Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 3, ASS-BOO

A S T R O N O xM Y.
Part II.
22
Hi (lory, vations are equally furprifing ; of which we (hall only liarly calculated to infpire an ardent defire of feeing fo Hiftory.
* v-~- ■ fay with Dr Prieftley*, “Mr Herfchel’s late difeo- great a feene a little more unfolded. Such difeoveries—
* Exper veries in and beyond the bounds of the folar fyftem, as thefe give us a higher idea of the value of our be-
vol vi ^ 8reat views that he has given of the arrangement ing, by raifing our ideas of the fyftem of uhich we are
I>rek • of the liars, their revolutions, and thofe of the im- a part ; and with this an earneft wifh for the continu-
menfe fyflems into which they are formed, are pecu- ance of it.”
PART II. OF THE APPARENT MOTIONS OF THE HEAVENLY BODIES.
WHEN we call our eyes up towards the heavens, we
perceive a vaft hollow hemifphere at an unknown di-
ftance, of which our eyes feem to conflitute the centre.
'The earth ilretches at our feet like an immenfe plain,
and at a certain diftance appears to meet and to bound
37 the heavenly hemifphere. Now the circle all around,
ance of the '"'here the earth and the heavens feem to meet and touch
heavens. each other, is called the horizon. We can fcarcely avoid
fuppoling, that befides the hemifphere which we per¬
ceive, there is another, exactly fimilar, concealed from
our viewr by the earth, and that the earth, therefore,
is fomehow or other fufpended in the middle of this
heavenly fphere, with all its inhabitants. A little ob-
fervation turns this fufpicion into certainty. For in a
clear evening the heavenly hemifphere is feen fludded
with liars, and its appearance is changing every inftant.
New liars are continually rifing in the call, while others
in the mean time are fetting in the wrell. Thofe liars,
that, towards the beginning of the evening, were juft
feen above the eadern horizon, late at night are feen
in the middle of the Harry hemifphere, and may be
traced moving gradually weft ward, till at laft they fink
altogether under the horizon. If we look to the north,
we foon perceive that many liars in that quarter never
fet at all, but move round and round, deferibing a com¬
plete circle in 24 hours. Thefe liars deferibe their
circles round a fixed point in the heavens; and the
circles are the fmaller, the nearer the liar is to the fix¬
ed point. This fixed point is called the north pole.
There mult be a fimilar fixed point in the fouthern he¬
mifphere, called the foulh pole. Thus the heavenly
fphere appears to turn round two fixed points, called
the poles, once every 24 hours. The imaginary line
which joins the points is called the axis of the world.
In order to have precife notions of the motions of
the heavenly bodies, it is necefiary to be able to affign
precifely the place in which they are. This is done
by means of feveral imaginary lines, or rather circles,
fuppofed deferibed upon the furface of the fphere $ and
thefe circles, as is ufual with mathematicians, are di¬
vided into 360 equal parts called degrees. Every
degree is divided into 60 minutes j every minute into
60 feconds, and fo on. That great circle of the fphere,
which is perpendicular to the axis of the world, and
of courfe 90° diftant from either pole, is called the
equator. The fmaller circles, which the ftars deferibe
in conftquence of their diurnal motions, are called
parallels, becaufe they are obvioufly parallel to the e-
quator.
The equator divides the heavenly fphere into two
equal parts, the north and the fouth j but to be able to
affign the pofition of the ftars, it is neceflary to have a-
nother circle, palling through the poles, and cutting
the equator perpendicularly. This circle is called a
meridian. It is fuppofed, not only to pafs through the
poles, but to pafs aifo through the point diredlly over
the head of the obferver, and the point of the fphere
exaftly oppofite to that. The firft of thefe points is
called the %enith, the fecond is called the nadir.
The meridian divides the circles deferibed by the
ftars into two equal parts ; and when they reach it they
are either at their greateft height above the horizon,
or they are at their leaft height. The fituation of the
pole is eafily determined ; for it is precifely half way
between the greateft and leaft height of thofe flars
which never fet. When we advance towards the north
We perceive that the north pole does not remain fta-
tionary, but rifes towards the zenith, nearly in propor¬
tion to the fpace w'e pafs over. On the other hand it
finks juft as much when we travel towards the foulb.
Hence wre learn that the furfaee of the earth is not plane,
as one would at firft: fuppofe, but curved.
All the heavenly bodies appear to deferibe a com¬
plete circle round the earth in 24 hours. But be¬
fides thefe motions which are common to them all,
there are feveral of them which poffefs motions pecu¬
liar to themfelves. The fun, the moil brilliant of all
the heavenly bodies, is obvioufly much farther to the
fouth during winter than during fummer. He does
not, therefore, keep the fame flation in the heavens, nor
deferibe the fame circle every day. The moon not
only changes her form, diminifhes, and increafes ; but
if we obferve the ftars, near which fhe is fituated one
evening, the next evening we fhall find her confider-
ably to the eaftward of them ; and every day {he re¬
moves to a ftill greater diftance, till in a month, fire
makes a complete tour of the heavens, and approaches
them from the weft. There are eight other ftars, be¬
fides, which are continually changing their place ; fome-
times we obferve them moving to the weftward, fome-
times to the eaftward, and fometimes they appear fta-
tionary for a confiderable time. Thefe ftars are called
planets. There are other bodies which appear only
occafionally, move for fome time with immenfe celerity,
and afterwards vanifh. The bodies are called comets.
But the greater number of the heavenly bodies always
retain nearly the fame relative diftance from each o-
ther, and are therefore called fixed fars. It will be
neceflary for us to confider the nature and apparent
motions of all thefe bodies. We fhall, therefore, di- 38
vide this firft part of our treatife, into the following Arrangc.
heads : metu.
1. Of the Sun. 4. Of the Comets.
2. Of the Moon. 5. Of the Fixed Stars.
3. Of the Planets. 6. Of the figure of the Earth.
Thefe topics fhall be the fubje&s of the following
chapters.
Chap,