AST 
from the year 163,5 to 1645, who firft obferved the tranfit 
of Venus over the fun in the year 1639. They were fol¬ 
lowed by Flamffeed, Caflini the father and Ion, Halley, 
de la Hire, Roemer, and Kirchius. The obfervations of 
the celebrated Dr. Bradley have not yet been publilhed, 
though long expelled. We have alfo now publilhed, from 
time to time, the accurate observations of the prelent Bri- 
tifit Aftronomer Ro : as, \» thole of the French and 
other obfervatories/ with the ..jblervations of many inge¬ 
nious private altronomers, which are to be found in the 
Tranfactions and Memoirs of the various Philofophical 
Societies. There have been all'o obfervations of many 
other eminent allronomers; as, Galileo, Huygens, and 
our countryman Harriot, whole very interelting obferva¬ 
tions have lately been brought to light by the earl of 
Egremont, and count Bruhl, by whole means they may 
come to be publilhed. Other publications of celeftial ob¬ 
fervations, are thole of Callini, La Caille, Monnier, &c. 
Astronomical Place of a planet or flar, is the lon¬ 
gitude or place in the ecliptic, reckoned from the begin¬ 
ning of Aries, according to the natural order of the figns. 
Astronomical Quadrant, is an inltrument framed 
and fitted with telefcopes, &c. to take obfervations of the 
moon or flars. 
Astronomical Sector, a very ufeful mathematical 
inltrument, made by tiie late ingenious Mr. Graham ; a 
defcription of w hich is given in the courfe of the article 
Astronomy. 
ASTRONO'MICALLY, adv. [from ajlronomical. ] In 
an alfronomical manner. 
ASTRONO'M 1 CALS, are fexagefimal fractions, fo call¬ 
ed becaufe anciently they were wholly tiled in alfronomical 
calculations, 
ASTRO'NOMY, [ [ AJlronomie , Fr. Aflrovomia, Lat. of 
esrpovor'.ia, of arpoi'fR (tar, and toy. c;, Gr. the law, rule, &c. ] 
Is that branch of natural philolophy which treats of the 
heavenly bodies, their motions, periods, eclipfes, magni¬ 
tudes, diftances, and other phenomena. The determi¬ 
nation of their magnitudes, diltances, and the orbits which 
they deferibe, is called plane or pure allrononty ; and the 
inveftigation of the caufes of their motions, is called phy- 
Jical aftrononiy. The former is determined from obferva¬ 
tions on their apparent magnitudes and motions ; and the 
latter from analogy, by applying thole principles and laws 
of motion by which bodies on and near the earth are go¬ 
verned, to the other bodies in the fyftem. But, before we 
enter upon the general view or hiftorical detail of the rile 
and progrefs of agronomy, it w ill be proper to give a con- 
cife arrangement and explanation of thole various aftro- 
nomical terms, which conlfitute the fundamental'principles 
of the fcience ; and which we have collected from the ela¬ 
borate work of the Rev. S. Vince, Plumian Profelfor of 
A ft fo no my in the University of Cambridge. 
DEFINITIONS. 
A great circle of a fphere is that whofe plane palTes 
through its center ; and a / mall circle is that whofe plane 
does not pafs through its center. 
A diameter of a fphere perpendicular to any great cir¬ 
cle is called the axis of that circle ; and the extremities of 
the diameter are called it s. poles. Hence the pole of a great 
circle is 90 0 from every point of it upon the furface of the 
fphere ; but, as the axis is perpendicular to the circle when 
it is perpendicular to any two radii, a point on the furface 
©f a Sphere, 90°- diftant from any two points of a great 
circle, will be the pole. 
All angular diftancss on the furface of a fphere, to an 
«ye at the.center, are meafured by the arcs of great circles; 
for they, being arcs to equal radii, will be as the angles. 
Hence all triangles formed upon the furface of a fphere, 
for the foluticn of fphericai problems, mull be formed by 
the arcs of great circles. And all great circles mult bifeift 
<tach other; for.pa (Ting through the center of the fphere 
their common fection mult be a diameter, which bifebts 
all circles. 
