937 



CIRCLE, ASTRONOMICAL. 



CIRCLE, ASTRONOMICAL. 



938 



vertical to the other. The changes of the level, of the instrument, or 

 of the time-keeper, may occasion greater errors than can well arise 

 from the divisions of the circle, not to mention the greater length of 

 time occupied by the observation, and the chance of disappointment 

 from change of weather. Lastly, the time may be derived from this 

 instrument, when the lower circle is clamped, and the telescope is 

 made to move in the meridian like a transit. The transit will be 

 explained hereafter, but the adjustment to the meridian is greatly 

 facilitated in this instrument by the divided horizontal circle. Either 

 the azimuths of a star may be observed when it has the same altitude 

 on the east and west of the meridian, in which case the middle reading 

 of the azimuths will be the reading corresponding to the meridian ; or 

 the tune may be got very nearly from a star near the zenith, and then 

 the error of the time-keeper being known, Polaris or any near circuni- 

 polar star, may be bisected, by moving the instrument in azimuth, at 

 the time when, by the clock, it should be on the meridian ; or Polaris, 

 i >r any known circumpolar star, may be observed at its greatest elonga- 

 tion, when its azimuth is known by computation; or, generally, a 

 known circumpolar star may be observed anywhere, and its azimuth 

 be computed for the known time of the observation, when the instru- 

 ment can be shifted this quantity. For the mode of correcting errors 

 of level, &c., and of deducing the longitude from the passage of the 

 moon over the meridian, &c., see TRANSIT. 



When the direction of the meridian is required with great accuracy, 

 the process used in the ' Trigonometrical Survey,' vol. i. p. 242, may 

 be followed. The azimuth of Polaris was observed when at its 

 greatest elongation to the east and to the west of the meridian, and the 

 mean of these was taken for the reading of the north point. The 

 cross level should be applied to the transit axis and the error carefully 

 noted before every observation, and the instrument should be reversed 

 once at least during the series. 



Little has been said of the error of collimation in altitude, because in 

 truth the determination of this error is scarely ever required in a 

 separate shape by an accurate observer. It may, however, be deter- 

 mined thus : Observe a star with the divided face of the circle to the 

 east, and then with face west, near the meridian, exactly as has been 

 already described, correct each zenith distance to the meridian and for 

 the indication of the fixed level. Then the difference between these 

 zenith distances, if any, shows that the microscopes do not read 0, 

 when the telescope is in the zenith and the vertical axis is correct : 

 hah 5 the difference is to be added to all observations made with the 

 instrument when the face is one way, and to be subtracted from all 

 observations when the face is the other way, as the case may be. 



If an observer should wish to use an instrument of this kind for 

 making a catalogue of unknown stars, he may place it accurately in the 

 meridian, and observe transits and zenith distances at once, using an 

 index error for the latter, deduced from known or standard stars, as 

 we have described above. Or the zenith distances may be determined 

 by circum-meridian observations, which is perhaps a more accurate and 

 certainly a more independent mode. The peculiar advantage of the 

 altitude and azimuth circle is, however, for observing phenomena when 

 out of the meridian, and therefore not within the reach of the prin- 

 cipal instruments : for instance, in determining the place of a comet, 

 Ac. Here the time must be noted when the object is bisected by the 

 crossing of the horizontal and vertical wires, and the upper and lower 

 microscopes and fixed level read off. The instrument is now to be 

 reversed and the operation repeated. Then if the object is not near 

 the meridian and the interval is short, the mean of the zenith distances 

 and the mean of the azimuthal readings correspond very nearly to the 

 middle time, and as the azimuthal reading of the meridian is, or easily 

 may be known, the azimuth and zenith distance of the object at a 

 ijiwn tune are known, which, with the colatitude, are two sides, and 

 the included angle of the triangle z r s, which can therefore be solved, 

 and the polar distance r s, and the horary angle z p s be found. If the 

 observations should be made near the meridian, the corrections as 

 found in circum-meridian observations must be applied. 



The observer, who is not afraid of working spherical trigonometry, 

 will find the following a very exact method of ascertaining the place of 

 a comet or planet : Observe the altitude and azimuth, noting the time, 

 of a known star, as near the comet as may be, and then the altitude 

 and azimuth, also noting the time, of the comet. The computed 

 altitude and azimuth of the star, when compared with that observed, 

 will give an index error'for the instrument, in altitude and in azimuth; 

 which, when applied to the observed places of the comet, afford correct 

 data for computing its horary angle and polar distance. This method 

 U only an application of the principle of measuring differences, rather 

 than absolute quantities, and admits of great exactness, even with an 

 indifferent instrument, especially if several stars, on different sides of 

 the comet, are used ; but the labour is considerable. 



An instrument of this kind will show very clearly the effects of re- 

 fraction and parallax, though it cannot be expected to increase our 

 knowledge on these points. On comparing the observed zenith dis- 

 tance with the zenith distance computed from the polar distance, and 

 the horary or azimuthal angle, the difference will be the effect of 

 refraction, or of refraction and parallax, as the case may be. The 

 determination of the law and quantity of refraction was one of the 

 purposes for which Piazzi's circle was principally designed. 



The instrument-maker ought to mark the value of the parts oil the 



scales of the levels, but it is advisable to ascertain these independently. 

