ASTRONOMY. 
even with one another, at twelve ; they both turn round 
the fame way, but the minute-hand turns round in a ftiort- 
er time than the hour-hand; when the minute-hand has 
completed one rotation, and is come round to twelve, the 
hour-hand will be before it, or will be at one ; fo that the 
minute-hand mud move more than once round, in order 
to overtake the hour-hand, and be even with it again. 
As this fubjeCt is of fome importance, we fhall endea¬ 
vour to render it more clear, by placing it in a different 
point of view. The diameter of the Earth’s orbit is but 
a phyfical point in proportion to the diftance of the Ears; 
for which reafon, and the Earth’s uniform-motion round 
its axis, any given meridian will revolve from any (far to 
the fame ftar again, in every abfolute turn of the Earth 
upon its axis, without the lead perceptible difference of 
time being (hewn by a clock which goes exactly true. If 
the Earth had only a diurnal, without an annual, motion, 
any given meridian would revolve from the Sun to the -Sun 
again in the fame quantity of time as from any ftar to the 
fame far again; becaufe the Sun would never change his 
place with refpeCt to the ftars. But as the Earth advan¬ 
ces almoft a degree eaftward in its orbit, in the time that 
it turns eaftward round its axis, whatever ftar paffes over 
the meridian on any day with the Sun, will pafs over the 
fame meridian on the next day, when the Sun is almoff a 
degree fhort of it, that is, 3' 56" fooner. If the year con¬ 
tained only 360 days, the Sun’s apparent place, fo far as 
his motion is equable, would change a degree every day, 
and then the fidereal days would be juft four minutes fhort- 
er than the folar. 
Let ABCDEFGH, fig. 4, in the fame Plate, be the 
Earth’s orbit, in which it goes round the Sun every year, 
according to the order of the letters, that is, from weft to 
eaft, and turns round its axis the fame way, from the Sun 
to the Sun again in every twenty-four hours. Let S be 
the Sun, and R a fixed fiar conceived to be at an immenfe 
diftance, fo that the diameter GC of the Earth’s orbit 
beats no fenlible proportion to that diftance; Not?z, the 
Earth in different points of its orbit. Let Nut be any par¬ 
ticular meridian of the Earth, and N a given point, or 
-place, lying under that meridian. When the Earth is at 
A, the Sun S hides the ftar R, which would always be 
hid if the Earth never moved from A ; and confequently, 
as the Earth turns round its axis, the point N would al¬ 
ways come round to the Sun and the ftar at the fame time. 
But, when the Earth has advanced through the eighth 
part of its orbit, or from A to B, its motion round its axis 
will bring the point N an eighth part of a day, or three 
hours, fooner to the ftar than to the Sun. For the ftar 
will come to the meridian in the fame time as though the 
-Earth had continued in its former fituation at A, but the 
-point N muff revolve from N to N, before it can have the 
Sun upon its meridian. The arc N n being therefore the 
fame part of a whole circle as the arc AB, it is plain that 
any ftar which comes to the meridian at noon, with the 
Sun, when the Earth is at A, will come to it at nine o’clock 
in the forenoon, when the Earth is at B. When the 
Earth has paffed from A to C, one-fourth part of its or¬ 
bit, the point N will have the ftar upon its meridian, or 
at fix in the morning, fix hours fooner than it comes round 
to the Sun: but the point N muff revolve fix hours more 
-before it has mid-day by the Sun: for now the angle ASD 
is a right angle, and fo is ND« ; i. e. the Earth has advanced 
•90°on its axis, to carry the point N fromthertartotheSun; 
for the ftar always comes to the meridian when Not is pa¬ 
rallel to RSA ; becaufe DS is but a point in refpedt to 
RS. When the Earth is at D, the ftar comes to the me¬ 
ridian at three in the morning at li, the Earth having gone 
half round its orbit; N points to the ftar at midnight, it 
being then directly oppofite to the Sun; and, therefore, 
by the Earth’s diurnal motion, the ftar comes to the me¬ 
ridian twelve hours before the Sun, and then goes on, till 
at A it comes to the meridian with the Sun again. 
