ENCYCLOPEDIA
AMERICANA
V.2, P. 571, 1986
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مكتبة يافث، الجامعة الأميركية في
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CALL NUMBE: R 031 E 56 ency.
Changing
Views of the Universe. For a while the belief developed that the sun holds the
central place in the universe, a truly heliocentric outlook. This outlook was
based on the fact that star counts seemed to reveal that in every direction the
stars thin out. Careful studies of photographs confirmed this observation. In
less than 100 years, however, the whole cosmological picture changed. Thus, in
1918, Harlow Shapley demonstrated that the center of stellar concentration was
not the sun and that in fact that the sun is more than 35,000 light years away
from the center of the stellar universe as it was known at that time.
The heliocentric view was dead; it was replaced by the
galactocentric view. The center of the universe was now taken to be the vast
group, or galaxy, of stars to which the sun belongs and which is known as the
Milky Way.
The galactocentric view was in turn quickly abandoned.
In 1924, Edwin Hubble demonstrated that the so-called Andromeda Nebula actually
is not a nebula (a cloud of gas and dust) but another galaxy more than a
million light years distant from the Milky Way. Subsequently many more star
systems were identified that rival or exceed the glory of our own galaxy.
Spectroscopic studies of the light from other galaxies,
begun by Vesto Slipher, revealed that the more distant galaxies are all moving
away from our galaxy. Thus it appeared for a while that the Milky Way might be
the center of the universe. But astronomers had learned that such assumptions
should be mistrusted. With the development of relativistic mechanics a simple
cosmology evolved which held that the universe appears essentially the same
from any galaxy in the universe. This might be called the universal view as
opposed to the heliocentric or galactocentric view. With this view, astronomers
could now ask critical questions about the composition, size, and age of the
universe with reasonable hope of finding meaningful answers.
THE NEW
ENCYCLOPEDIA BRITANNICA
V.1, P. 656, 1986
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CALL NUMBER: R 030 E 56e
During the 2nd century AD Ptolemy (Claudius
Ptolemaus), one of the most celebrated of the ancient Greek astronomers, put
forth a conception of an Earth-centred (geocentric) universe that influenced
astronomical thought for more than 1,300 years.
THE CAMBRIDGE ATLAS OF ASTRONOMY
P.306
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CALL NUMBER: R 520 G 751 ga
Picture of our Galaxy viewed from outside. This diagram comes from a computer simulation made by J.N. Bahcall. It shows how our Galaxy would look to an exterior observer. At the centre of the galactic disk is the bulge. The 200 million stars in the Galaxy are not uniformly distributed in the disk ; the stellar density increases towards the centre and towards the central plane of the disk.
These arms spiral around the centre, turning around an axis perpendicular to the plane of the disk.
The various components of the Galaxy do not, however, rotate with the same angular velocity; the Galaxy dose not behave like a solid body. From the centre outwards, the angular velocity increases out to a radius of about 8 kiloparsecs, and then decreases. The Galaxy is thus continuously deformed. While the Sun takes 200 million years to complete a circuit around the Galaxy, a star located 5 Kiloparsecs from the centre takes less than half the time. The space between the spiral arms is far from empty; the density of gas and of stars is only a factor of 2 or 3 lower than in the spiral arms.The third component of the Galaxy is the halo, which occupies a spherical volume of radius about 15 kiloparsecs, containing globular clusters and, in general, the oldest stars. The halo contains very little matter compared with the disk and the bulge.
ACADEMIC
AMERICANA ENCYCLOPEDIA
V. 7, P. 18, 1989
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مكتبة يافث ، الجامعة الأميركية في
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CALL NUMBER: R 031 A 168 a
Earth, motions of
The Earth rotates about its spin axis in a period of 1
Day and revolves about the sun, in a plane called the ECLIPTIC, in a period of
1 YEAR. Its path is a nearly circular ellipse, with the sun at one focus (see
KEPLER'S LAWS). The Earth is nearest the sun (perihelion) about January 3 and
farthest (aphelion) about July 4 (see ORBITAL ELEMENTS). The orbital speed
averages 30 km/sec (18.6 mi/sec); it is greatest at perihelion and least at
aphelion. The spin axis intersects the Earth's surface at the NORTH POLE and
SOUTH POLE. The great circle 90º from the poles is the EQUATOR. It intersects
the ecliptic at a nearly constant angle of about 23.5º, at points called the
vernal and autumnal EQUINOXES. The Sun apparently passes through these points
within a day or two of March 20 and September 23, respectively. The Sun is most
northerly about June 21 and most southerly about December 22; for an observer
in the Northern Hemisphere, these events are the summer and winter SOLSTICES,
respectively.
Because of its rotation, the Earth has an equatorial
bulge, upon which the gravitational attractions of the Sun and Moon act. The
Earth, as a result, moves like a spin axis keeps a nearly constant inclination
to the ecliptic.