Elementary chapters in astronomy / from the 'Traité élémentaire d'astronomie physique' ... Edited [and translated] by the Rev. Harvey Goodwin.

  • Jean-Baptiste Biot
Date:
1850
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Licence: Public Domain Mark

Credit: Elementary chapters in astronomy / from the 'Traité élémentaire d'astronomie physique' ... Edited [and translated] by the Rev. Harvey Goodwin. Source: Wellcome Collection.

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    altogether, we conclude that their setting is the effect of the magnitude of the circles which they describe, and that they complete them under the horizon, below the earth, when they disappear from our eyes. The most obvious analogy leads us to extend this explanation to stars situated in that part of the heavens which is opposite to the north, and which we call the south. These stars, after setting, also complete their revolu¬ tion below the earth, to reappear, like the sun, in the east. If then, for distinctness of conception, we conceive a mathe¬ matical line, or axis of rotation, round which all this motion takes place, it will be necessary to conceive, that in Europe this axis appears elevated towards the north, and oblique to our horizon, that is to say, to the plane which, passing through our eyes and touching the earth’s surface, separates the visible portion of the heavens from that which is concealed from us. It is not necessary to represent this axis as anything mate¬ rial, really existing in space ; it is only a geometrical concep¬ tion, which defines the series of points in space which appear fixed in the midst of the general motion. It is the same with the circles which the stars appear to describe round this axis of their diurnal revolution ; we are only to understand by these the points of space which we perceive them successively to occupy. As to the supposition that this diurnal motion is really and exactly circular, we shall employ it here merely as an hypothesis which expresses the appearances ; for, although it is true in all geometrical strictness, the complete proof can only be given by means of very precise measurements, and by the help of considerations which cannot be introduced at present. 3. If we examine the heavens during a great number of nights, we notice certain heavenly bodies which change their places amongst the stars ; these do not always form part of the same constellations : each day they approach some, and become more distant from others by an almost imperceptible quantity. This phenomenon is much more manifest in the case of the moon ; for the relative position of that body varies, very sensibly, in the course of a single night. But, even for the other bodies of which we are speaking, their small dis¬ placements become at length perceptible, and carry them at length into entirely different parts of the heavens. On this 1—2
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