Transit Circle, an astronomical instrument for determining the absolute positions of the heavenly bodies. As these positions are given by two independent elements, the right ascension and declination, corresponding to geographical longitude and latitude, so this instrument is a combination of two independent constructions, each giving its share to the name of the whole, and each furnishing its corresponding element by independent and yet simultaneous observation. The transit circle now forms an essential part of the equipment of every well constituted observatory. The two constructions which have here combined their powers are the transit instrument and the meridian or vertical circle. The former consists of a telescope whose tube is composed of two slightly conical portions firmly secured at their bases to opposite sides of a hollow central cube, from two other opposite sides of which proceed also equal cones of more massive make, generally indeed cast in the same piece with the cube, and forming an axis at right angles with the telescope.
At or near the extremities of this axis are two perfectly cylindrical, highly finished pivots of hardened steel, corresponding in position to sockets resting upon stone columns which, based firmly in the ground, exactly east and west of each other, and rising to a convenient height, support the instrument so that the telescope revolves freely between them in the plane of the meridian. This gives the simple transit instrument, by which and its necessary accompaniment, the clock, is observed the time of meridian passage (the transit) of the star whose place is to be determined. If now we attach firmly to the axis a finely graduated circle which will revolve with the telescope, we shall be enabled, by means of its divisions, to measure also the precise altitude of the star at the same instant of culmination; and thus the transit circle will give, by the first observation, the desired right ascension, and by the second, the desired declination of the object. This combination is entirely of modern date.
Transit instruments and meridian arcs and circles have been used ever since the days of Roemer and Picard, but the first real conjunction of the two dates from the close of the first quarter of the present century. - The sockets of the transit circle receive the pivots and determine the position of the instrument. They are not formed, as might be supposed, of circular "boxes" accurately fitting the pivots, but are simply solid little pieces of gun metal, cut away at the upper surface by two planes inclined to each other like the sides of the letter V, from which letter they take their technical and convenient name. In these V's the pivots revolve smoothly and truly, touching the inclined sides at but two points, and consequently without the lateral play which it would be impossible to avoid in circular boxes, however truly ground. Again, the stone piers upon which the instrument rests, even though wrought into perfect symmetry and equality in every respect, and though posited in such a manner as to furnish no apprehension of relative change, will yet continually manifest such change, sometimes under the influence of varying temperature from day to night and night to day, but more frequently from causes even more irregular and less known than this.
In order therefore to be able to keep the axis of the instrument duly east and west and truly horizontal, the V's are not permanently bedded in the stone, but are so held by strong plates of the same material, themselves permanently fastened, as to allow of small changes of position, one in a horizontal and the other in a vertical direction. Passing next to the telescope, we notice that the narrower ends of the tapering tubes are terminated by flat rings of precisely the same dimensions, upon which are fitted caps containing, one the object glass and the other the eye tube with its mechanisms. These caps are exactly of equal weight, and, partially entering the ends of the tube, their centres of gravity fall truly in the line of junction with the telescope, thus the instrument is not only perfectly counterpoised, but also, the caps being convertible, the object glass and eye tube may be and should be periodically interchanged, in order to eliminate from an average result the effect of a possible flexure of the tube. The object glass presents nothing worthy of especial remark. The construction of the eye piece is peculiar.
The term "eye piece" is generally, though incorrectly, applied to the whole mechanism at the eye end of the telescope, which consists of a small tube sliding in the end cap, and carrying not only the eye piece proper, which is of the form known as Ramsden's (see Telescope), but also a conveniently shaped box containing two thin metallic plates. These plates, called diaphragms, are made with central openings, across which are stretched the threads used to mark the star's position in or its progress through the field. One of these diaphragms is used for the observation of transits, and is securely held in place by fine " antagonist" adjusting screws. Across its opening and precisely through the centre of the field is stretched vertically a most delicate thread of spider's web, which, as the instrument revolves, represents to the observer's eye the meridian as a visible line across which the heavenly bodies are seen to pass at the moment of culmination. In order to gain more accuracy in this observation (for the instant of transit is required to be known within a small fraction of a second), other threads are also introduced parallel with the central one and symmetrically disposed on either side of it, so that, by noting the time of crossing each and taking the average, a very great degree of accuracy is attained.
Ordinarily the transit diaphragm contains either five or seven threads, all at equal intervals; but for special purposes their number and arrangement are adapted to the circumstances. With a telegraphic method of registry, as practised with the large transit instrument of the Washington observatory, five different sets or tallies, with five threads in each, are sometimes used. Across the same diaphragm is stretched horizontally another fixed thread, as a guide to the observer in placing the telescope so that the star shall traverse the centre of the field. The second diaphragm, carrying only a single horizontal thread, is movable in a vertical direction between truly fitting guides, and by means of a finely wrought micrometer screw. As the first plate belongs to the transit portion of the twofold construction, so this one belongs to and cooperates with the circle, and the office of the screw which carries it is to measure the exact distance of the star, as it traverses the field, either from the fixed horizontal thread, or from some other definite starting point, which may be represented upon the scale of the screw without being necessarily visible.