Thus it is plain, that one abfolute revolution of the 
Earth on its axis (which is always completed when any 
particular ftar comes to be parallel to its fituation at any 
time of the day before) never brings the fame meridian 
round from the Sun to the Sun again; but that the Earth 
requires as much more than one turn on its axis, to finifh 
a natural day, as it has gone forward in that time, which, 
at a mean ftate, is a 365th part of a circle. 
From hence we obtain a method of knowing by the ftars 
whether a clock goes true or not: for, if through a frnall 
hole in a window-ftiutter, or in a thin plate of metal fixed 
to a window, we obferve at what time any ftar difappears 
behind a chimney, or corner of a houfe, at a little diftance; 
and, if the fame ftar difappears the next night 3' 56" foon¬ 
er, by the clock, and on the fecond 7' 52" fooner, the third 
night n' 48" fooner, and fo on every night, it is an infal¬ 
lible fign that the machine goes true; ctherwife it does 
not, and muff be regulated accordingly. This method 
,may be depended on to nearly half a fecond. 
The fum of what has been ftated amounts to this: that 
the fidereal or ajlral year, is the time in which the Sun 
completes its revolution amongft the fixed ftars, or in the 
ecliptic; andconfiftsof 365 da. 6h. 9'. 17". And that the 
folar or tropical year, is the time the Sun takes in palling 
-through the zodiac, from one tropic or equinoctial point 
till it comes to the fame again, which it performs in 365 da. 
5I1. 48'. 48". This is the true or natural year, and on. 
-which the return of the feafons depends. 
Of the Anoma.listic Year. 
The year, called the anomaliflic year, is fometimes ufed 
by aftronomers, and is the time from the Sun’s leaving its 
apogee till it.returns to it. Now, the motion of the Sun’s 
apogee is 1' 2" every year, in longitude, or in refpeCt to 
the equinox, according to M. de la Lande, therefore 1' 2" 
— 5 o"-25 = 1 1 "* 75 , the progreffive motion of the apogee 
in a year; and hence the anomaliftic muft be longer than 
the fiderial year by the time the Sun takes in moving over 
1 1 "• 75 of longitude at his apogee; but, when the Sun is 
in its apogee, its motion in longitude is 58' 13" in twenty- 
four hours ; hence 58' 13" : ii"- 75 :: 24I1 : 4 ' 5 of", which 
added to 363d. 6h. 9'. 1 ii". gives 363d. 6h. 14'. 2L". for the 
length of the anomaliftic year. M. de la Lande determi¬ 
ned this motion of the apogee, from the obfervations of 
M. de la Hire and thofe of Dr. Mafkelyne. Caflini made 
it the fame. Mayer made it 1' 6" in his Tables. 
Of the Precession of the Equinoxes. 
The preceffion of the equinoxes is a very flow motion, 
by which they change their place, going from eaft to weft, 
or backward, in antecedentia, or contrary to the order of 
the figns. From the late improvements in aftronomy it 
appears, that the pole, the folftices, the equinoxes, and all 
-the other points of the ecliptic, have a retrograde motion, 
and are conftantly moving from eaft to weft, or from Aries 
towards Pifces, &c. by means of which, the equinoctial 
points are carried farther and farther back, among the pre¬ 
ceding figns or ftars, at the rate of about 50^" each year; 
which retrograde motion is called the prccejion, recejfion\ 
or retrcccjfion, of the equinoxes. Hence, as the flars remain 
immoveable, and the equinoxes go backward, the ftars 
will feem to move more and more eaftward with refpeCt to 
them ; for which reafon the longitudes of all the ftars, be¬ 
ing reckoned from the firft point of Aries, or the vernal 
equinox, are continually increafing. From this caufe it is, 
that the confteilations feem all to have changed the places 
alTigned to them by the ancient aftronomers. In the time 
of Hipparchus, and the oldeft aftronomers, the equinoctial 
points were fixed to the firft ftars of Aries and Libra: but 
the figns do not now anfwer to the fame points; and the 
ftars which were then in conjunction with the Sun when he 
was in the equinox, are now a whole fign, or thirty de¬ 
grees, to the eaftward of it: fo, the firft ftar of Aries is 
nt>w in the portion of the ecliptic called Taurus, and the 
ftars of Taurus are now in Gemini, and fo on. 
This feeming change of place in the ftars was firft ob- 
ferved by Hipparchus of Rhodes, who, 128 years before 
Chrift, 
