| Date: | 1863 |
| Author: | Anonymous |
| Title: | A night at Greenwich Observatory |
| About: | Published as an article in The Cornhill Magazine, Vol VII, (London, Smith Elder & Co.), pp.381–389, March 1863. |
| Images: | None |
| Click here to read the account in its original format |
[From The Cornhill Magazine, Vol VII, (London, Smith Elder & Co.), pp.381–389, March 1863]
A night at the Greenwich Observatory
Greenwich, long known to history, and well-beloved of cockney holidaymakers, is most important in our time for a certain edifice in which the longest calculations, the deepest thought, and the minutest care, are in operation day and night. This is the Observatory on the hill; a building chiefly remarkable to the untaught visitor for a large clock dial, the minute hand of which advances by a series of jerks, as though stepping onwards towards eternity as a matter of private business. The hour hand of this clock may, perhaps, indicate that the time is about half-past twenty-two o'clock—an announcement that somewhat puzzles the untaught visitor, until a volunteer philosopher, who enjoys instructing the ignorant, and who prides himself on knowing this one thing, informs him that here there is no such thing as one, two, or three o'clock in the morning, but that time is counted from noon to noon, and from 0 to 24. Thus, one o'clock A.m. is thirteen o'clock, four o'clock A.m., sixteen o'clock, and so on.
If, desirous of seeing the interior of this mysterious building, we knock at the door, and ask permission to view the establishment, we shall be politely but decidedly informed that "no visitors are admitted." This seclusion is absolutely necessary, for the staff is a hard-worked one, and is not to be interrupted. The instruments are most delicate, and a touch— the resting of a hand on a screw or lever, or even breathing on a portion which is liable to rust—might cause damage or delay, which could not possibly be afforded.
In long calculations, perfect quiet is also necessary, and it is not improbable that the visitation of an occasional organ-grinder near the Observatory might lead to the wreck of some half-dozen ships, which had erroneously calculated their positions by data influenced for ill in consequence of the computer's nerves being tormented. Thus "no admission except on business" is an order rigidly enforced.
If, however, the person desirous of visiting the Observatory be a student, and anxious to acquire a knowledge of the system adopted here, he has merely to obtain the necessary introductions, and he will be received with every politeness and attention. The whole mystery and process of the “observations" and "reductions" will be shown him, with a freedom from reserve that at once indicates the soundness of the principles here adopted. The absence of all mystery,—a condition which too frequently conceals ignorance or defect,—speaks of the desire rather to impart knowledge than to preserve it amongst a select few.
Greenwich Observatory was commenced in 1675. The site selected by Wren was within view of all vessels passing up or down the Thames, and thus information could be readily telegraphed to these vessels from the Observatory. Many additions and improvements have been made to the building since its first erection, the greatest progress having been made in the instruments used for astronomical purposes.
The aim of all the time and labour given at the Royal Observatory, is to give accurately the position of the various heavenly bodies, and, from past and present observations, to be enabled to foretel for two, three, or four years in advance, the exact position of the sun, moon, and stars, at any instant during the twenty-four hours.
Upon the information thus afforded depends the accuracy of all large surveys in various parts of the world. The correct position of ships at sea, the true places also of various dangerous rocks or shoals at sea, can only be found by the aid of the data supplied from Greenwich Observatory. Thus the safety of much that is most valuable is actually dependent, in a great measure, upon the calculations made within the building in Greenwich Park.
Although to afford the information referred to is the chief object of the Observatory, still it is not the only one. A constant watch is maintained on sun, moon, planets and stars, in order to discover the slightest indications of any changes which might be occurring to them; whilst the announcement of a visitor to our system in the shape of a comet, at once entails a fresh series of observations and calculations, in order to determine the distance and course of the stranger.
To watch for the arrival of comets, or to be perpetually searching for new asteroids or planets, forms no part of the labours at the national Observatory; business in the form of observation and calculation being the main object.
Upon entering the small gate upon the left of the clock-face, a courtyard is seen, whilst on the left is the main building. In this building is the principal observing room, in which is the large " transit-instrument" This transit-instrument is the most important thing in the establishment, and deserves a special notice, for with it the greater portion of the work is done.