Attached to the screw and revolving with it is a small disk or "head," whose edge is divided into 100 equal parts, so as to measure very accurately the fractions of a revolution, while the whole number of turns necessary to carry the thread to any part of the field is registered upon a convenient scale usually placed within the eye piece and visible with the threads themselves. In order to render thread and scale visible by night, various contrivances are used, the most common of which is to introduce a flat oval ring with whitened surface into the central cube, and with its plane inclined at an angle of 45° with the axis, so that, receiving light thrown in through an orifice in the pivots, it will reflect sufficient into the field to show the threads as black lines upon a bright ground. Sometimes also the illumination is thrown upon the threads themselves, when they appear as bright lines upon a dark ground; and in the great transit circle at Greenwich a very ingenious combination of prisms enables the observer to produce either effect at pleasure. - Upon each half of the axis, between the cube and the pivots, is a circle whose diameter is usually from one third to one half the length of the telescope.
These circles with their several radii and cross bars are generally cast each in a single piece, to insure greater firmness and avoid unequal tensions. But the six-foot circles of the Greenwich instrument just mentioned, weighing about 300 lbs. each, are made of two castings, the rim in one, and the whole system of radii and braces in another, the two being afterward firmly bolted together at 12 equidistant points. Upon a narrow band of silver inserted near the circumference of the circles are cut the graduations required for the special office of each; one, used only for pointing the telescope in any given direction, is divided so as to read with a vernier to single minutes, which is abundantly sufficient; the other circle, intended for the exact measurement of angles, is divided with the most scrupulous accuracy into arcs of two, three, or five minutes, as the case may be, and, once fixed upon the axis, should never during observations be handled or subjected to unequal pressure or strain of any sort.
Assuming now that these division marks are truly cut, we next look for the means of subdividing the small arcs into seconds and fractions of seconds, and find this accomplished by a system of " reading microscopes." These are microscopes of the ordinary compound construction, but each provided with a micrometer screw carrying, as in. the German instruments, a pair of close parallel threads between which the image of the division under consideration can be placed with great accuracy, or, as in Troughton's form, two threads crossing each other at a very acute angle, which may be bisected by the division. The microscopes are so made that one revolution of the screw is equal to a minute, and the micrometer head is divided into 60 equal parts, each of which therefore represents a second. There are usually four of these microscopes placed 90 degrees apart; but sometimes as many as six are used for greater certainty, both from the greater number of readings and from the probable reduction of the systematic errors of the primary division. The proper method of supporting these microscopes to insure their perfect stability has been a subject of much study.
A favorite plan has been to place them on the periphery of another smaller circle which rests, accurately fitting, upon the axis itself, but is prevented from revolving with it by a small projecting bar caught below between two screws attached to the pier. Experience, however, seems to have decided in favor of securing firmly and independently upon the pier itself, near the V-plate, a solid block of metal which serves as the centre of a strong square frame at whose corners the microscopes are attached by adjusting screws. The microscopes are thus entirely disconnected from the circle; and although every new adjustment of the axis will show itself in their record of the graduations, yet this produces no effect whatever upon the mean of readings of opposite microscopes. In the Greenwich instrument, whose piers are broader than the circles themselves, the microscopes are very long, and are passed through the pier itself, converging from the rim of the circle until their eye pieces are collected within a very small space, where the observer reads them with convenience and ease.
The graduated limb is bevelled to suit this arrangement, and from another point near the observer a small gas-burner radiates light through other openings in the pier in such a manner as to illuminate uniformly the field of each microscope; a matter of very high practical importance. - To bring the instrument into its proper place in the meridian, it is necessary that the middle vertical thread of the fixed diaphragm be placed truly in the optical axis of the telescope, which is the central line of the cone of rays converging from the object glass. This may be effected by turning the telescope to a very distant fixed object, noting the exact position of this middle thread with reference to the images in the field, and then reversing the instrument, when the thread will probably occupy a different position, whereupon it must be brought by the adjusting screws of the diaphragm to a point midway between the first and second places, and the operation repeated until no change appears upon reversal. Next, by means of a spirit level and the vertical adjusting screws of one of the V-plates, the axis of the instrument is rendered truly horizontal; and finally, the approximate sidereal time being known, the telescope is directed to some star, also known, very near the pole of the heavens, and the axis moved by the horizontal adjusting screws of the other V-plate, until at the right moment the star and thread coincide exactly.
