Rudolf Steiner

The celestial sphere is a imaginary Sphère of unspecified ray and whose center is occupied by the Ground. This astronomical concept , inherited the Antiquity, makes it possible to represent all the stars such as one sees them since the Ground. Thus, it is possible to position the stars in the sky in their allotting single coordinates.

Genesis of the concept

The notion of sphere celestial was invented by the Greek philosophers of Antiquity. Liked the idea the Greek philosophers for whom the sphere was a perfect geometrical figure. All Greek cosmology was thus based on a model of concentric spheres around the Earth, of which the sphere of fixed the which corresponds more or less to the current celestial sphere.

During the Antiquity and the the Middle Ages, the star S were thus regarded as equidistant of the Earth and this sphere like an exact representation of the Univers. Even if one knows now that this model is false, it remains a very useful abstraction. Indeed, all that we see in the sky is very distant from us and it distance is impossible to measure just while looking at. Since these distances are unspecified, it is only necessary to know the direction of the object to locate it in the sky. In this direction, the model of the celestial sphere is a very practical tool for the Astronomie of position.

The system of representation

While the Earth turns on its axis, the objects on the celestial sphere seem to turn around the celestial poles of 24 hours: it is the diurnal Mouvement apparent. For example, the Sun always seems to rise to the east and to lie down in the west, just like the stars, the Planet S and the the Moon. However, because of the difference existing between the solar Time and the sidereal Time, each night a given star will rise 4 minutes earlier than it rose the night before.

In the system of representation of the celestial sphere, the Earth is regarded as motionless and it is the celestial sphere which turns around our planet. The axis of rotation passes by the geographical poles, and its projection on the celestial sphere is defined as correspondent with the celestial poles. α Ursae Minoris, more known under the name of pole star , is so close to the celestial north pole which it seems motionless in the sky. Same manner, the celestial equator corresponds to the projection of the terrestrial equator on this sphere. This equator divides the celestial sphere in two parts: the northern celestial hemisphere and southern celestial hemisphere. Generally, one can project any point of the Earth on the celestial sphere. The vertical of a point of the Earth, except for the poles, progressively traces a parallel on this sphere of its rotation.

All the stars can be also represented on the celestial sphere, including the Sun, and one calls ecliptic the projection of the trajectory of the Sun on the celestial sphere. The slope of the axis of the Earth compared to the ecliptic makes that Ci does not merge with the celestial equator. The intersections from these two Grand circle S correspond to the equinox S, which mark the beginnings of spring and the autumn. These crossings correspond to the moment when the duration of the days is equal to the duration of the nights on the equator. The crossing of spring is called Point vernal; the meridian line of the vernal point is also called origin of the right ascensions .

The frame of reference

All the stars can be projected on the celestial sphere. They are then identified there by a couple of coordinates made up of the variation and right ascension. These coordinates are the equivalents of the Latitude and the Longitude on Earth.

the variation is a Angle, positive or negative, which corresponds to the apparent distance between the star in question and the celestial equator; it is measured in degree S, minutes and seconds of arc.
the Right ascension is a size of time calculated in hours, minutes and second S. It corresponds to the time which would spend the celestial sphere so that the projection of a fixed point on Earth passes from the meridian line of the vernal point to the meridian line on which the star observed is, the celestial sphere being of course divided into twenty-four hours.

See too

celestial Frame of reference.

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