Vo l, II. No. 74, 
AST 329 
Secondaries to a great circle are great circles which pafs 
through its poles. Hence fecondaries mult be perpendi¬ 
cular to their great circle ; for, if one line be perpendicu¬ 
lar to a plane, any plane palling through that line will alfo 
be perpendicular to it; therefore as the axis of the great cir¬ 
cle is perpendicular to it, and is the common diameter to 
all the fecondaries, they mult all be perpendicular to the 
great circle. 
The axis of the earth is that diameter about which it 
performs its diurnal motion ; and the extremities of this 
diameter are called its poles. 
The terrejlrial equator is a great circle of the earth per¬ 
pendicular to its axis. Hence the axis and poles of the 
earth are the axis and poles of its equator. That half of 
the earth which lies on the fide of the equator which we 
inhabit is called the northern hemijphere , and the other the 
fouthe.rn ; and the poles are relpebtively called the north and 
Jout'a poles. 
The latitude of a place on the earth’s furface is its an¬ 
gular diltance from the equator, meafured upon a fecon¬ 
dary to it. Thefe fecondaries to the equator are called 
meridians. The longitude of a place on the earth’s furface 
is an arc of the equator intercepted between the meridian 
palling through the place, and another, called the firlt me¬ 
ridian, palling through that place from which you begin 
to meafure. 
If the plane of ihe terrejlrial equator be produced to the 
fphere of the fixed liars, it marks out a circle called the 
cclrjiial equator ; and if the axis of the earth be produced 
in like manner, the points in the heavens to which it is 
produced are called pol j, being the poles of the celeltial 
equator. The (tar nearelt to each pole is called the pole 
ltar. Secondaries to the celeltial equator are called circles 
of declination ; of thefe, twenty-four which divide the 
equator into equal parts, each containing 15 0 , are called 
hour circles. Small circles parallel to the celeltial equator 
are called parallels of declination. 
The jenfiblc horizon is that circle in the heavens whofe 
plane touches the earth at the Ij ectator. The rational ho-* 
rizon is a great circle in the heavens, palling through the 
earth’s center, parallel to the fenfible horizon; Almacantcr 
is a (mall circle, parallel to the horizon. 
If the radius of the earth to the place where the fpec- 
tator (lands be produced both ways to the heavens, that' 
point vertical to him is called the zenith , and the oppolite 
point the nadir. Hence the zenith and nadir are the poles 
of the rational horizon ; for the radius produced, being 
perpendicular to the fenfible, mult alfo be perpendicular to 
the rational, horizon. 
Secondaries to the horizon are called vertical circles, be¬ 
caufe they are perpendicular to the horizon ; on thefe cir 
cles therefore the altitude of a- heavenly body is meafured. 
A fecondary common to the celeltial equator and the ho¬ 
rizon of any place, and which therefore palfes through the 
poles of each, is the celeltial meridian of that place. Hence 
the plane of the celeltial meridian of any place coincides 
with the plane of tine terreffrial meridian of the fame place. 
That direction w hich pafies through the north pole is called 
north, and the oppolite direction is called Jouth. Hence 
the meridian mult cut the horizon in the north and fonth 
points ; and hence the meridian of any place divides the 
heavens into two hemilpheres lying to the ealt and well; 
that lying to the eaft is called the cajlern hemifphere, and 
the other lying to the welt is called the wejlern hemifphere. 
■ The vertical circle which cuts the meridian of any place 
at right angles is called the prime vertical ; and the points 
where it cuts the horizon arc called the cq/Tand wjl points. 
Hence the ealt and weft points are 90 0 diftant from the 
north and (outh. Thefe four are called the cardinal points. 
The azimuth of an heavenly body is its diltance on the 
horizon, wheiv referred to it by a fecondary, from the 
north or fonth points. The amplitude is its diltance from 
the ealt or welt point. 
The ecliptic is that great circle in tire heavens which the 
fun appears to deferibe in the courfc of a year. The eclip- 
4.0 lift. 
