The Practical Astronomer / Comprising illustrations of light and colours--practical descriptions of all kinds of telescopes--the use of the equatorial-transit--circular, and other astronomical instruments, a particular account of the Earl of Rosse's large telescopes, and other topics connected with astronomy

by Thomas Dick

CHAPTER III.

ON OBSERVATORIES.

In order to make observations, with convenience and effect, on the heavenly bodies, it is expedient that an observatory, or place for making the requisite observations, be erected in a proper situation. The following are some of the leading features of a spot adapted for making celestial observations: 1. It should command an extensive visible horizon all around, particularly towards the south and the north. 2. It should be a little elevated above surrounding objects. 3. It should be, if possible, at a considerable distance from manufactories, and other objects which emit much smoke or vapour, and even from chimney-tops where no sensible smoke is emitted, as the heated air from the top of funnels causes undulations in the atmosphere. 4. It should be at a distance from swampy ground or valleys that are liable to be covered with fogs and exhalations. 5. It should not, if possible, be too near public roads, particularly if paved with stones, and frequented by heavy carriages, as in such situations, undulations and tremulous motions may be produced, injurious to the making of accurate observations with graduated instruments. 6. It is expedient that the astronomical observer should have access to some distant field within a mile of the observatory, on which a meridian mark may be fixed, after his graduated instruments are properly adjusted. The distance at which a meridian mark should be erected will depend in part on the focal length of the telescope generally used for making observations on the Right Ascensions and declinations of the stars. It should be fixed at such a distance that the mark may be distinctly seen without altering the focus of the telescope when adjusted to the sun or stars, which, in most cases, will require to be at least half a mile from the place of observation, and more if it can be obtained.

Observatories may be distinguished into public and private. A private observatory may be comprehended in a comparatively small building, or in the wing of a building of ordinary dimensions for a family, provided the situation is adapted to it. Most of our densely-peopled towns and cities, which abound in narrow streets and lanes, are generally unfit for good observatories, unless at an elevated position at their extremities. Public observatories, where a great variety of instruments is used, and where different observers are employed, require buildings of larger dimensions, divided into a considerable number of apartments. The observatory of Greenwich is composed principally of two separate buildings—one of which is the observatory properly so called, where the assistant lives and makes all his observations; the other is the dwelling-house in which the astronomer-royal resides. The former consists of three rooms on the ground-floor, the middle of which is the assistant’s sitting and calculating room, furnished with a small library of such books only as are necessary for his computations, and an accurate clock made by the celebrated Graham, which once served Dr. Halley as a transit-clock. Immediately over this is the assistant’s bed-room, with an alarum to awake him to make his observations at the proper time. The room on the eastern side of this is called the transit-room, in which is an 8 feet transit instrument, with an axis of 3 feet, resting on 2 pieces of stone, made by Mr. Bird, but successively improved by Messrs. Dollond, Troughton and others. Here is also a chair to observe with, the back of which lets down to any degree of elevation that convenience may require. On the western side is the quadrant room, with a stone pier in the middle running north and south, having on its eastern face a mural quadrant of 8 feet radius, by which observations are made on the southern quarter of the meridian, through an opening in the roof, of 3 feet wide, produced by means of two sliding shutters. On the western face is another mural quadrant of 8 feet radius, the frame of which is of iron, and the arch of brass, which is occasionally applied to the north quarter of the meridian. In the same room is the famous zenith sector, 12 feet long, with which Dr. Bradley made the observations which led to the discovery of the nutation of the earth’s axis and the aberration of the light of the fixed stars. Here are also Dr. Hooke’s reflecting quadrant and three time-keepers by Harrison. On the south side of this room a small wooden building is erected for the purpose of observing the eclipses of Jupiter’s satellites, occultations of stars by the moon, and other phenomena which require merely the use of a telescope, and the true or mean time. It is furnished with sliding shutters on the roof and sides to view any part of the hemisphere from the Prime Vertical down to the southern horizon. It contains a 40-inch achromatic, with a triple object-glass; and also a 5 feet achromatic by Messrs. John and Peter Dollond—a 2 feet reflecting telescope by Edwards, and a 6 feet reflector by Herschel. Above the dwelling-house is a large octagonal room, which is made the repository for certain old instruments, and for those which are too large to be used in the other apartments. Among many other instruments, it contains an excellent 10 feet achromatic by Dollond, and a 6 feet reflector by Short. Upon a platform, in an open space, is erected the great reflecting telescope constructed by Mr. Ramage of Aberdeen, on the Herschelian principle, which has a speculum of 15 inches diameter, and 25 feet focal length, remarkable for the great accuracy and brilliancy with which it exhibits celestial objects. Various other instruments of a large size, and of modern construction, have of late years been introduced into this observatory, such as the large and splendid transit instrument constructed by Troughton, in 1816—the two large mural circles by Troughton and Jones—the transit clock, by Mr. Hardy, and several other instruments and apparatus which it would be too tedious to enumerate and describe.

