ASTRONOMY. 
nights lengthen, till the Sun arrives at the tropic of Ca¬ 
pricorn ; but, when the Sun is arrived there, it is then at 
its utmoft fbuthern limit, and returns in the ecliptic to the 
equator again. During this return, its diftance from our 
pole leffens, and confequently the days will lengthen, as 
the nights will tliorten, till they become equal, when the 
Sun is come round to the vernal equinoctial point. 
Our fummer is nearly eight days longer than the win¬ 
ter. By fummer is meant here the time that pafl'es 
between the vernal and autumnal equinoxes; by winter, 
the time between the autumnal and vernal equinox. The 
ecliptic is divided into lix northern and fix fouthern figns, 
and interfeCts the equator at the fir ft of Aries and the firft 
of Libra. In our fummer, the Sun’s apparent motion is 
through the lix northern, and in winter through the fix 
fouthern, figns; yet the Sun is i S 6 da. nh. 51'. in palling 
through the fix firft; and only i78da. 17I1. 58'. in palling 
through the fix lad. Their difference, 7 da. 17b. 53'. is 
the length ot time by which our fummerexceeds the winter. 
This may be farther illuftrated by referring back to the 
la ft figure in page 368, where ABCD reprefents the Earth’s 
orbit, and S the Sun in one of its foci. When the Earth is 
at B, the Sun appears at H, in the firft point of X ; and, 
whi 1 ft the Earth moves from B through C to D, the Sun 
appears to run through the fix northern figns, from X 
through s to i at F. When the Earth is at D, the Sun 
appears at F, in the firft point of ; and, as the Plarth 
moves trom D through A to B, the Sun appears to move 
through the fix fouthern figns, from through vf to 7^ 
at H. Hence the line FH, drawn from the firft point of 
T through the Sun at S, to the firft point of divides 
the ecliptic into two equal parts ; but the fame line divides 
the Earth’s elliptical orbit into two unequal parts. The 
greater part, BCD, is that which the Earth defcribes in the 
lummer, while the Sun appears in the northern figns. The 
lelfer part is DAB, which the Earth defcribes in winter, 
while the Sun appears in the fouthern figns. C, the Earth’s 
aphelion, where it moves (lou'eft, is in the greater part; 
A, its perihelion, is in the lelfer part, where the Sun moves 
hifteli. There are, therefore, two realons why our lum¬ 
mer is longer than our winter ; firft, becaufe tiie Sun con¬ 
tinues in the northern figns, while the Earth is defcribing 
the greater part of its orhit; and fecondly, becaufe the 
Sun’s apparent motion is (lower while it appears in tiie 
northern figns, than whilft it appears in the fouthern ones. 
The Sun’s apparent diameter is greater in our winter 
than in fummer, becaufe the Earth is nearer to the Sun 
when at A in the winter, than it is when at C in the fum- 
mer. The Sun’s apparent diameter, in winter, is 31' 47" ; 
in lummer, 31'40". But, if the Earth is farther from the 
Sun in fummer than in winter, it may be alked, Why our 
winters are fo much colder than our fummers ? To this it 
may be anfwered, that our fummer is hotter than the win¬ 
ter, firft, on account of the greater height to which the 
Sun rifes above our horizon in the fummer; fecondly, the 
greater length of the days. The Sun is much higher at 
noon in fummer than in winter ; and confequently, as its 
rays in fummer are lefs oblique than in winter, more of 
them will fall upon the lurfaceof the Earth. In the fum¬ 
mer the days are very long, and the nights very fhort; 
therefore the Earth and air are heated by the Sun in the 
day-time more than they are cooled in the night; and, 
upon this account, the heat will keep increafing in the 
lummer, and for the fame reafon will decreale in winter, 
when the nights lengthen. 
It would exceed the limits of this treatife, were we to 
inquire into the feveral concurring caufes of the tempera¬ 
tures that obtain in various climates ; it may be fufficient, 
therefore, to obferve what a remarkable provifion is made 
in the world, and the feveral parts of it, to keep up a per¬ 
petual change in the degrees of heat and cold. Thefe 
two are antagonifts, or, as lord Bacon calls them, ‘ the 
very hands of nature with which fire chiefly vvorketh 
the one expanding, the other contracting, bodies, fo as to 
jpaintain an ofcilJutory motion in all their parts; and fo fer- 
3?* 
viceable are thefe changes in the natural world, that they 
are promoted every year, every hour, every moment. 
