Celestial sphere
In
astronomy and
navigation, the celestial sphere is an imaginary
rotating sphere of "gigantic radius", concentric and
coaxial with the
Earth. All objects in the
sky can be thought of as lying upon the sphere. Projected, from their corresponding
geographic equivalents, are the
celestial equator and the celestial poles. The celestial sphere projection is a very practical tool for positional astronomy.
The celestial sphere can be used
geocentrically and topocentrically. The former means that it is centred around an imaginary observer in the centre of the Earth, and no
parallax effects need to be taken into account.
Encyclopedia
In
astronomy and
navigation, the
celestial sphere is an imaginary
rotating sphere of "gigantic radius", concentric and
coaxial with the
Earth. All objects in the
sky can be thought of as lying upon the sphere. Projected, from their corresponding
geographic equivalents, are the
celestial equator and the celestial poles. The celestial sphere projection is a very practical tool for positional astronomy.
The celestial sphere can be used
geocentrically and topocentrically. The former means that it is centred around an imaginary observer in the centre of the Earth, and no
parallax effects need to be taken into account. In the latter case it is centred around an observer on the surface of the Earth and then horizontal parallax cannot always be ignored; especially not for the Moon.
In the
Aristotelic and Ptolemaic models, the celestial sphere was imagined as a physical reality rather than a geometrical projection .
The celestial sphere is divided by projecting the
equator into space. This divides the sphere into the north celestial hemisphere and the south celestial hemisphere. Likewise, one can locate the Celestial Tropic of Cancer, Celestial Tropic of Capricorn, North Celestial Pole, and South Celestial Pole. The directions toward various objects in the sky can be quantified by constructing a celestial coordinate system.
As the Earth rotates from
west to
east around its axis once every 23 hours 56 minutes, the celestial sphere and all objects on it appears to rotate from
east to
west around the celestial poles in the same time. This is the diurnal motion. Therefore stars will rise in the east, culminate on the north-south line and set in the west, . Next night a particular star will rise again, but with our normal clocks running a 24 hour 0 minutes cycle, it will do so 4 minutes earlier. The following night that is already 8 minutes. And so forth every night again.
The reason for this apparent misadjustment of our clocks is that the Sun is not standing still on the celestial sphere, as the stars do, but moves about 1° per day eastwards over a
great circle known as the
ecliptic . As an angle of 1° corresponds to 4 minutes in time , we need therefore 4 extra minutes of diurnal motion to see the Sun back on the meridian again, making the duration of one rotation just 24 hours exactly
Normal clocks therefore indicate
solar time.
Astronomers studying the movements of stars may want to have clocks indicating
sidereal time, going around once in 23h56m .
A celestial sphere can also refer to a physical model of the celestial sphere. Also known as a star globe, this sort of celestial sphere will indicate which constellations are visible at a given time and place.
See also
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External links