The transit-instrument is a large telescope, possessing an object-glass of great size, but not of very high magnifying power, the aim being not to magnify objects very much, but to be able to see them clearly and distinctly. In this telescope are placed several small and fine spiders' webs, by means of which the time of a star's passage across the centre of the object-glass can be more accurately ascertained.
Although possessing the appearance of a large cannon, yet so well arranged are the works belonging to this instrument, that one man, by the aid of a finger only, can turn it upwards or downwards as occasion may require.
It is essential to the accuracy of the observations, that the transit instrument should, upon being turned from the zenith towards the horizon, sweep down the exact north or south line in the heavens, and hence when horizontal, that it should point exactly north or south. The slightest deviation, even to the amount of a hair's breadth,—unless the value of this is known—will cause errors in the observations; and thus the greatest precautions are used to place and maintain the instrument in a proper position.
Formerly there was a mark on the Essex coast, near Chingford, which was exactly north of the transit-instrument, and the centre spider's web in the telescope being directed on to this, the instrument could be placed horizontally north and south. To ensure its obtaining a vertical position, it was placed, as nearly as could be guessed, vertically, and a large trough of mercury arranged beneath the object-glass. This mercury would reflect the spiders' lines, which ought then to appear to coincide when seen by direct vision and by reflection. If they should not, a slight alteration of the instrument would be requisite in order to place it in a vertical position.
If the amount of the transit's deviation be known, either from the vertical or north and south position, then all observations made with the instrument may have a correction applied to them, which will ensure as much accuracy as though the instrument had been moved.
Instead of the mark near Chingford being now used to place the transit north and south, two tubes are fixed into the walls of the transit-room, and contain cross hairs, by means of which the value of the deviation can be ascertained, and consequently allowed for.
A very comfortable-looking chair rests on a small railway beneath the transit, and on this the observer reclines, and can move himself backwards or forwards by a mere push. Directly opposite to him is a clock, whose beats are audible, decided, and business-like; and we merely now require some celestial object, and an observer, in order to make " an observation.” We will, however, leave this room at present, and will return to it at night, when the usual business is being carried on.
West of the transit-room we enter the computing-room, where we find some half-dozen computers, who are employed in "reductions; " that is, they are applying certain corrections to the observed time of a star or planet's passage across the centre wire of the transit. We notice that the mean of the times of transits over nine wires is first taken, and then corrections are applied for collimation; that is, for the centre wire, or spider's web, not being exactly in the diameter of the telescope; for level error, that is, in consequence of the trunnions of the great gun telescope not being exactly level; and for azimuthal error, that is for the amount that the transit deviates from the north and south line.
Each star that is observed requires different corrections to be applied for these three items, and thus when, during a long winter night, some hundred stars are observed, the work entailed on the following morning is very considerable. A densely cloudy or foggy night is holiday time at the Observatory, and during the next morning arrears of work can be got up.
Every Monday morning it is usual to examine the transit, in order to test its adjustment, and thus probably a fresh value for each item mentioned above may have to be applied from week to week. So delicate arc the various parts of the instrument, that a sudden change in the temperature, the intrusion of the sun's rays, and often an unexplained cause, will throw it out of adjustment.
From the computers' room we ascend a narrow staircase, and enter the chronometer-room. Here we are welcomed by a hive-like hum of ticking clocks. Upwards of one hundred valuable chronometers are arranged round this room, some on shelves, others in cases. They are all passing an examination, in order to test their regularity, and seem to be under the command of a large fixed clock, which, elevated above them, beats loudly.
The great value of a chronometer is regularity of rate. It is of no great consequence whether the instrument gain or lose, as long as it does this regularly and uniformly. If the chronometer gain, however slightly, one week, and lose the next, it is not to be trusted for astronomical purposes.
Several jets of gas are admitted into a large chest, and the chronometers are thus taken into the tropics, whilst a daily inspection of them serves to discover how they like it.
