Themeridian circle is an instrument for timing of the passage ofstars across the localmeridian, an event known as aculmination, while at the same time measuring their angular distance from thenadir. These are special purposetelescopes mounted so as to allow pointing only in themeridian, thegreat circle through the north point of the horizon, the northcelestial pole, thezenith, the south point of the horizon, the south celestial pole, and thenadir. Meridian telescopes rely on the rotation of the sky to bring objects into theirfield of view and are mounted on a fixed, horizontal, east–west axis.
The similartransit instrument,transit circle, ortransit telescope is likewise mounted on a horizontal axis, but the axis need not be fixed in the east–west direction. For instance, a surveyor'stheodolite can function as a transit instrument if its telescope is capable of a full revolution about the horizontal axis. Meridian circles are often called by these names, although they are less specific.
For many years, transit timings were the most accurate method of measuring the positions of heavenly bodies, and meridian instruments were relied upon to perform this painstaking work. Beforespectroscopy,photography, and the perfection ofreflecting telescopes, the measuring of positions (and the deriving oforbits andastronomical constants) was the major work ofobservatories.[1][2][3]
Fixing a telescope to move only in themeridian has advantages in the high-precision work for which these instruments are employed:
The state of the art of meridian instruments of the late 19th and early 20th century is described here, giving some idea of the precise methods of construction, operation and adjustment employed.[4][5]
The earliest transittelescope was not placed in the middle of the axis, but nearer to one end, to prevent the axis from bending under theweight of the telescope. Later, it was usually placed in the centre of the axis, which consisted of one piece ofbrass orgun metal with turned cylindrical steel pivots at each end. Several instruments were made entirely ofsteel, which was much more rigid than brass. The pivots rested on V-shapedbearings, either set into massive stone or brick piers which supported the instrument, or attached to metal frameworks on the tops of the piers.[6] The temperature of the instrument and local atmosphere were monitored by thermometers.[7]The piers were usually separate from the foundation of the building, to prevent transmission of vibration from the building to the telescope. To relieve the pivots from the weight of the instrument, which would have distorted their shape and caused rapid wear, each end of the axis was supported by a hook or yoke withfriction rollers, suspended from alever supported by the pier,counterbalanced so as to leave only a small fraction of the weight on the precision V-shaped bearings.[6] In some cases, the counterweight pushed up on the roller bearings from below.[8] The bearings were set nearly in a true east–west line, but fine adjustment was possible by horizontal and vertical screws. Aspirit level was used to monitor for any inclination of the axis to the horizon. Eccentricity (an off-center condition) or other irregularities of the pivots of the telescope's axis was accounted for, in some cases, by providing another telescope through the axis itself. By observing the motion of an artificial star, located east or west of the center of the main instrument, and seen through this axis telescope and a small collimating telescope, as the main telescope was rotated, the shape of the pivots, and any wobble of the axis, could be determined.[9]
Near each end of the axis, attached to the axis and turning with it, was a circle or wheel for measuring theangle of the telescope to the zenith or horizon. Generally of 1 to 3 feet or more in diameter, it was divided to 2 or 5arcminutes, on a slip of silver set into the face of the circle near the circumference. Thesegraduations were read bymicroscopes, generally four for each circle, mounted to the piers or a framework surrounding the axis, at 90° intervals around the circles. By averaging the four readings the eccentricity (from inaccurate centering of the circles) and the errors of graduation were greatly reduced. Each microscope was furnished with amicrometer screw, which movedcrosshairs, with which the distance of the circle graduations from the centre of the field of view could be measured. The drum of the screw was divided to measure single seconds of arc (0.1" being estimated), while the number of revolutions were counted by a comb like scale in the field of view. The microscopes were given such magnification and placed at such a distance from the circle that one revolution of the micrometer screw corresponded to 1 arcminute (1') on the circle. The error was determined occasionally by measuring standard intervals of 2' or 5' on the circle. The periodic errors of the screw were accounted for.[10] On some instruments, one of the circles was graduated and read more coarsely than the other, and was used only in finding the target stars.
