The Determination of the Equator-Point for Observing Star Declinations

• Published in: International Astronomical Union Colloquium
• Main Author: Mikhailov, A. A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1975
• Subjects: South Pole; North Pole; Zero Point; South pole
• Online Access: http://dx.doi.org/10.1017/s0252921100051125

The deolinations of the stars are by definition reckoned from the celestial equator. However this great circle is not marked on the sky; therefore, instead of dejclinations, the polar distances of the stars are measured, as the position of the pole can be found by observing a star in upper and lower culmination and taking the mean zenith distance, corrected for changes in the position of the star and observer during the elapsed interval of time. Thus the zero point of the polar distance to be measured is established and it remains to determine the scale, which is given by the divisions of the circle. These divisions must be carefully investigated as to their strict equality and they must go around the circle without a gap or overlapping. In this way, the scale has only one fixed point in the pole, while the other end near the equator hangs loosely. This leads to a noticeable discrepancy near the celestial equator when the measurements starting from the north pole meet those from the south pole. It would be very valuable if we could determine the position of an intermediate point, say on the equator, for then the measurements would be established at both ends and would consist of an interpolation between two fixed points. This can be achieved by determining the exact absolute declinations of a number of stars situated close to the equator with different right ascensions. We shall explain how this can be done.