The process of rating is carried on each day by two assistants, and to the uninitiated seems very mysterious; for, instead of waiting to compare the smaller chronometer until the second hand of the large clock is at any well-defined position of its circle, such as 0, 15, or 30 seconds, the one is quickly read and compared with the other by a glance. Thus the hundred chronometers are compared in about half an hour.
Several of these instruments are purchased by the Admiralty for use in the Navy, whilst, for a small sum, any maker may have his instrument tested at Greenwich.
Leaving the chronometer-room, we pass across the small open court and enter that portion of the building to which the ball belongs. It wants some minutes to one o'clock, and, standing beside the computer, we see him wind up the ball at five minutes to one. Just before us is a large clock-face, the hands of which are moved by electricity, an important aid to modern astronomy. A sort of trigger is pointed out to us, by pressing on which the ball is released, and intimation thus given to hundreds of watchers that it is one o'clock by “Greenwich mean time." Intimation is also sent to other localities, by telegraph, of the instant at which on* o'clock occurs at Greenwich. To Deal, for example, where the ball is released from its temporary elevation by the electric current, sent on its message by the computer from Greenwich. The ball at Deal on falling, completes the electric circuit; and a return electric current to Greenwich announces there that the ball has safely descended to the bottom of its pole.
When the second hand of the clock approaches near the fiftieth second, J the computer watches each beat, and, with fingers extended over the trigger, stands ready to fire:—58, 59, and off goes the signal. "Look out for the return," we are told, whilst we hear the grating of the ball above us, as it descends; and in a few seconds, a small “hand” gives a slight wave, indicating that the ball at Deal is safely down.
A glance at the library, and at some other astronomical instruments, several of which are old and now scarcely used, completes our examination of the establishment, and we defer further investigations until the night.
Before the evening has advanced beyond that condition popularly spoken of as twilight, the computer on duty for the night has commenced his operations.
Beside him is a list of stars which it is requisite that he observe during the night. To each of these is affixed certain data, indicating the instant of {'me that the star ought to pass across the wires of the telescope, and the elevation that the instrument ought to have in order to see the particular star in the centre of the object-glass.
A few seconds before the star is expected, the observer looks at a clock opposite to him, and takes from it one of the second beats, which he then repeats, counting on 32, 33, 34, &c., without further looking at the time-piece. Practice soon enables a person to go on counting for a minute without getting wrong, and even whilst making memoranda upon other subjects; but these individuals must be those in whom the organ of time is largely developed. As the star approaches and passes each wire of the telescope, the instant of its passage is written down in ink, not only the second, but also the decimal of a second. This decimal of a second may be estimated in the following manner :—
If upon counting 32, for example, the star had not reached a given wire, but upon counting 33 it had passed it, then if the star were equally as much short of the wire at the 32 as it was past it at the 33, then we should write 32-5 for the time of transit over that wire. Whereas if it had been three times as far beyond the wire at the instant of counting 33 as it was short of it when we counted 32, then 32'25 would be the time of transit.
The transit of an important star having been observed and registered, we, as a special favour, are allowed to take the transit of the next member of the stellar host. Having arranged ourselves on the couch in a very comfortable attitude, we apply our eye to the telescope, and are surprised by the lightness of the view within. The plane of view is cut up by a number of rather coarse looking iron bars, which might serve as protections against burglars, so stout and so strong do they appear: these are the? finest spiders' threads, which are arranged within the telescope. One or two trials are made in order to practise counting the seconds correctly when looking away from the chronometer, but we soon find that to count in a trustworthy manner we must sit down for three or four days near a chronometer, whilst we count and count again—for you must count mechanically as you would walk, and not by any mental exertion, or you can never become a practical astronomer. We, however, manage to count —after a fashion—and now "Look out" is the word.
Instantly darting into the field of view, a little fiery, twinkling object comes galloping onwards towards the wires: so rapidly does it come, that counting seconds as we are, and with the mind strained in order to remember each necessary step, we make a dash at the time of passage over the first wire and again look at the clock, for whilst writing down our estimation of time, we lost count of the seconds. We were again only just in time, for the little star was gliding onwards, and once more we scribbled down seconds and portions of seconds. Again we glance at the clock, and again we estimate, and so on until nine estimations have been made.
