Dictionary of Physics or an Attempt at an Explanation of the Most Important Concepts and Technical Terms of Natural Philosophy

oblique desc. = right asc. + desc. diff.
where, in the case of a negative value for the descensional difference, one must subtract instead of add.

Deviation, astronomical or Declination of stars, Declinatio, Declinaison, is called in astronomy the distance of the stars from the equator, measured by the arc of a great circle. If in Plate I, Fig. 5, a great circle P S D p is drawn through the star S and the two celestial poles P and p, which stands perpendicular to the equator A Q because it passes through its poles, then this circle is called the star's circle of deviation or Declination circle. The arc of this circle S D contained between the star S and the point of the equator D is the star's deviation.

If the star is between the equator and the North Pole P, its deviation S D is called northern (borealis), but southern (australis) if the star is situated between the equator and the South Pole. In the formulas, one can set northern deviations as positive and southern as negative. The deviation of a star standing in the equator itself is = 0; the deviation of one standing at the pole would be = 90°. It is also clear that no deviation can exceed 90°.

Through the deviation S D and the right ascension V D (see Ascension), the place of a star in the sky is determined and distinguished from the places of all other stars. It is therefore a very important task for the astronomer to investigate the deviations of the stars through observations.

However, one finds the deviations of the stars very easily through observations of their meridian altitudes. At the moment in which a star passes through the meridian, its declination circle coincides with the meridian, which always passes through the celestial poles and, in this moment, also through the star. Thus, the deviation is equal to the arc of the meridian contained between the star and the equator, which at