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

24
ASTRO N O M Y.
Part II.
Apparent
Motions of
the Heaven¬
ly Bodies.
43
Ecliptic.
•4364479—Of 23e
quired.
' The orbit in which the fun moves is called the
ecliptic. It does not coincide with the equator, but
cuts it, forming with it an angle, which in the year
1769 was determined by Dr Maflcelyne, at 23 28 10 ,
or 230 .46944. This angle is called the obliquity of the
44 ecliptic.
Seafons es- The different feafons of the year are occafioned by
plained. combination of this proper motion of the fun with
■his diurnal motion. The two points in which the
ecliptic cuts the equator, are called the equinoxes, or
equinoctial points ; becaufe on the days that the fun
is in them, he deferibes by his diurnal motion the
equator, which being divided into two equal parts by
the horizon, the day is then equal to the night in every
part of the earth. One of thefe equinoxes is called
.the vernal, becaufe the fun is in it about the 20th of
March, or the beginning of the fpring. As the lun
advances in his orbit from that point, his meridian al¬
titude becomes greater and greater every day. The
vifible arches of the parallels which it deferibes, be¬
come continually greater and with them the length
of the day increafes, till the fun reaches his greatelt
altitude, or diftance from the equator : then the day
is the longeft of the year. And as at that period the va¬
riations in the fun’s altitude are fcarcely fenfible for
fome time, as far at leaft as it affefts the length of the
day ; the point of the orbit, where the fun’s altitude is
a maximum, has for that reafon been called the fummer
foljlice. The parallel which the fun deferibes when in
that point, is called the tropic of Cancer. From the fol-
ftice the fun defeends again towards the equator, croffes
it ao-ain at the autumnal equinox, and goes fouthward
till its altitude becomes a minimum. This point of the
orbit is called the winter foljlice. The day is then the
thorteft of the year, and the parallel which the fun de¬
feribes, is called the tropic of Capricorn. From the
winter folftice the fun again approaches the equator,
and returns to the vernal equinox.
Such is the conftant courfe of the fun and of the
feafons. The interval between the vernal equinox and
the fummer lolftice, is called the fpring; the interval
between this folftice and the autumnal equinox, is called
fummer; that between the autumnal equinox and the
winter folftice, is autumn ; and that between this folftice
and the vernal equinox, is winter.
The different altitudes of the pole in different cli¬
mates, occafion remarkable peculiarities in the fea¬
fons, with which it is proper to be acquainted. At
the equator the poles are fituated in the horizon, which
laft circle cuts all the parallels into two equal parts.
Hence the day and the night are coitilantly of the
fame length all the year round. On the equinoxes the
fun is in the zenith at noon. His altitude is the leaft
poflible at the folftices, and is then equal to the com¬
plement of the inclination of the ecliptic. During the
fummer folftice, the. ftiadows of bodies illuminated by
the fun are dire&ed towards the fouth j but they are
diretted towards the north at the winter folftice j
changes which never take place in our northern cli¬
mates'. Under the equator then there are in reality
two fummers and two winters. The fame thing takas
place in all countries lying between the ironies. Be¬
yond t)ieai there on^7 one ,’jramcr and one winter
34' 43'/, the angle E re- in the year. The fun is never in the zenith. The Apparent
length of the longeft day increafes, and that of the Motions of
fhorteft day diminithes, as we advance toward the poles i
and when the diftance between the zenith and the pole .
is only equal to the inclination of the ecliptic, the fun
does not fet at all on the days of the fummer folftice,
nor rife on that of the winter folftice. Still nearer the
pole, the period in which he never fets in fummer, and
never rifes in winter, gradually increafes from a few
days to feveral months \ and, under the pole itfelf, the
equator then coinciding with the horizon, the fun never
fets when it is upon the fame fide of the equator with
the pole, and never rifes Avhile it is in the oppofite
fide. . _ 45
The intervals of time between the equinoxes and Motion not
folftices are not equal. There are about feven days more un^orin’
between the vernal and autumnal equinox, than between
the autumnal and Vernal. Hence we learn, that the
motion of the fun in its orbit is not uniform. Nume¬
rous obfervations, made with precifion, have afeertained,
that the fun moves fafteft in a point of his orbit fitu¬
ated near the winter folftice, and floweft in the oppofite
point of his orbit near the fummer folftice. When in
the firft point, the fun moves in 24 hours i°.oi943 ;
in the fecond point, he moves only o0.953i9. The
daily motion of the fun is conftantly varying in every¬
place of its orbit, between thefe two points. The me¬
dium of the two is o0.98632, or 59' ll'', which is the
daily motion of the fun about the beginning of Ofto-
ber and April. It has been afeertained, that the va¬
riation in the angular velocity of the fun, is very near¬
ly proportional to the mean angular diftance of it
from the point of its orbk where its velocity is
greateft.
It is natural to think, that the diftance of the fun 4. .
from the earth varies as well as its angular velocity. var;es#
This is demonftrated by meafuring the apparent dia¬
meter of the fun. Its diameter increafes and diminilhes
in the fame manner, and at the fame time, with its an¬
gular velocity ; but in a ratio twice as fmall. About
the beginning of January, his apparent diameter is about
32' 39", and at the beginning of July it is about 31'34",
or more exaflly, according to De la Place, 32' 35"=:
195 5" in the firft cafe, and 31' in the fe¬
cond. _ 47
Opticians have demonftrated, that the diftance of sun’s di-
any body is always reciprocally as its apparent diame-ftance va-
ter. The fun muft follow the fame law' ; therefore, ries.
its diftance from the earth increafes in the fame pro¬
portion that its apparent diameter diminilhes. That
point of the orbit in which the fun is neareft the earth,
is called perigeon, or perigee ; and the point of the or¬
bit in which that luminary is fartheft diftant from the
earth, is called apogee. When the fun is in the firfi:
of thefe points, his apparent diameter is greateft, and
his motion fwifteft 5 but rvhen he is in the other point,
both his diameter and the rapidity of his motion are
the fmalleft ppflible.
From thefe remarks it is obvious that if the orbit of
the fun be a circle, the earth is not fituated in the cen¬
tre of that circle, otherwife the diftance of the fun
from the earth would remain always the fame, which
is contrary to faft. It is poflible therefore, that the
variation in his angular velocity may not be real, but
only apparent. Thus in fig. 3. let AMPN be the'
orbit