"Seven minutes' interval now," is announced by our companion, who proceeds to examine our memorandum, which, having been added up and divided, shows that our estimation was correct to within two seconds—a result of which we were not a little proud, until we are assured that unless we can approach to within one-tenth of a second our observation would be rejected.
"Do you know what is the value of your personal equation?" is a question which follows the summing up of our observed transit Not being in the least aware what is meant by this question, we can merely guess that it is something connected with our opinion on the stars in general. So we reply that we do not know its value, but would like to know what was his.
We are informed in answer, that "his for stars is about '37 from the standard." Believing from this reply that the said peculiarity cannot be anything of a very heinous description, we make further inquiries, and then discover that every observer his about his eyes an individuality which causes him to invariably perceive a phenomenon, such as the transit of a star, a little before or a little after another observer, and that this " personal equation" is allowed for in all observations; a standard observer being arbitrarily fixed upon.
We could not with any certainty ascertain whether the personal equation was found to be the same both before and after dinner, or if the state of the digestion had anything to do with the fact. The simple statement, however, showed us that here was a singular phenomenon well worthy of the inquiry of psychologists; for it would appear as though there was a variation in the rate at which the external senses telegraphed to the seat of reason.
What is even more singular is that the "personal equation " for stars is not the same with the same person as it is for the moon, and thus it is also probable that each planet might give a different result, and a most complex problem is placed before us.
An interval of some ten minutes enables my companion to show me a very ingenious contrivance for registering the time of a star's transit over a series of wires. A sort of drum revolves slowly and is moved by clockwork; round this drum are long rolls of paper fitting closely. The electric clock pricks a small hole in the paper at every second, and thus time is measured on the paper by distance. A wire connects this drum with an electric battery, and the observer at the transit-instrument, by striking a sort of trigger, can prick holes on the revolving drum. Thus, instead of counting seconds and estimating the portions thereof, the observer strikes the trigger at the instant the star is behind the wire, and the measure (by scale) of the mark thus made will give exactly the portion of a second, as well as the second of time, at which the star passed behind the wire.
Electricity, as before mentioned, is a most important aid to astronomy: it serves to preserve a series of clocks in the various rooms of a large observatory in exact coincidence, each beat being performed at the same instant. By its aid, also, the relative position of two distant observatories can be accurately ascertained, the method being as follows:—
The speed of the electric current, or the time occupied by a signal passing from one observatory to another, is first ascertained; for electricity does take some time to pass along a wire, and the rate in all cases is not the same; then the chronometers at each observatory are compared by signal, and the difference, if any, is thus found. Both transit-instruments being adjusted, the time of a star's passage is telegraphed by the most eastern observatory to the western; after a certain interval (depending on the longitude) the same star passes the western observatory; the interval of time between the star's two passages gives the exact longitude, which, before the introduction of electricity, was a long and difficult item to obtain.
During some five or six hours of darkness a multitude of stars and two or three planets are registered in the note-book; clouds occasionally interrupting the view, and giving the observer a little leisure. As a rule, the observations are confined to those stars which are to the south of Greenwich, or immediately to the north of the zenith, and the greatest dependence is placed upon observations made on stars which pass near the zenith of the Observatory, because refraction, the astronomer's bugbear, produces no deviation of a ray of light from an object in the zenith. Thus, y Draconis, 0 and n Ursae Majoris, are valuable stars for Greenwich Observatory.
Having commenced observations at about six o'clock in the evening, we were so engaged in noticing the second-beats of the clock, that some time elapsed before we perceived that the minute and hour hands indicated a little short of one o'clock. This discovery led to a dissertation on the subject of time, and we were given much information, of which the following is a rough sketch:
The clock that showed one hour was an astronomical clock, indicating sidereal time, sidereal time being counted from one round to twenty-four hours. It is 0 o'clock, sidereal time, when a certain point in the heavens, called the first point of Aries, is due south of the Observatory, this first point of Aries being that point in the heavens in which we see the sun about the 21st of March, when it is exactly over the equator. Thus the hour hand of a sidereal clock, if placed horizontally, would move round regularly, and always point towards that part of the heavens called the first point of Aries.