The three errors thus corrected are denominated the errors in collimation, level, and azimuth respectively. And now, by help of stellar observations under properly varied circumstances, we are able not only to determine with great precision the small outstanding values of these errors, which by no means remain constant for any length of time, but also to judge the clock that aided us, and finally the very places of the stars that have served as our I guides. The errors of instrument and clock having been thus determined, it is possible, bythe aid of formulas and methods which have been so thoroughly developed and systematized as to be applicable with the greatest facility, to obtain by a single observation of any new object its right ascension within a very small fraction of a second of time. - But, as we have intimated, the chief value of the instrument consists in its power of furnishing at the same culmination not only the right ascension but also the declination of the object, and it accomplishes the latter most simply in the following manner.
While the observer is noting the progress of the star across the transit threads, he at the same time, by a delicate movement of the telescope in altitude, places it so that the star appears to run along the fixed horizontal thread; and then, the transit observation having been completed, he reads, even to the fraction of a second, from the circle microscopes the precise point corresponding to the apparent altitude of the star. Or, a still more accurate determination is obtained by placing the telescope so that the star will traverse the field at a little distance above or below the fixed thread; and there is ordinarily time enough to bring the movable thread several times into coincidence with the star's image by means of the micrometer screw, always noting its indications and afterward taking the mean of all. The small distance from the fixed thread, as thus measured, must of course be duly applied as a correction to the readings of the microscopes, and thus we derive one extremity of the desired arc, and then proceed to find the other. In order to know the star's declination, we must first have its altitude above the horizon.
This can sometimes be obtained by a double observation of the star's image, first as reflected from a quicksilver surface, and then as seen directly, in which case the arc included between these two directions is obviously equal to twice the altitude of the star; but this course is not always applicable. We have however a readier and exquisitely beautiful method of obtaining with very great accuracy the direction of the vertical line, from which we can count the star's zenith distance. The telescope being turned so as to look directly downward, we place immediately beneath it a vessel of quicksilver; and if then, by means of a small plate of thin glass held at an angle of 45°, we reflect a strong light down the telescope, it will be reflected back by the quicksilver, and, looking through the glass from above, we shall see not only the threads in the eye piece, but also the reflected image of each; and by moving the instrument carefully until the fixed horizontal thread coincides with its own image, we shall have the telescope mathematically vertical, and may read from the circle the corresponding-second point of the desired arc, whereby we obtain the apparent altitude, and thence, correcting for refraction, the true altitude, and finally the desired declination. - A few words must be added respecting the use of transit instruments in the prime vertical, that is, so placed that the great circle described by the collimation axis is in the prim© vertical.
Bes-sel first suggested this method of mounting a transit instrument, for the purpose of determining with special accuracy the latitude of the place of observation. It is manifest that any star which has a north declination less than l, where l is the latitude, crosses the prime vertical at equal altitudes on the eastern and western quadrants. If the interval in time between these passages be noted as=2t, it is manifest from the right-angled spherical triangle having for its angles the pole, the zenith, and the star's place in either quadrant of the prime vertical, that tan l=tan δ sec t. This method of determining the latitude has a great advantage in the readiness with which instrumental errors may be got rid of, by using the instrument alternately in opposite positions as respects the rotation axis. The adjustments for a transit instrument in the prime vertical relate, like those of the meridional transit instrument, to the three points, collimation, level, and azimuth. In collimation the adjustment resembles that of the ordinary transit instrument. The instrument is brought nearly into the prime vertical by directing it to a star of small northerly declination at the calculated time of the star's passage of the prime vertical.
When this has been done, the rotation axis must be carefully levelled, and a fresh adjustment made by means of another star. For the small adjustment thus rendered necessary provision is made by allowing one of the V's a small motion in azimuth. Another method is to have the instrument provided with a graduated horizontal circle, and then, having adjusted it in the meridian, to revolve it through 90° in azimuth. When the rotation axis is in the meridian but inclined to the horizon, a correction can be readily made for this inclination, because the great circle described by the collimation axis crosses the horizon at the true east and west points, but passes slightly to the north or to the south of the true zenith; and the latitude found by means of the instrument corresponds to the latitude of the point where the great circle thus swept out crosses the meridian. Thus the only required consideration of the level correction is that this correction should be applied directly to the latitude found from the instrument used as if correctly adjusted.
But if the rotation axis is neither in the meridian nor level, or if the middle thread is not in the collimation axis, the correction is less simple. (See Chauvenet's "Spherical and Practical As-tronomv," vol. ii., p. 242).