« PreviousContinue »
of catalogues, or for special purposes. Certain instrumental errors, such as flexure of the telescope, errors of circle divisions, and want of circularity of pivots, are determined occasionally. Nothing is supposed for a moment to be in perfect adjustment;
no error is mechanically corrected ; everything is supposed to be always wrong, and to be corrected numerically in the after reduction of the observations.
But the Transit Circle has an important auxiliary in the Altitude and Azimuth, or, as it is briefly called, the Altazimuth instrument, which was designed specially for position observations of the moon. Viewing the primary importance of these
observations, and having regard to the impossibility of seeing the moon on the meridian during that critical part of her orbit when she is near the sun, and also to the frequent loss of meridional observations from temporary cloudiness of the sky, Sir George Airy deemed it highly desirable to devise means of securing observations off the meridian comparable in accuracy with those made on the meridian. The only form of instrument competent for this purpose was an altazimuth of massive construction. He accordingly designed and mounted (in 1847) the instrument here figured. It consists of a telescope (4 inches aperture) solidly fixed into a broad-rimmed vertical or altitude circle which has a divided silver limb read by four microscopes. This circle turns between two semi-cylinders of cast-iron connected by top and bottom plates to form a frame which turns on vertical pivots, the lower one borne by a stone pier and the upper one by a triangular framework of iron bars. The vertical frame carries on one side the four microscopes which read the altitude circle, and around its base other four which read a horizontal circle fixed to the stone pier for azimuthal measures. There are levels for showing the inclinations of both axes; and they, and the microscopes, are carried by supports which are cast upon (not screwed to) the parts they spring from. The nightly observations with this instrument consist of an altitude and azimuth of the moon, azimuths of stars for zero of the azimuth circle and collimation error of the telescope, and observations of a distant terrestrial mark for zenith-point error of the vertical circle. All observations are made in two (reversed) positions of the telescope; and all times of exact position-determinations are registered by the chronograph. The azimuths and altitudes are in effect reduced to Right Ascension and Polar Distances by after-calculations, which are very laborious, but are, in common with every computation in the observatory, facilitated by skeleton forms of which about two hundred, simple and elaborate, are in constant use.
The Altazimuth secures about two hundred observations of the moon in a year, the Transit-Circle only one hundred ; and a large percentage of the former are of the highest value as affording tests of the lunar tables for parts of the orbit when the moon is near conjunction with the sun, and cannot be touched by a meridian instrument.
Another auxiliary instrument of the exact class is the Reflex Zenith Telescope, for measuring with great precision the small zenith distance of the star y Draconis, which passes near the zenith of Greenwich, and is favourably situated for determination of the amount of stellar aberration. It consists of a horizontal object-glass, capable of semi-rotation about a yertical axis, with a trough of mercury at half its focal distance below. The rays from the star, after passing through the object-glass, are received upon the mercury and reflected up through the glass again, forming a focal image just above it. In the focal plane are the wires of a micrometer by which the star's image is twice bisected, the object-glass being reversed between the bisections. A measure of the star's zenith distance is thus obtained which is remarkably free from instrumental errors. During the past two years a zenithal telescope of 3-feet focus, filled with water, has been used upon the same star to determine whether the aberration is altered in amount by the passage of the star's light through a considerable thickness of a refracting medium-a question which has been raised by some continental astronomers. The observations hitherto have negatived the anticipated alteration.
