Жұлдыздар өтуін уақытты өлшеуге арналған аспап

Astronomical instrument for timing of the passage of stars

The meridian circle is an instrument for timing of the passage of stars across the local meridian, an event known as a culmination, while at the same time measuring their angular distance from the nadir. These are special purpose telescopes mounted so as to allow pointing only in the meridian, the great circle through the north point of the horizon, the north celestial pole, the zenith, the south point of the horizon, the south celestial pole, and the nadir. Meridian telescopes rely on the rotation of the sky to bring objects into their field of view and are mounted on a fixed, horizontal, east–west axis. The similar transit instrument, transit circle, or transit telescope is likewise mounted on a horizontal axis, but the axis need not be fixed in the east–west direction. For instance, a surveyor's theodolite 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. Before spectroscopy, photography, and the perfection of reflecting telescopes, the measuring of positions (and the deriving of orbits and astronomical constants) was the major work of observatories.

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.

The earliest transit telescope was not placed in the middle of the axis, but nearer to one end, to prevent the axis from bending under the weight of the telescope. Later, it was usually placed in the centre of the axis, which consisted of one piece of brass or gun metal with turned cylindrical steel pivots at each end. Several instruments were made entirely of steel, which was much more rigid than brass. The pivots rested on V shaped bearings, either set into massive stone or brick piers which supported the instrument, or attached to metal frameworks on the tops of the piers. The temperature of the instrument and local atmosphere were monitored by thermometers. 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 with friction rollers, suspended from a lever supported by the pier, counterbalanced so as to leave only a small fraction of the weight on the precision V shaped bearings. In some cases, the counterweight pushed up on the roller bearings from below. The bearings were set nearly in a true east–west line, but fine adjustment was possible by horizontal and vertical screws. A spirit 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. Near each end of the axis, attached to the axis and turning with it, was a circle or wheel for measuring the angle of the telescope to the zenith or horizon. Generally of 1 to 3 feet or more in diameter, it was divided to 2 or 5 arcminutes, on a slip of silver set into the face of the circle near the circumference. These graduations were read by microscopes, 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 a micrometer screw, which moved crosshairs, 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. 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. 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 prevent flexure. The connection to the axis was also as firm as possible, as flexure of the tube would affect declinations deduced from observations. The flexure in the horizontal position of the tube was determined by two collimators—telescopes placed horizontally in the meridian, north and south of the transit circle, with their objective 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. The collimators were often permanently mounted in these positions, with their objectives and eyepieces fixed to separate piers. 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 that eyepiece 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. 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. 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.

Meridian circles required precise adjustment to do accurate work. The rotation axis of the main telescope needed to be exactly horizontal. A sensitive spirit 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 of mercury, 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 a circumpolar 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 are zenith telescopes designed to point straight up at or near the zenith for extreme precision measurement of star positions. They use an altazimuth 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 a micrometer.

The idea of having an instrument (quadrant) fixed in the plane of the meridian occurred even to the ancient astronomers and is mentioned by Ptolemy, but it was not carried into practice until Tycho Brahe constructed a large meridian quadrant. Meridian circles have been used since the 18th century to accurately measure positions of stars in order to catalog them. This is done by measuring the instant when the star passes through the local meridian. Its altitude above the horizon is noted as well. Knowing one's geographic latitude and longitude these measurements can be used to derive the star's right ascension and declination. 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 the atomic clock this was the most reliable source of accurate time.

In the Almagest, 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 the ecliptic.

A meridian circle enabled the observer to simultaneously determine right ascension and declination, 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 an astronomical sextant being preferred. These methods were very inconvenient, and in 1690, Ole Rømer invented the transit instrument.

The transit instrument consists of a horizontal axis in the direction east and west resting on firmly fixed supports, and having a telescope fixed at right angles to it, revolving freely in the plane of the meridian. At the same time Rømer invented the altitude and azimuth 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. This latter idea was, however, not adopted elsewhere, although the transit instrument soon came into universal use (the first one at Greenwich being mounted in 1721), and the mural 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 by Jesse Ramsden, who also improved the method of reading off angles by means of a micrometer microscope as described below.

The making of circles was shortly afterwards taken up by Edward Troughton, who constructed the first modern transit circle in 1806 for Groombridge's observatory at Blackheath, the Groombridge Transit Circle (a meridian transit circle). Troughton afterwards abandoned the idea and designed the mural circle to take the place of the mural quadrant. 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 by Johann Georg Repsold and Georg Friedrich von Reichenbach were mounted at Göttingen, and one by Reichenbach at Königsberg. 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 of Harvard College, Cambridge University and Edinburgh University had large circles by Troughton and Simms. The Airy Transit Circles at the Royal Greenwich Observatory (1851) and that at the Royal Observatory, Cape of Good Hope (1855) were made by Ransomes and May of Ipswich. The Greenwich instrument had optical and instrumental work by Troughton and Simms to the design of George Biddell Airy.

A modern day example of this type of telescope is the 8 inch (~0.2m) Flagstaff Astrometric Scanning Transit Telescope (FASTT) at the USNO Flagstaff Station Observatory. Modern meridian circles are usually automated. The observer is replaced with a CCD 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:
The CCD can collect light for as long as the image is crossing it, allowing a dimmer limiting magnitude to be reached. The data can be collected for as long as the telescope is in operation – an entire night is possible, allowing a strip of sky many degrees in length to be scanned. Data can be compared directly to any reference object which happens to be within the scan – usually a bright extragalactic object, like a quasar, with an accurately known position. This eliminates the need for some of the painstaking adjustment of the meridian instrument, although monitoring of declination, azimuth, and level is still performed with CCD scanners and laser interferometers. Atmospheric refraction can be accounted for automatically, by monitoring temperature, pressure, and dew point of the air electronically. Data can be stored and analyzed at will. The first automated instrument was the Carlsberg Automatic Meridian Circle, which came online in 1984.