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

28 ' ASTRO
Apparent and as both funs come at the fame moment to the point
Motions ot W? they come to the meridian at the moment of noon
the Hea ven-ky .jie clock.
jy Bodies ^ jn jep3rting from Libra, through the third qua¬
drant, the real fun going through MNOPQ^ towards
at R, and the fidthious fun through mn o pq towards
r, the former comes to the meridian every day fooner
than the latter, until the real fun comes to ©, and the
fidlitious to r, and then they come both to the meridian
at the fame time.
Laftly, As the real fun moves equably through
STUVW, from 0 towards } and the fidlitious fun
through s t u v w, from r towards «Y’, the former comes
later every day to the meridian than the latter, until
they both arrive at the point </», and then they make it
noon at the fame time with the clock.
Having explained one cauie of the difference of time
fhown by a well-regulated clock and a true fun-dial,
fuppofing the fun, not the earth, as moving in the
ecliptic j we now proceed to explain the other caufe of
this difference, namely, the inequality of the fun’s ap¬
parent motion ; which is floweft in fummer, when the
fun is fartheft from the earth, and fwifteft in winter
when he is neareft to it.
If the fun’s motion were equable in the ecliptic, the
whole difference between the equal time as fhown by
the clock, and the unequal time as fhown by the fun,
would arife from the obliquity of the ecliptic. But the
fun’s motion fometimes exceeds a degree in 24 hours,
though generally it is lefs : and jvhen his motion is
iloweft, any particular meridian will revolve fooner to
him than when his motion is quickefl; for it will over¬
take him in lefs time when he advances a lefs fpace than
when he moves through a larger.
Now, if there were two funs moving in the plane of
the ecliptic, fo as to go round it in a year ; the one
defcribing an equal arc every 24 hours, and the other
defcribing fometimes a lefs arc in 24 hours, and at
other times a larger, gaining at one time of the year
what it loft at the oppofite ; it is evident, that either
of thefe funs would come fooner or later to the meri¬
dian than the other, as it happened to be behind or
before the other j and when they were both in con-
jun&ion, they would come to the meridian at the fame
moment.
As the real fun moves unequably in the ecliptic, let
us fuppofe a fiftitious fun to move equably in a circle
coincident with the plane of the ecliptic. Let ABCD
(fig. 8.) be the ecliptic or orbit in which the real
fun moves, and the doted circle a b c d imaginary or¬
bit of the fi&itious fun ; each going round in a year
according to the order of letters, or from Aveft to eaft.
Let HIKL be the earth turning round its axis the
feme way every 24 hours 5 and fuppofe both funs to
ftart from A and o, in a right line with the plane of the
meridian EH, at the fame moment: the real fun at
A, being then at his greateft diftance from the
earth, at which time his motion is floweft j and the
fiftitious fun at o, whofe motion is always equable,
becaufe his diftance from the earth is fuppofed to be
always the fame. In the time that the meridian re¬
volves from H to H again, according to the order of
the letters HIKL, the real fun has moved from A to
¥ j and the fidlitious with a quicker motion from n to
fj through a large arc: therefore, the meridian EH
N O M Y. Part II.
will revolve fooner from H to ^ under the real fun at E, Apparent
than from HE to k under the fictitious fun at/; and Motions of
confequently it will then be noon by the fun-dial fooner Heaven-
than by the cluck. _ q to
As the real fun moves from A towards C, the fwift-
nefs of his motion increafes all the way to C, where it
is at the quickeft. But notwithftanding this, the fic¬
titious fun gains fo much upon the real, foon after his
departing from A, that the increafing velocity of the
real fun does not bring him up with the equally-moving
fictitious fun till the former comes to C, and the latter
to c, when each has gone half round its refpeCtive or¬
bit ; and then being in conjunction, the meridian EH,
revolving to EK, comes to both funs at the fame time,
and therefore it is noon by them both at the fame mo¬
ment.
But the increafed velocity of the real fun now being
at the quickeft, carries him before the fictitious one j
and therefore, the fame meridian will come to the ficti¬
tious fun fooner than to the real: for whilft the ficti¬
tious fun moves from a to g, the real fun moves through
a greater arc from C ^ G : confequently the point K
has its noon by the clock when it comes to / but not
its noon by the fun till it comes to /. And although
the velocity of the real fun diminilhes all the way from
C to A, and the fictitious fun by an equable motion
is ftill coming nearer to the real fun, yet they are not
in conjunction till the one comes to A and the other
to o, and then it is noon by them both at the fame
moment.
True time is obtained by adding or fubtraCting this
equation to the mean time. 1 he mean and apparent
folar days are never equal, except Avhen the fun’s daily
motion in right afeenlion is 59' 8" ; this is nearly the
cafe about April 15th, June 15th, September ift, and
December 24th : on thefe days the equation is nothing,
or nearly fo j it is at the greateft about November ift*
Avhen it is 16 m. 14. fee. . ^
The return of the fun to the fame equinox marks the Year,
years, in the fame way as his return to the fame meri¬
dian indicates the days. It has been afeertained, that
before the fun returns again to the fame equinox, an in¬
terval of 365.242222 days elapfes, or 365 days, 5 hours,
48 minutes, and 47 leconds. T his is called the tropi¬
cal year : The fun takes a larger interval of time to re¬
turn again to the fame ftar. The. Jidcreal year is the
interval Avhich the fun employs to return from one ftar
to another. It is greater than the tropical year by
0.014162 days, or 20 m. 23 fee. j therefore the length
of the fidereal year is 365 days, 6 h. 9 m. and 10 fee.
From this it follows, that the equinoxes do not retain
the feme place in the ecliptic, but that they have a re¬
trograde motion, or contrary to that of the fun, in con-
fequence of which they deferibe every year an arc equal
to the mean fpace Avhich the fun paffes over in 20' 23",
or about 50" *, fo that they Avould make a complete re¬
volution in 25972 years. This is called the precejjion
of the equinoxes. 60
Dr Mafkelyne has invented a rule for computing Method of
the equation of time, in which the preceflion of the^Fj^S
equinoxes, as well as the two caufes mentioned a^0^e»tion of
are included. Let APL£), fig. 9' ec^pHc’ time.
AL£) the. equator, A the firft point of Aries, P the
point where the fun’s apparent motion is lloweft, S
any place of the fun j draAV S v perpendicular to the
equator.