Equatorially-mounted telescopes have always been regarded as of secondary importance at Greenwich. When Sir George Airy entered office he found the equipment in this department, to use his own word, “ contemptible.” The best equatorial then was one presented by Sir G. Shuckburgh, made by Ramsden, with a 4-inch object-glass, a shaky mounting, and in a very bad situation. In 1837 a 64-inch glass was presented to the Observatory by the late Rev. R. Sheepshanks; it was a somewhat inferior one, and it was mounted with circles too small for accurate measurements. So about fifteen years ago Sir George Airy planned the Great Equatorial, deeming it essential that the Observatory should have a first-rate instrument of the class, even if it was for a time regarded as a luxury. The instrument (without the covering dome) is figured in the annexed cut. Its object-glass, by Merz, of Munich, is of 12-inch aperture, and 17 feet 6 inches focus. The mounting is of the English form, which admits of large circles, and such were regarded as essential; the hour circle is 6 feet diameter, that for Declination 4 feet. The polar frame is formed of iron columns firmly braced, and connected at top and bottom by oval iron frames. Except the tube, which is of wood, every large part is of iron, and the whole was designed with the highest engineering skill. Gas is carried to all the micrometers about the instrument, and to the eye-piece; and galvanic wires are led to a chronographic touch-piece at the eye-end and to a chronometer there also, which is always in front of the observer at the telescope, and is controlled by the Sidereal Motor Clock. An observing chair, moving up and down upon a travelling frame, enables the observer to command all positions without leaving his seat. · The driving-clock is mounted in a lower apartment; it consists of a water turbine controlled by a conical pendulum.
large circles et diameter, that columns fir
We have spoken hitherto solely of the tools of the Greenwich observers. Let us now pass in review the works that have been actually accomplished thereby in furtherance of the objects for which the Observatory was constituted. We begin with the formation of star catalogues, for these are the foundations of exact astronomy. Flamsteed formed a catalogue of 3,310 stars, that remained the standard work of its kind for half a century, and has, in respect of selection and nomenclature of the stars it comprised, served as the basis of every catalogue since produced. Halley did nothing in this department. Bradley worked wonders; he laid the foundations of the present fabric
of mensurative astronomy. In 1750, when bis new instruments were mounted, he began the daily course of meridional observations of principal stars, sun, moon, and planets which has continued without interruption (save by bad weather) to the present time. His star observations were incorporated into a catalogue (which comprised 3,222 stars) by Bessel, and, with values of the constants of refraction, aberration, precession, and other elements deduced therefrom, was brought forth by the illustrious Königsberg astronomer in a great work, whose title expresses its character-we allude to the “ Fundamenta Astronomiæ.” The rich harvest that Bradley reaped has not yet had all its good grain thrashed out, for at this moment several German astronomers are engaged in a re-reduction of his observations, believing that they can get from them even more than Bessel obtained. Maskelyne did not aim at producing a great catalogue; he confined himself to thirty-six of the principal stars, whose places he sought to fix with the utmost possible exactness, in order that they might serve as reference, points of the first order. Pond accumulated a long series of observations, and produced a catalogue of 1,112 stars, which was the most valuable contribution to the sidereal astronomy of the time, and is second in accuracy to no modern catalogue. The present Astronomer Royal has already produced four large catalogues ; the first from the observations during the twelve years 1836–47 ; the second from the six years 1848–53; the third from the seven years 1854–60, and the fourth from the seven years 1861-67. The numbers of stars in these catalogues average over 2,000 each; each contains all the fundamentals, and together they include the greater portion if not all the stars visible at Greenwich down to the fifth magnitude, and the majority of the sixth. They are sought all the world over, wherever accurate astronomy is pursued for its own sake or for geographical or geodetical purposes, and they have served and will serve again as the best materials for researches upon the proper motion of the stars or of the solar system.
Then as to the moon ; Greenwich has alone sufficiently supplied the investigators of her movements from the time of Newton to the present. At so critical a period was the Observatory established that Flamsteed actually fed Newton, so to speak, from hand to mouth with places of the moon for perfecting the lunar theory which the latter was then deducing from his theory of universal gravitation; and there is an entry in Flamsteeds note-book of the author of the “ Principia" coming down to Greenwich one Sunday evening for twelve observations of the moon of which he was in urgent need. Halley, though he did no star work, made some moon observations, and compared them with his own lunar tables; it was the only good piece of regular work that he did. He was the exception we previously spoke of to the consentaneous character of the Royal Astronomers; and his case shows that a man may be a great astronomer, and yet fail in the office of Royal Observator, which requires the exercise of no small amount of self-denial. The lunar observations of Bradley, Maskelyne, and Pond, have to be spoken of connectedly, for they were reduced en masse by the present Astronomer Royal, and they form the basis of the Lunar Tables that are now in almost universal use. But before this stupendous uniform reduction, the Greenwich Observatory had furnished 1,200 moon observations to improve