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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    sequence of this proper motion, the sun appears to move through the whole circuit of the heavens, going from west towards east. 5. Again, in examining the heavens by night, at different seasons of the year, we find their face altogether changed. There are no longer the same stars visible ; they are arranged and disposed differently. This is a very simple consequence of the sun’s proper motion. In that portion of the heavens •where the sun happens to be, the brightness of its light pre¬ vents us from seeing the stars with the naked eye ; with telescopes it is possible to see them even in the day-time. But, as the sun leaves these stars in consequence of its proper motion, as it constantly travels eastward, they arrive above the horizon during the night and so become visible. We may see in this circumstance already the inaccuracy of the rough notions to which the first sight of the phenomena gives rise, and in virtue of which we should be led to believe that the heavens are divided into two portions which appear above the horizon successively, and of which one is occupied by stars, the other by the sun. Exact and continued obser¬ vation of celestial phenomena will lead us to recognize many other illusions, and teach us to renounce them. Those stars only which lie towards the north, and which never set, remain constantly visible in the heavens at night at all seasons of the year. But, at the same period of the night, we see them successively in different positions, ac¬ cording to their situation with reference to the sun ; and in this way the proper motion of that body will be sensible even by reference to these stars. 6. The proper motion of the sun is not directed exactly from west to east, for every one is aware that at certain times the sun rises much more above our heads than at others, a fact which becomes very sensible in the variations of its heat, whence results the change of the seasons. It may be recognized also by noting the points of its rising and setting, which do not always coincide on the horizon with the same terrestrial objects. But the first proper motion of the sun, from west eastward, is the most considerable, since in virtue of that it makes the entire circuit of the heavens ;
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