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The telescope consisted of two tubes screwed to the central cube of the axis. The tubes were usually conical and as stiff as possible to help preventflexure. The connection to the axis was also as firm as possible, as flexure of the tube would affectdeclinations deduced from observations. The flexure in the horizontal position of the tube was determined by twocollimators—telescopes placed horizontally in the meridian, north and south of the transit circle, with theirobjective lenses towards it. These were pointed at one another (through holes in the tube of the telescope, or by removing the telescope from its mount) so that the crosshairs in their foci coincided.[11] The collimators were often permanently mounted in these positions, with their objectives and eyepieces fixed to separate piers.[12] The meridian telescope was pointed to one collimator and then the other, moving through exactly 180°, and by reading the circle the amount of flexure (the amount the readings differed from 180°) was found. Absolute flexure, that is, a fixed bend in the tube, was detected by arranging thateyepiece and objective lens could be interchanged, and the average of the two observations of the same star was free from this error.
Parts of the apparatus, including the circles, pivots and bearings, were sometimes enclosed in glass cases to protect them from dust. These cases had openings for access. The reading microscopes then extended into the glass cases, while their eyepiece ends and micrometers were protected from dust by removable silk covers.[13]
Certain instrumental errors could be averaged out by reversing the telescope on its mounting. A carriage was provided, which ran on rails between the piers, and on which the axis, circles and telescope could be raised by a screw-jack, wheeled out from between the piers, turned 180°, wheeled back, and lowered again.[11]
The observing building housing the meridian circle did not have a rotating dome, as is often seen at observatories. Since the telescope observed only in the meridian, a vertical slot in the north and south walls, and across the roof between these, was all that was necessary. The building was unheated and kept as much as possible at the temperature of the outside air, to avoid air currents which would disturb the telescopic view. The building also housed the clocks, recorders, and other equipment for making observations.
At thefocal plane, the eye end of the telescope had a number of vertical and one or two horizontal wires (crosshairs). In observing stars, the telescope was first directed downward at a basin ofmercury[14] forming a perfectly horizontal mirror and reflecting an image of the crosshairs back up the telescope tube. The crosshairs were adjusted until coincident with their reflection, and the line of sight was then perfectly vertical; in this position the circles were read for thenadir point.
The telescope was next brought up to the approximatedeclination of the target star by watching the finder circle. The instrument was provided with a clamping apparatus, by which the observer, after having set the approximate declination, could clamp the axis so the telescope could not be moved in declination, except very slowly by a finescrew. By this slow motion, the telescope was adjusted until the star moved along the horizontal wire (or if there were two, in the middle between them), from the east side of the field of view to the west. Following this, the circles were read by the microscopes for a measurement of the apparentaltitude of the star. The difference between this measurement and the nadir point was thenadir distance of the star. A movable horizontal wire or declination-micrometer was also used.[11]
Another method of observing the apparentaltitude of a star was to take half of the angular distance between the star observed directly and its reflection observed in a basin of mercury. The average of these two readings was the reading when the line of sight was horizontal, thehorizontal point of the circle. The small difference inlatitude between the telescope and the basin of mercury was accounted for.
