DETERMINATION OF THE PLACES OF THE PLANETS. 



By Robert T. A. Innes, F.R.A.S., F.R.S.E. 



In the Observatory for September 1910, Professor H, H. 

 Turner, F.R.S., of the University Observatory, Oxford, puts 

 forth proposals for measuring large arcs of the sky (say about 90° 

 or 120'^) by means of two fixed horizontal telescopes and a mirror. 

 The places of the heavenly bodies are at present referred to the 

 sphere by means of Right x^scensions measured along the equator 

 of the sky from the spring equinox (first point of Aries) and 

 Declinations measured from the equator towards each pole. The 

 Declinations depend on simple angular measurements made with 

 divided circles. Although such measurements are comparatively 

 simple, there are subtle difficulties well-known to the practical 

 astronomer which Professor Turner duly notes. The angles in- 

 volved in the Right Ascensions are measured in quite a different 

 way. Advantage is taken of the rotation of the Earth. Each 

 star will roughly transit the meridian once in 24 sidereal hours ; 

 hence if one star transits at say noon and another 12 hours later, 

 we know that they are separated in Right Ascension by half a 

 circumference or 180°. This method is simple enough; the diffi- 

 culties arise from the fact that during sunlight, the stars are 

 invisible, or if the stars are bright enough to be seen, they are 

 observed under different conditions of temperature from those ob- 

 served at night, and we can only tell when 12 hours have elapsed 

 by means of clocks which suffer more or less from temperature 

 effects ; again to find the spring equinox or zero-point of the Right 

 Ascensions, it is necessary to find the Sun's position amongst the 

 stars. These difficulties become very great in practice, and it is 

 considered that all stars' Right Ascensions suffer from errors of 

 the type 



A sin R.A. +B cos R.A. 



If Professor Turner's proposals can be carried into effect, it 

 should be possible to eliminate errors of time-keeping, but the 

 great practical difficulty of referring the Sun to the stars will 

 always remain. Professor Turner has expressed his willingness 

 to essay his new ideas, and we hope that means will be at hand 

 to enable him to do so. 



I believe the late Monsieur Loewy endeavoured to use the bent- 

 equatorial of the Paris Observatory with an object-glass prism in 

 somewhat the same way, the ostensible object being- the deter- 

 mination of the constant of aberration ; but the results, if any, 

 are unknown to the writer. 



Professor Turner's proposals have led me to a vein of thought 

 which is worked in what follows. 



As just mentioned the Right Ascensions of the heavenly bodies 

 are determined by the rotation of the Earth. Is this the right 

 way, and what would happen, as is alleged to be the cases with 

 the neighbouring planets Venus and Mercury, if the Earth did 

 not rotate with reference to the Sun? If the Earth rotated very 

 slowly or not at all the precise measurement of time would become 

 exceedingly difficult if not actually impossible. The Sun would 

 remain fixed in the sky within a line less than 4° long, along which 