 Place the instrument so that the third foot-screw, that with the slow 

 motion apparatus, is in the plane of the altitude circle, raise the foot- 

 screw till the end of the bubble towards the observer is near the end of 

 the scale, bisect a well-defined object with the telescope, and read the 

 upper microscopes and the ends of the bubble. Then lower the foot- 

 screw till the bubble is towards the other end of the scale, bisect the 

 object again, and read off the microscopes (bringing the crosses to the 

 same divisions) and bubble as before. You have thus a given number 

 of parts of the scale, those through which the bubble has travelled 

 equal to the difference of the readings of the microscopes in' the two 

 positions, which is therefore known in seconds of spaces. This should 

 be done several times, and the temperature noted, as it will be found 

 that not only the length of the bubble, but the value of the parts, 

 varies with the temperature. A table may then be made for future 

 use. By attaching the cross level to the altitude circle it may be 

 examined in the same way. 



There are several variations in the form of the altitude and azimuth 

 circle. The vertical axis is sometimes depressed below the azimuth 

 circle, which gives the instrument a greater compactness of form. In 

 a few instances, instead of a pair of microscopes upon a fixed support, 

 there are three or more which can be placed anywhere on the circum- 

 ference of a ring, parallel to and concentric with the vertical circle. 

 This is a very essential improvement, as besides getting rid of eccen- 

 tricity, three microscopes at 120 distance, or a pair at right angles, 

 destroy the effect of any change of figure corresponding to ellipticity. 

 (See ' Monthly Notices of the Astron. Soc.,' vol. ii. p. 96.) The errors 

 of division may also be gradually eliminated by changing the positions 

 of the readings. Three or more microscopes are sometimes applied to 

 the horizontal circle. 



The circle here drawn and described is divided by an engine ; in 

 instruments of higher pretensions one or both circles are divided by 

 hand, generally according to Troughton's method. Sometimes both 

 faces of the vertical circle are divided. There is a figure and descrip- 

 tion of a very beautiful circle, generally known by the name of the 

 Westbury circle, in a paper by Mr. Pond, 'Phil. Trans.,' 1806, p. 420, 

 plate xx., and of another in Pearson's 'Astronomy,' vol. ii .p. 434, 

 plate xix. A vertical circle, of eight feet diameter, moving freely in 

 azimuth, planned by Ramsden and finished by Berge, is at the Obser- 

 vatory of Dublin, which, so far as we know, has not yet been described, 

 though well known by the deductions of Dr. Brinkley. (See ' Trans. 

 Royal Irish Academy,' vol. xii. p. 33.) 



The transit circle is very shortly described, as it is only the upper 

 circle of the last-mentioned instrument, generally on a larger scale. 

 Thte, when fixed in the plane of the meridian, may be used both as a 

 transit and as a meridian circle at the same time. The supports should 

 be of stone, to which the reading microscopes should also be attached. 

 A very beautiful transit circle, of four feet diameter, and divided on 

 both faces, was constructed by Mr. Troughton in 1804. This was for 

 many years in the possession of the late Stephen Groombridge, Esq., of 

 Blackheath, and was employed by him in forming a catalogue of stars 

 within 50 of the north pole. It is described and engraved in Pearson's 

 ' Astronomy,' vol. ii. p. 402, plate xvii. The artist himself was dis- 

 satisfied with the weakness of the axis, a capital fault in an instrument 

 for observing transits ; and we conceive that there was a still greater 

 oversight, though one more easily repaired, in the unsteady fixing of 

 the reading microscopes, which are mounted on slender bars of brass 

 instead of stout stone crosses; the consequence was, that the index 

 error was always changing. The meridian circle of Roeiner, and those 

 which resemble it, have been already mentioned. This form is per- 

 haps preferable to that of Groombridge's, the axis being less subject to 

 flexure ; but its unsymmetrical appearance is disagreeable, and make 

 some of the ordinary modes of adjustment impracticable. The mi- 

 equal bearing on the pivots may also require caution. The meridian 

 circle of Reichenbach is like the ordinary transit telescope, with the 

 circle and verniers close to one of the pivots. It is too complicated to 

 be described in a few words, and is not, we believe, to be met with in 

 this country ; but most of the Continental observatories east of the 

 Rhine are furnished with them. In the hands of Bessel, Gauss, &c., 

 the circle of Reichenbach is undoubtedly a very powerful and accurate 

 instrument ; but we think not so perfect, certainly not so fit for ordi- 

 nary observers, as a closer copy of Roomer's would be, while it is much 

 less simple, and the telescope is more liable to injurious flexure. 



The adjustments of the transit circle are those of a mural circle 

 and of a transit combined. A very ingeniously contrived plumb-line 

 was applied to Groombridge's circle, to level the cross axis and 

 adjust the collimation in altitude ; but the methods already described 

 were found to be more accurate and much less troublesome. Indeed, 

 the plumb-line apparatus of the mural circle is superseded by the use 

 of observations by reflection. 



* In 1851 the mural circle and transit instrument of the Royal Obser- 

 tory, Greenwich, were replaced by a transit circle, the design of which 

 is due to Mr. Airy, the present astronomer-royal. This fine instrument 

 was constructed by Messrs. Ransonie and May, of Ipswich, and Mr. 

 Simms, of London. The telescope has an aperture of 8 inches, and a 



* This paragraph and the one which follows has been added to the original 

 article by the Rev. Richard Sheepshanks. 



