929 



CIRCLE, ASTRONOMICAL. 



CIRCLE, ASTRONOMICAL. 



placing the line of eight at right angles to the axis, may be performed 

 by two distant marks to the north or south of the circle, which have 

 been previously correctly placed by means of a transit; or by a col- 

 limating transit [COLLIMATOR] ; and finally, the instrument may be 

 placed in the meridian, either by a mark, or by observations of circum- 

 stars above and below pole, or by observations of high and low 

 stars, just as a transit. [TRANSIT.] Or the position of the plane 

 Jescnbed by the line of sight with respect to the meridian, may be 

 determined by comparing the passages of stars over the meridian as 

 urved with the circle and with the transit at the same time : when 

 i law of the errors being known, and also the amount of their 

 sum, their respective values are easily ascertained and corrected. It 

 may, however, be remarked, that small errors in any or all these 

 adjustments will not vitiate the results; and that the moon is the 

 only celestial body sensibly affected by ordinary variations from the 

 true meridian. 



The mural circle is used in measuring angular distances on the 

 meridian, and the observation is performed thus : The telescope is 

 pointed nearly in the direction of the object, and the circle fixed by 

 the clamp in that position : then the instrument is moved by the slow 

 motion-screw of the clamp, until the horizontal wire exactly cuts the 

 star in two, or bisects it, at or near the meridian wire. If the object 

 be the sun or moon, or a planet having a considerable disc, the wire is 

 made a tangent to its illuminated edge. With the sun, a dark glass is 

 put on the eye-end to protect the sight, and at night a lamp placed 

 opposite the centre of the telescope throws light through a glazed 

 aperture, which is again reflected by a polished metal ring placed 

 at an angle of 45 with the axis of the telescope, so as to illuminate 

 the field of view and show the wire distinctly. This light can be 

 modified at the pleasure of the observer, and the apparatus for this 

 purpose is shown on the telescope. The mural circle has been used 

 somewhat differently by different observers. When Troughton pro- 

 posed a mural circle for Greenwich, he said that all observations should 

 be considered as distances from the pole ; and that the place of the 

 pole on the instrument (that is, the reading of the circle when the 

 telescope points to the pole) should be determined from the successive 

 upper and lower culminations of Polaris, and other close circumpolar 

 stars. The north polar distances of stars might thus be measured and 

 registered with the help of a correct table of refraction, without an 

 accurate knowledge of the latitude of the place, which is only wanted 

 for converting polar distances into altitudes or zenith distances, for 

 computing refraction and parallax. The latitude of the observatory 

 might be subsequently deduced by comparing the polar distances of 

 stars near the zenith, for instance, of 7 Draconis at Greenwich, with 

 the zenith distance of the same star observed by the zenith sector or 

 the zenith tube. [ZENITH SECTOR.] The observations could be checked 

 by combining them with observations by reflection, when the star is 

 seen reflected from the surface of a trough of quicksilver. In a 

 climate so variable as that of England, Mr. Pond, then astronomer 

 royal, found it advisable to modify the plan thus suggested. He first 

 formed an approximate catalogue of the north polar distances of several 

 stars by these or other means, which he perfected as follows : Assuming 

 the approximate catalogue to be correct, every future observed place, 

 compared with the place computed from the catalogue, presented a 

 difference, which he called the index error of the instrument, and from 

 the observations of several stars he obtained a mean index error. This 

 mean index error was then applied to each of the observations, and a 

 corrected catalogue thus produced, which by repeating the process 

 leaves no constant error except such as may be common to all the stars 

 employed ; such, for instance, as an original error in assuming all the 

 polar distances too large or too small. Now an error of this nature 



930 



clear, the instruments continuing in the same state, that if from the 

 observation of any other star, by Jones, you subtract m", you will have 

 the reading which Troughton would have shown, barring ale denTl 

 error 



errors an vm rrf, that on adding m" to any obsrvation by 

 Troughton, you will get the corresponding observation, with the same 

 exceptions, which would have been made by Jones. Again, let a star 

 m group B be taken, and let D be the angle read off by Jones the star 

 being then seen directly and R the angle by Troughton, the star betg 

 seen by reflection Then, according to what has been said above 

 D T m, is the angle which Troughton would have given by direct 

 vision and consequently the angle between the star seen directly and 

 by reflection, which is twice the altitude of the star, = R D + m" , 



or the altitude = ^Tl2 + 2L*. Again, as the reading which corresponds 



to the horizontal position of the telescope is evidently D + altitude of 

 the star by Jones and R- altitude or depression of the star by 

 Troughton, we shall find for that reading, or the horizontal point, as it 



is called, in Jones, !L + ", and in Troughton, R ^_^. I n 



2 22' 



