The Liverpool Standard and General Commercial Advertiser.
Liverpool, Merseyside, England, 1842-09-02 00:00:00

value :

ABERRATION, (a deviation.)—Light occupies Bm. 13s. (8
minute's 13 seconds or time) in passing from the sun to the
earth, and' during this time the earth has moved 20s. (20
seconds of a degree, the entire orbit of the earth round the
sun being recleaned as a circle of 360 degrees). At any in-
stant, therefore, the rays of light which reach the earth are
these which left the sun Bm. 13s. previous, when the earth
was in a different position with the sun to that which it oc-
cupies at the time of observation. This' phenomenon is
called aberration, and the luminous circle which may be seen
around the real disc (face) of the sun (caused by the aberra-
tion of the solar rays) is called the crown of aberration.
And as the akerration of the sun is owing to the motion of
the earth in its orbit, so the aberration of a planet is due to
the combined motions of that planet and the earth in their
respective orbiti. It is therefore said that the aberration of
a planet is equal to the geocentric (see Geocentric) motion of
the planet, or the space it appears to move over as seen from
the earth, during the time the light occupies in passing from
the planet to the earth. For the method of estimating the
amonnt of aberration see mensuration.
ClRCLES.—Astronomers imagine many great circles in the
heavens, but three principal ones, from whiCh they form the
bases of all observations of the heavenly bodies. Every
circle is called Great, the plane of which passes through the
centre of the earth. The plane of a circle is an imaginary
number of radii filling up the circle : thus the spokes of a
wheel are the radii of the circle of the wheel, and the radii
with the axle compose the plane—the parchment of a drum
is the plane of the circle on which the parchment is stretched.
The first of the three great circles is the Rational Horizon.
That circle made round our field of view at sea, by the water
apparently touching the heavens, is called the apparent or
visible horizon ; a circle drawn horizontally, or perfectly
level, from the eye of an observer is the sensible horizon,
and a circle whose plane passes through the centre of the
earth is the rational horizon. The second great circle is the
Equinoctial, or Celestial Equator, a circle of the heavens the
plane of which is in continuation with the plane of the
equator of the earth. The sun is on the equinoctial about
the 21st of March and 21st of September, when equal days
and nights are over the earth. The equinoctial is traced by
observing certain remarkable stars, to which the moon in her
course seems to approach, and also by obgerving the position
of the planets. The third great circle is the Ecliptic, or ap-
parent annual path of the sun, which also is traced by ob-
serving certain remarkable stars, the distance of which from
the ecliptic have been accurately ascertained.
TIDES.—The rotation of the earth on its axis, combined
with the attraction of the sun and moon exerted on the fluid
portion of the globe, occasions the phenomena of the tides.
If the earth were at rest between the sun and moon, the
waters immediately under the two luminaries would be raised
by their attraction and remain permanently elevated ; but as
the relative positions of all three bodies are continually un-
dergoing a change, so the attraction on the waters is conti-
nually shifting in position ; the attraction of the sun and
moon being exerted in the same direction when the moon is
new and full (or in conjunction with and opposition to the
sun), forming the high or spring tides, and the attraction
being divided in amount between that due to the sun and
that due to the moon when the moon is in her first and third
quarters ; the tides being then neap or low. The elevation
of the tide due to the sun is two feet and to the moon five
feet; when, therefore, both the attractions are exerted to-
gether, the tidal wave near the equinoctial line, where the
attraction is greatest, is seven feet high, and when the sun
and moon are in quadrature (one-fourth of a great celestial
circle apart), the height is about three feet, being the eleva-
tion due to the moon lessened by the attraction of the waters,
in an angular direction, due to the sun.
This article concludes with explanations of the great tidal
waves, the theory of which is plainly shown. The "winds"
are also explained in a similarly clear manner, and the utility
of the barometer, particularly at sea, as affording indication
of coming storms, is strongly urged; but we have not room
for further extracts, nor can we quote from those articles
which require diagrams for elucidation ; and we therefore
refer our astronomical readers to the work itself.