The vertical wires were used for observing transits of stars, each wire furnishing a separate result. The time of transit over the middle wire was estimated, during subsequent analysis of the data, for each wire by adding or subtracting the known interval between the middle wire and the wire in question. These known intervals were predetermined by timing a star of known declination passing from one wire to the other, thepole star being best on account of its slow motion.[11]\Timings were originally made by an "eye and ear" method, estimating the interval between two beats of a clock. Later, timings were registered by pressing a key, the electrical signal making a mark on astrip recorder. Later still, the eye end of the telescope was usually fitted with animpersonalmicrometer, a device which allowed matching a vertical crosshair's motion to the star's motion. Set precisely on the moving star, the crosshair would trigger the electrical timing of the meridian crossing, removing the observer'spersonal equation from the measurement.[15]
The field of the wires could be illuminated; the lamps were placed at some distance from the piers in order not to heat the instrument, and the light passed through holes in the piers and through the hollow axis to the center, whence it was directed to the eye-end by a system ofprisms.[11]
To determine absolute declinations or polar distances, it was necessary to determine the observatory'scolatitude, or distance of thecelestial pole from thezenith, by observing the upper and lower culmination of a number ofcircumpolar stars. The difference between the circle reading after observing a star and the reading corresponding to the zenith was the zenith distance of the star, and this plus the colatitude was the north polar distance. To determine the zenith point of the circle, the telescope was directed vertically downwards at a basin ofmercury, the surface of which formed an absolutely horizontal mirror. The observer saw the horizontal wire and its reflected image, and moving the telescope to make these coincide, its optical axis was made perpendicular to the plane of the horizon, and the circle reading was 180° + zenith point.[14]
In observations of starsrefraction was taken into account as well as the errors of graduation and flexure. If the bisection of the star on the horizontal wire was not made in the centre of the field, allowance was made for curvature, or the deviation of the star's path from a great circle, and for the inclination of the horizontal wire to the horizon. The amount of this inclination was found by taking repeated observations of the zenith distance of a star during the one transit, the pole star being the most suitable because of its slow motion.[16]
Attempts were made to record the transits of a star photographically. Aphotographic plate was placed in the focus of a transit instrument and a number of short exposures made, their length and the time being registered automatically by a clock. The exposing shutter was a thin strip of steel, fixed to the armature of an electromagnet. The plate thus recorded a series of dots or short lines, and the vertical wires were photographed on the plate by throwing light through the objective lens for one or two seconds.[16]
Meridian circles required precise adjustment to do accurate work.[17]
The rotation axis of the main telescope needed to be exactly horizontal. A sensitivespirit level, designed to rest on the pivots of the axis, performed this function. By adjusting one of the V-shaped bearings, the bubble was centered.
The line of sight of the telescope needed to be exactly perpendicular to the axis of rotation. This could be done by sighting a distant, stationary object, lifting and reversing the telescope on its bearings, and again sighting the object. If the crosshairs did not intersect the object, the line of sight was halfway between the new position of the crosshairs and the distant object; the crosshairs were adjusted accordingly and the process repeated as necessary. Also, if the rotation axis was known to be perfectly horizontal, the telescope could be directed downward at a basin ofmercury, and the crosshairs illuminated. The mercury acted as a perfectly horizontal mirror, reflecting an image of the crosshairs back up the telescope tube. The crosshairs could then be adjusted until coincident with their reflection, and the line of sight was then perpendicular to the axis.
The line of sight of the telescope needed to be exactly within the plane of the meridian. This was done approximately by building the piers and the bearings of the axis on an east–west line. The telescope was then brought into the meridian by repeatedly timing the (apparent, incorrect) upper and lower meridian transits of acircumpolar star and adjusting one of the bearings horizontally until the interval between the transits was equal. Another method used calculated meridian crossing times for particular stars as established by other observatories. This was an important adjustment, and much effort was spent in perfecting it.
In practice, none of these adjustments were perfect. The small errors introduced by the imperfections were mathematically corrected during the analysis of the data.
Some telescopes designed to measure star transits arezenith telescopes designed to point straight up at or near thezenith for extreme precision measurement of star positions. They use analtazimuth mount, instead of a meridian circle, fitted with leveling screws. Extremely sensitive levels are attached to the telescope mount to make angle measurements and the telescope has an eyepiece fitted with amicrometer.[18]
The idea of having an instrument (quadrant) fixed in the plane of the meridian occurred even to theancientastronomers and is mentioned byPtolemy, but it was not carried into practice untilTycho Brahe constructed a large meridian quadrant.[6]
Meridian circles have been used since the 18th century to accurately measure positions of stars in order tocatalog them. This is done by measuring the instant when the star passes through the local meridian. Itsaltitude above the horizon is noted as well. Knowing one's geographiclatitude andlongitude these measurements can be used to derive the star'sright ascension anddeclination.