Every observatory, whether public or private, should be furnished with the following instruments. 1. A transit instrument for observing the meridian passage of the sun, planets and stars. 2. A good clock whose accuracy may be depended upon. 3. An achromatic telescope, at least 44 inches focal distance, with powers of from 45 to 180 for viewing planetary and other phenomena—or, a good reflecting telescope at least 3 feet long, and the speculum 5 inches diameter. 4. An equatorial instrument, for viewing the stars and planets in the day-time, and for finding the Right Ascension and declination of a comet, or any other celestial phenomenon. Where this instrument is possessed, and in cases where no great degree of accuracy is required, the equatorial may be made to serve the general purposes of a transit instrument.

A private observatory might be constructed in any house which has a commanding view of the heavens, provided there is an apartment in it, in which windows may be placed, or openings cut out fronting the north, the south, the east and the west. The author of this work has a small observatory erected on the top of his house, which commands a view of 20 miles towards the east, 30 miles towards the west, and north-west, and about 20 miles towards the south, at an elevation of above 200 feet above the level of the sea, and the banks of the Tay, which are about half a mile distant. The apartment is 12½ feet long by 8½ wide, and 8½ feet between the floor and the roof. It has an opening on the north by which observations can be made on the pole-star; a window on the south by which the meridian-passages of the heavenly bodies may be observed; another opening towards the east, and a fourth opening, consisting of a door, towards the west. There is a pavement of lead on the outside, all around the observatory-room, enclosed by a stone parapet 3½ feet high, the upper part of which is coped with broad flat stones, in certain parts of which groves or indentations are made for receiving the feet of the pedestal of an achromatic telescope, which form a steady support for the telescope in the open air, when the weather is calm and serene, and when observations are intended to be made on any region of the heavens. By placing an instrument on this parapet, it may be directed to any point of the celestial canopy, except a small portion near the northern horizon, which is partly intercepted by a small hill. In the following ground-plan, fig. 91. AAA, is the parapet surrounding the observatory-room; BBB, a walk around it nearly 3 feet broad, covered with lead. O is the apartment for the observatory, having an opening C to the north, another opening D to the east, E is a window which fronts the south, and F is a door fronting the west, by which an access is obtained to the open area on the outside. GHI is an area on the outside towards the south, covered with lead, 15 feet long from G to H, and 6½ feet from E to I, from which a commanding view of the southern, eastern and western portions of the heavens may be obtained: eeee are positions on the top of the parapet where a telescope may be conveniently placed, when observations are intended to be made in the open air. The top of this parapet is elevated about 30 feet from the level of the ground. On the roof of the observatory, about 12 feet above its floor, on the outside is a platform of lead, surrounded by a railing, 6 feet by 5, with a seat, on which observations either on celestial or terrestrial objects may occasionally be made. K is a door or hatchway, which forms an entrance into the observatory from the apartments below, which folds down, and forms a portion of the floor.

figure 91.