this way each of the stars in group B gives a fresh value for the 

 horizontal point m each instrument, and the mean of the determinations 

 lor each circle is taken as the basis for reducing every observation bv 

 that circle. Thus the reading of each direct observation of the stars 

 in group A with Troughton, being subtracted from the mean value of 

 its horizontal point, will give the true altitude of the stars in group A 

 and the same value of the horizontal point being subtracted from everv 

 reflected observation of group B, by this circle, leaves in like manner 

 the true altitude, which is all the astronomer demands from the circle 

 1 his method requires the groups A and B to be sufficiently numerous 

 tor eliminating the chance errors of mere observation, and to be dis- 

 tributed over various parts of the circle to destroy faults of form or of 

 division. It is also supposed that the circles are instruments of nearly 

 equal goodness, and that each is as perfect as art can make it. If they 

 should be defective, the discrepancies in the partial determinations of 

 the horizontal point will show it, and the astronomer must then 

 attempt to discover the law and the cause of, the defect. The combi- 

 nation of two circles to form, as it were, one instrument has only 

 been tried at Greenwich, but there it has proved eminently successful 

 Mr. Ponds opinion, after long experience, being, that the pair were 

 more than twice as powerful as a single one. The same astronomer 



affecta all the stars at their superior passage alike, and makes them all 

 equally appear too near to or too far from the pole ; but it affects the 

 superior and inferior passages of the circumpolar stars by equal and 

 contrary quantities. Hence the difference between the mean of two 

 sets of observations of Polaris and other circumpolar stars, one set 

 being deduced solely from upper, and the other solely from lower 

 culminations, is clearly equal to twice the mean error in the assumed 

 place of the pole with respect to the stars ; and this correction being 

 applied to all the stars, a new and more correct standard catalogue was 

 formed, to be again corrected and improved by future observations. 

 It js evident that the errors of the originally assumed catalogue are 

 now wholly eliminated. By shifting the place of the telescope on the 

 instrument, fresh divisions were brought into use for each star. The 

 accuracy of the tables of refraction employed might have been tested 

 by the agreement of near and distant circumpolar stars, in assigning 

 the same value for this correction to the polar point. The only defect 

 in this method seems to be, that it assumes the accuracy of the instrument 

 or the accuracy of the tables of refraction ; and that if either or both be 

 imperfect, the final results are not in a form suited to investigation. 



Another mode was adopted by Mr. Pond after 1824, when a second 

 mural circle, made by Mr. T. Jones, of Charing Cross, was erected at 

 Greenwich. Suppose several stars to be observed the same evening in 

 two groups, which call A and B. All the stars in group A are observed 

 by both circles by direct vision ; the stars of group B are observed 

 directly by one circle, suppose Jones, and by reflection, by Troughton. 

 Let the mean of the readings of group A, by Jones, exceed the mean 

 of the readings of the same group by Troughton, by m" ; then it is 



ARTS AND HOT. DIV. VOL. II. 



different nights (the change of place in the stars and in the refraction 

 being allowed for) will give you the variation or change of place of the 

 instrument and microscopes; and by applying this to the stars B, 

 observed on either of the nights, you will have observations of the 

 same stars on different nights, by direct and reflected vision, but with 

 an unchanged instrument. Hence the double altitudes of B are found, 

 and also the horizontal point for one of the nights which when 

 increased or diminished by the variation above found, is the horizontal 

 point for the other. This method, or one similar to it, is, we believe 

 followed at Edinburgh and the Cape of Good Hope ; and it is found 

 that these instruments are so steady and unfluctuating, that the same 

 value of the horizontal point results from the observations of several 

 days. In this system the observations of one day are made dependent 

 on those of another, when the atmospheric circumstances may be 

 widely different. It has, however, one advantage over the previous 

 method with two circles, that it places more directly in evidence the 

 individual errors of the instrument. The differences which may be 



fnnnrl Vipt/urpipn +.1-io ni)-+iil /-la'l-a-nwiiTini-irmr, r*t *!, T I j.-i __ . 



found between the partial determinations of the horizontal point on 

 the same night, and on different sides of the zenith, will show what 

 reliance is to be placed on it and on the observer. 



When'"the mural circle was erected at Cambridge, Professor Airy 

 selected the following plan of determining the horizontal or zenith 

 point. The micrometer attached to the eye end of the telescope has 

 already been mentioned. The value of a revolution of this micrometer 

 having been very exactly measured, and the parallelism of its moveable 

 wire with the horizontal wire carefully ascertained, and also the 

 reading of the divided head when the two wires coincide, the telescope 

 is set pretty nearly in the position at which a star by reflection would 

 be on the horizontal wire, the circle is then firmly clamped, and the 

 microscopes are read off. When the star is at a convenient distance 

 from the meridian wire, it is bisected by the micrometer wire without 

 stirring the circle. When this is done, and the time noted, the circle 

 is undamped, and the star bisected by direct vision on the fixed hori- 

 zontal wire, by turning the whole circle round, and the time is again 

 noted. If the star had been observed in both instances upon the 

 meridian and upon the fixed wire, it is clear that the reading corre- 

 sponding to the horizontal position of the telescope would be half way 



T? , T-v 



between the readings of elevation and depression, or . The cor- 



2t 



rection to the meridian is easily found and applied, the times being 

 known. Again, if the distance between the fixed and micrometer wire 

 be equal to n", it is clear that the reading of R is greater or less by n" 

 than it would have been if the observation had been made with the 



3o 