Once good star catalogs were available a transit telescope could be used anywhere in the world to accurately measure local longitude and time by observing local meridian transit times of catalogue stars. Prior to the invention of theatomic clock this was the most reliable source of accurate time.
In theAlmagest, Ptolemy describes a meridian circle which consisted of a fixed graduated outer ring and a movable inner ring with tabs that used a shadow to set the Sun's position. It was mounted vertically and aligned with the meridian. The instrument was used to measure the altitude of the Sun at noon in order to determine the path of theecliptic.[19]
A meridian circle enabled the observer to simultaneously determineright ascension anddeclination, but it does not appear to have been much used for right ascension during the 17th century, the method of equal altitudes by portable quadrants or measures of the angular distance between stars with anastronomical sextant being preferred. These methods were very inconvenient, and in 1690,Ole Rømer invented the transit instrument.[6]
The transit instrument consists of a horizontal axis in the direction east and west resting on firmly fixed supports, and having atelescope fixed at right angles to it, revolving freely in the plane of the meridian. At the same time Rømer invented the altitude andazimuth instrument for measuring vertical and horizontal angles, and in 1704, he combined a vertical circle with his transit instrument, so as to determine both co-ordinates at the same time.[6]
This latter idea was, however, not adopted elsewhere, although the transit instrument soon came into universal use (the first one atGreenwich being mounted in 1721), and themural quadrant continued until the end of the century to be employed for determining declinations. The advantages of using a whole circle, it being less liable to change its figure and not requiring reversal in order to observe stars north of the zenith, were then again recognized byJesse Ramsden, who also improved the method of reading off angles by means of amicrometermicroscope as described below.[6]
The making of circles was shortly afterwards taken up byEdward Troughton, who constructed the first modern transit circle in 1806 forGroombridge'sobservatory atBlackheath, theGroombridge Transit Circle (a meridian transit circle). Troughton afterwards abandoned the idea and designed the mural circle to take the place of the mural quadrant.[6]
In the United Kingdom, the transit instrument and mural circle continued until the middle of the 19th century to be the principal instrument in observatories, the first transit circle constructed there being that at Greenwich (mounted in 1850). However, on the continent, the transit circle superseded them from the years 1818–1819, when two circles byJohann Georg Repsold andGeorg Friedrich von Reichenbach were mounted atGöttingen, and one by Reichenbach atKönigsberg. The firm of Repsold and Sons was for a number of years eclipsed by that of Pistor and Martins in Berlin, who furnished various observatories with first-class instruments. Following the death of Martins, the Repsolds again took the lead and made many transit circles. The observatories ofHarvard College,Cambridge University andEdinburgh University had large circles byTroughton and Simms.[6]
The Airy Transit Circles at theRoyal Greenwich Observatory (1851) and that at theRoyal Observatory, Cape of Good Hope (1855) were made byRansomes and May of Ipswich. The Greenwich instrument had optical and instrumental work by Troughton and Simms to the design ofGeorge Biddell Airy.
A modern-day example of this type of telescope is the 8 inch (~0.2m) Flagstaff Astrometric Scanning Transit Telescope (FASTT) at theUSNO Flagstaff Station Observatory.[20] Modern meridian circles are usually automated. The observer is replaced with aCCD camera. As the sky drifts across the field of view, the image built up in the CCD is clocked across (and out of) the chip at the same rate. This allows some improvements:[21]
The first automated instrument was theCarlsberg Automatic Meridian Circle, which came online in 1984.[22]
{{cite web}}: CS1 maint: archived copy as title (link)Attribution:
This article incorporates text from a publication now in thepublic domain: Dreyer, John Louis Emil (1911). "Transit Circle". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 27 (11th ed.). Cambridge University Press. pp. 181–183.