From the oblique pofition of the ecliptic, the Earth con¬ 
tinually prefents a different face to the Sun, and never re¬ 
ceives his rays two days together in the fame direction. In 
the day and night the differences are fo obvious, that they 
need not be mentioned, though they are moll remarkable 
in thole climates where the Sun at his letting makes the 
greateft angle with the horizon. Every hour of the day, 
the heat varies with the Sun’s altitude/ is altered by the 
interpofition of clouds, and the action of winds ; and there 
is little room to doubt, but that the various changes that 
thus take place concur in producing many of the lmaller 
and greater phenomena of nature. Be this as it may, it 
is certain that the various irregularities and intemperature 
of the elements, which feem to deftroy nature in one fea- 
Ion, fevve to. revive it in another : the immoderate heats of 
fummer, and the exceili-ve cold of winter, prepare the 
beauties of the fpring, and the rich fruits of autumn. 
Thefe viciflitudes, which feem to fuperficial minds the ef¬ 
fects of a fortuitous concourfe of irregular caufes, are re¬ 
gulated according to weight and meafure by the fovereign 
wildom, who weighs the Earth as a grain of land, the-- 
fea as a drop of water! 
Of Solar and Siderial Time. 
We have (hewn, that the daily motion of the Sun from 
caff to weft is not a real, but an apparent, one, which is 
owing to the rotation of the Earth round its axis. Now, 
if the Sun had no other motion but this apparent one, it 
would feem to go once round the Earth, in the time of one 
complete rotation, or in 23I). 56'; which is the cafe with 
any of the fixed (tars, and is therefore the length of a fi- 
derial day. But the Sun is found to take up a longer time 
to complete its apparent revolution; for, if it is in tire 
fouth of any particular place at twelve o’clock at noon to¬ 
day, it will not complete an apparent revolution, fo as to 
return to the fouth of that place again, till twelve o’clock 
at noon on the next day, and confequently the time of this 
apparent revolution is twenty-four hours. We lhall render 
this fubject- clearer, by defining, in other words, the nature 
of the folar and fiderial day. 
The Jolar day is that fpace of time which intervenes be¬ 
tween the Sun’s departing from any one meridian, and its 
return to the lame circle again; which (pace is alfo called 
a natural day ; or it is the time from the noon of one day 
to the noon of the next. 'I he fiderial day is the (pace of 
time which happens between the departure of a liar from, 
and its return to, tire fame meridian again. We are now 
to fee why thefe days differ in length, or why the time, 
that the Sun takes up to complete one revolution, is long¬ 
er than the time that the Earth takes to revolve onee upon 
its axis. 
This difference arifes from the Sun’s annual motion. 
For the Sun docs not continue always in the fame place in 
the heavens, as tire fixed ftars do: but, if it is feen at M, 
fig. 3, in the preceding Aftronomical-Plate 111. on one- 
day, near the fixed ftar R, it will have drifted its place the 
next day, and will be near to loirre other fixed ftar at L. 
This motion of the Sun is from weft to eaft, and one en¬ 
tire revolution is completed in a year. Suppofe, there¬ 
fore, that the Sun, when at M, near to the fixed ftar R, 
appears in the fouth of any particular place S; and then- 
imagine the Earth to turn once round upon its axis from 
weft to eaft, or in the direction STVW, fo that the place 
may be returned to the ianre duration; after this rotation 
is completed, the ftar R will be in the fouth of the place as 
before ; but the Sun having, in the mean time, moved eaft- 
vvards, and being near to the ftar L, or to the eaft of R, 
will not be in tire fouth of tire place S, but to the eaftvvai d 
of it: upon this account, the place S muff move ona lit¬ 
tle farther, and muff come to T, before it will be even 
with the Sun again, or before the Sun will appear exactly 
in the fouth. T his may be illuftrated by the following in- 
ftance: The two hands-of a-vvatch •are dole together, or 
even* 
