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The Celestial Sphere Model
The Egyptians and Babylonians did not produce models of the cosmos that could
account for the observed motions of the heavens or reveal the true shape of the
earth. A rational, theoretical approach to these problems began with the Greek
philosophers of the fifth century BC. People believed that the Sun, the Moon,
planets, and starts were embedded on the surfaces of several concentric spheres
centered at the center of the Earth, and that these spheres constantly revolved about
the Earth. This spherical model became a cornerstone of the Greek astronomy and
their civilization. The Greeks also had developed skills in spherical geometry that
enabled them to measure and map star positions.
We know that the stars are not set in one sphere. But for the purposes of
observation and mapmaking, this model works quite well. The Greek celestial
spherical model enabled astronomers and cartographers to construct globes and
armillary to show the stars, the poles, equator, ecliptic, and tropics.
Eudoxus of Cnidus first described many constellations into which we still use
today. Some constellations came from the Babylonians, such as the Scorpion,
the Lion, and the Bull. On the other hand, Perseus, Andromeda, and Hercules
are Greek mythic figures. These figures marked different regions in the sky. The
earliest representation of the classical constellations is the Farnese Atlas. The Museo
Nazionale in Naples houses a marble statue of the mythological character, Atlas,
who supports the heavens on his shoulders (See Fig. 2.10 ). Figure 2.11 shows some
constellation figures on the celestial globe. The hands on either side are the hands
of Atlas.
Figures 2.12 and 2.13 are star maps of the 48 classical constellations in the
Northern and Southern Hemisphere, respectively, published in the 1795 edition of
The Constellations of Eratosthenes by Schaubauch.
Celestial mapping relies on two fundamental inventions by Greek astronomers:
a spherical model of the heavens and star constellations in the sky. The symbol
of the ancient Greek astronomy is Ptolemy of Alexandria, who compiled a text
in the second century AD that remained fundamental to astronomy until the
sixteenth century. Known by its Arabic name, Almagest (Greatest), is a catalogue
identifying 1,022 of the brightest stars with their celestial coordinates, grouped
into 48 constellations. Ptolemy compiled this catalogue with the aid of naked-eye
sighting devices, but he was indebted to earlier catalogues such as that of the Greek
astronomer Hipparchus (146-127 BC). While Ptolemy specified how to draw the
stars and constellation figures on a globe, there is nothing in Almagest to suggest
that he made two-dimensional star maps (Whitfield 1999 ).
In order to draw accurate star maps in two dimensions, astronomers needed a
means of projecting a sphere of the sky onto a flat surface while still preserving
correct star positions. A star chart cannot be simply a picture of what is seen in the
sky because, at any given time of night, only about 40 % of the sky is visible.
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