
of the sphere C, there is drawn a line Q E, 
it will represent the edge of a great circle, at 
equal distances from both poles, and at right 
angles to the axis, called the equator, be- 
cause it divides the heavens into two equal 
parts. 
It PI O be the horizon, the highest point, 
or that immediately over our heads, as M, 
is called the zenith ; and the opposite point 
in the sphere, or lowest point N, is called 
the nadir. 
1 he rising and setting of the sun are the 
two most remarkable circumstances to be 
observed in the heavens. Pie rises in the 
east, mounts to the highest point in the arch 
which he describes, and descends in the west. 
1 he highest point to which he reaches, is 
naturally called the mid-day point. If a 
great circle is traced through this point and 
the zenith, it is called the meridian of the 
place ; and all the stars must cross this circle, 
or meridian, twice in the twenty-four hours ; 
but those that go below the horizon are seen 
only to cross it once, because when they 
cross it a second time they are invisible. 
Three great circles are now established in 
the heavens ; the horizon, the equator, and 
the meridian. The first determines the ris- 
ing and setting of the heavenly bodies ; and 
also the altitude of any of them, at anv time 
ot their course. For this purpose we must 
suppose another great circle lo pass through 
the star and the zenith ; it will consequently 
be perpendicular to the horizon. This is 
called a vertical circle, and upon this circle 
we reckon the number of degrees which the 
star is distant from the horizon. The quad- 
rant is an instrument for measuring the num- 
ber of degrees of altitude which any body 
has. 
The three great circles already mentioned 
form the basis of all observations upon the 
heavenly bodies, and to them all their situa- 
tions must be referred. It is necessary, 
therefore, to determine the relative situations 
ot these circles. If the polar star had been 
accurately at the pole of the heavens, no- 
thing more would be necessary, in order to 
obtain the altitude of the pole, than to take 
the altitude of this star; but this star is situ- 
ated two degrees distant from the pole ; tw6 
degrees must therefore be added to this alti- 
tude, to find that of the pole. 
The elevation of the pole being discover- 
ed, it is easy to find that of the equator. 
Thus, in the diagram (fig. 1.), II M O, or 
the visible part ot the heavens, contains 180 
degrees ; but it is 90 degrees from the pole 
P, to E the equator. If we take away P E 
from the semi-circle II M O, there remains 
90 degrees for the other two arcs ; or, in 
other words, the elevation of the pole and 
the equator, are together equal to 90 de- 
grees ; so that the one being known, and sub- 
tracted from 90 degrees, "it will give the 
other; therefore, the elevation of the pole at 
any place, is the complement of the elevation 
ot the equator, or what that elevation wants 
of 90 degrees. Ilence it follows, that the 
elevation of the equator is equal to the dis- 
tance from the pole to the zenith ; for the 
elevation of the equator is the difference be- 
tween that of the pole and 90 degrees : the 
same elevation subtracted from 90 degrees 
gives its distance from the zenith. A little 
attention will soon convince us that the sun 
does not always rise at the same point of th6 
heavens. Thus, if we commence our obser- 
vations on the sun, for instance in the be*- 
ginning of March, we shall find him appear 
to rise more to the northward every day, to 
continue longer above the horizon, and to be 
more vertical or higher at mid-day. This 
continues till towards the end of June, when 
he moves backward in the same manner, and 
continues this retrograde motion till near the 
end of December, when he begins to move 
forward, and so on. It is from" this change 
in the sun’s place, and from his height 
being so much • greater in summer than in 
winter, that the different length of the 
days and nights, and the vicissitudes of sea- 
sons, are owing. We cannot observe the 
sun’s motion among the fixed stars, because 
he darkens the heavens by his splendour, and 
effaces the feeble light oft hose stars that are 
in his neighbourhood ; hut we can observe the 
instant of his coming to the meridian, and 
his meridional altitude ; we can also compute 