This arrangement of time is particularly convenient for giving the position of- the stars, for they are said to be one hour, two hours, and so on, from the first point of Aries, just as we say a city or town is one, two, or more hours of longitude east or west of Greenwich. Having, then, a list of stars, it is merely necessary to look at the astronomical clock in order to see which are then near to the meridian.
Instead of saying a star is " one hour from the first point of Aries," the term " right ascension" is used, and a star is said to have one hour of right ascension, and for the sake of brevity this is written 1 AR.
When, then, we saw the astronomical clock indicating one o'clock nearly, stars having about 1 AR were due south of us.
Another clock in the establishment indicates Greenwich mean time, that is, portions of the common time shown by our clocks and watches. In consequence of the sun moving during the year in a sort of oblique course around the earth, and from its not being in the exact centre of the earth's annual course, it does not come to the south of any place at exactly equal intervals of time—that is, from noon of one day to the noon of the next is not an uniform quantity of time. It would be very inconvenient for all purposes to have a variable length of day, and in the present days of railway travelling regulating the clocks would be a very difficult matter. For business, as well as for scientific purposes, therefore, a day of uniform length is adopted, and this day is the mean of all the variable days throughout the year, and is hence called a mean day, and portions thereof " mean time."
The sidereal clock and the mean-time clock only indicate the same time once during the year, viz., on or about the 21st of March; after this date the sidereal clock moves on more rapidly, and about June 21st would indicate six hours P.M. when the mean-time clock pointed to noon. One of the items for calculation at Greenwich is the transferral of Greenwich mean time into sidereal, and vice versa, but by the aid of tables this work is rendered very simple.
We will now briefly consider the practical results derived from the Greenwich Observatory, for these results are essentially practical.
We are now on board a valuable ship, and somewhere on the Atlantic Ocean; the sun has not been visible for three days, and a heavy gale has driven us we know not where. During the night a slight opening in the clouds reveals some half-dozen stars: two of these are recognized, and the height of each above the horizon is carefully measured with the sextant. One is exactly south, the other south-west. At the instant that the observation on the south-west star was made, the time shown by the ship's chronometer, and which had been rated at Greenwich, is accurately registered; upon referring to the Nautical Almanac, which work contains the results of the Greenwich labours, we find the correct position in the heavens of these two stars.
By the aid of the star in the south, the ship's latitude is at once obtained, whilst by the aid of the second, the sidereal time of the observation is obtained: this sidereal time can, by the aid of a table and data supplied from Greenwich, be converted into mean time, which will be the mean time of the ship. The chronometer shows Greenwich time, and hence the difference between the time at the two localities gives the longitude of the ship, and hence its exact position on the ocean.
Again, from some unexplained cause the chronometer has stopped, and we know not that essential to our calculations, viz., Greenwich time. Our loss, however, can be remedied by the aid of the Greenwich observations, for there, in the southern sky, is the moon, and to the west of it a bright star. Sextant in hand, the mariner measures the height of the star and moon above the horizon, and the distance in degrees between the moon and star, the time by a hack watch, or the restarted chronometer, is noted at the instant of observation, and the measurements being corrected for certain items, it is found that the moon's centre was when observed just 40° 10' 10" from the star.
Upon reference to the information supplied two years previously from Greenwich, we find that it was exactly ten minutes and four seconds past nine by the Greenwich clock when the moon and the star were that distance apart. The chronometer is at once restarted correctly, and the mariner is confident that it is showing the same time as the clock on the exterior wall of Greenwich Observatory.
These are but a few of the benefits derived from this establishment, which serves, besides, as a sort of head-quarters for all practical astronomical information. It is not from it, however, that any important discoveries connected with the nature and constitution of the various celestial bodies are likely to emanate. The whole training and work of the various members partake entirely of the practical and mechanical. From independent observers it is most probable the next great advance will originate, though it will most likely be suggested by an examination of the facts collected and registered at the Greenwich Observatory.
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