In the perspective view of the building fronting the title-page, the position and general aspect of the observatory-part of the building may be more distinctly perceived.

In public observatories, where zenith or polar distances require to be measured, it is necessary that there should be a dome, with an opening across the roof and down the north and south walls. Should an altitude or azimuth circle, or an equatorial instrument be used, they will require a revolving roof with openings and doors on two opposite sides, to enable an observer to follow a heavenly body across all the cardinal points. The openings may be about 15 inches wide, and the roof needs not be larger than what is requisite for giving room to the observer and the instrument, lest its bulk and weight should impede its easy motion. There have been various plans adopted for revolving domes. Fig. 92 represents a section of the rotatory dome constructed at East Sheen by the Rev. Dr. Pearson. This dome turns round on three detached spheres of lignum vitæ, in a circular bed, formed partly by the dome, and partly by the cylindrical frame-work, which surrounds the circular room of 9 feet diameter. A section of this bed forms a square which the sphere just fills, so as to have a small play to allow for shrinking; and, when the dome is carried round, the spheres, having exactly equal diameters of 4¼ inches each, when placed at equal distances from one another, keep their relative places, and move together in a beautifully smooth manner. These spheres act as friction rollers in two directions at the four points of contact, in case any obstacle is opposed to their progressive motion by the admission of dirt, or by any change of figure of the wood that composes the rings of the dome, and of the gang-way. No groove is here made, but what the weight of the roof resting on the hard sphere occasions. The dome itself moves twice round for the balls once, and has, in this way, its friction diminished. The wood of this dome is covered by Wyatt’s patent copper, one square foot of which weighs upwards of a pound; and the copper is so turned over the nails that fix it at the parts of junction, that not a single nail is seen in the whole dome. This covering is intended to render the dome more permanent than if it had been made of wood alone. At the observatory at Cambridge the dome is made chiefly of iron. In the figure a, a represents one of the two oblong doors that meet at the apex of the cone, and a piece of sheet-copper bent over the upper end of the door which shuts last, keeps the rain from entering at the place of junction. The two halves of the dome are united by brass rods passing through the door-cheeks of wainscot at a and a by means of nuts that screw upon their ends, which union allows the dome to be separated into two parts when there may be occasion to displace it. The wooden plate bb, which appears in a straight line, is a circular broad ring to which the covering wainscot boards are made fast above the eaves, and cc is a similar ring forming the wall-plate or gang-way on which the dome rests and revolves.

figure 92.

figure 92*.

Fig. 92* shows a small door that lies over the summit of the dome, and may be separately opened for zenith observations; the rod of metal with a ring at the lower end passing through it, serves to open and shut this door, and at the same time carries upon its upper end a large ball that falls back on the roof when the door is open, and keeps the door in a situation to be acted upon by the hook of a handle that is used for this purpose. The doors aa being curved, are made to open in two halves, the upper one being opened first, on account of its covering the end of the other; and the observer may open one or two doors as may best suit his purpose. The weight of this dome is such that a couple of wedges, inserted by a gentle blow between the rings bb and cc, will keep it in its situation under the influence of the strongest wind.

It may not be improper to remark, that in all observatories, and in every apartment where celestial observations are made, there should, if possible, be a uniform temperature; and consequently a fire should never be kept in such places, particularly when observations are intended to be made, as it would cause currents of air through the doors and other openings, which would be injurious to the accuracy of observations. When a window is opened in an ordinary apartment where a fire is kept, there is a current of heated air which rushes out at the top, and a current of cold air which rushes in from below, producing agitations and undulations, which prevent even a good telescope from showing celestial objects distinct and well defined; and, I have no doubt, that many young observers have been disappointed in their views of celestial phenomena, from this circumstance, when viewing the heavenly bodies from heated rooms in cold winter evenings; as the aërial undulations before the telescope prevent distinct vision of such objects as the belts of Jupiter, the spots of Mars, and the rings of Saturn.