Popular Science Monthly/Volume 4/December 1873/Preparations for the Coming Transit of Venus
THE nature of a transit of one of the inferior planets (Mercury or Venus) is well understood, and the phenomena attending such a transit have been thoroughly discussed, and fully described in many places. The importance of the observation of these transits, and the general character of the results expected from the expeditions sent out to observe them, are probably understood by all, but it is thought that a brief account of the means that are to be employed to accomplish the desired end will be of interest.
The records of the plans which have been formed, and of the preparations which have been made by the different governments of the world and by private individuals, are, unfortunately for the general public, published only in proceedings of scientific societies, or in many cases they exist only in manuscript. When the expeditions return home after the observations are made, in astronomical Europe and America will resound the busy hum of preparation, and from the beginning of 1875 the reader of astronomical items will be sated.
At first will come a series of preliminary reports as the parties come in; then we shall have the final reports, giving numbers, data, descriptions of instruments, and the observations made at the transit, the longitudes and latitudes of the various stations, and, in short, every result which the practical astronomer will have derived.
These final reports will be eagerly looked forward to, for upon them depends the constant of solar parallax, and from them will be deduced the definitive result of all the astronomical work done on the globe on that day.
We know already that the final outcome of all these vast preparations which we are going to describe will be a number very near to 8“.848.
The whole world is united in an effort to know exactly how to change this; whether to write it greater or less. But the results of these expeditions, if they are successful (and we can hardly fail of success), will be, not simply the establishing of the earth's distance from the sun on a certain basis, but much more.
So many expeditions of trained scientific observers will bring back with them data only second in importance to the main object of their journeys. The latitude and longitude of many of the almost unknown islands of both oceans will be established with a certainty as great as the corresponding coordinates of most seaports on our own Atlantic coast. Observations for magnetic constants will be made at places widely separated, and much will be learned in this way. The line of Russian stations, and the American station in Siberia, will be connected by telegraphic wires to St. Petersburg, and possibly the stations in the Indian Ocean may likewise be joined with New York or Washington, so that independent longitude determinations by telegraph may be extended over seven-eighths of the globe.
Americans should not forget that our own Coast Survey has made three independent determinations of transatlantic longitude in the years 1867, 1870, and 1873, nor should they forget the wonderful agreement of the results obtained over three different cables, by different observers at different times. This agreement is so marvelous (considering the independence of the determinations), that the results are here quoted:
Longitude of Harvard College Observatory, west of Greenwich Observatory.
It must be remembered also that, incidentally as it were, the relative longitude of Paris and Greenwich Observatories was found: so that it is to American astronomers, working by a method of American invention, that the exact value of so important a coordinate is due.
Americans will have reason to be proud if equally exact determinations can be extended by them from the Indian Ocean to New York, and from Siberia to Greenwich.
These are only some of the incidental advantages which it may be hoped will be gained by the various expeditions for which the different governments have provided.
There are various ways in which the observation of the transit of Venus may be made, and, in order to describe the instruments, and the preparations which are making, it will be necessary to refer to these briefly:
1. There is the method of contacts, which consists in determining the time at which the limb or edge of Venus's disk is tangent to the limb of the sun. To make this observation, a small equatorial telescope is needed, provided with suitable colored glasses to protect the observer's eye, and with the usual appurtenances.
2. The micrometric method, which consists in measuring the distance apart of the bright horns of that part of the edge of the sun which Venus partly obscures as she is moving on or off. As Venus has a sensible diameter (about one minute of arc), it will take a sensible time for her disk to move from first contact (when her disk just touches the disk of the sun exteriorly) to second contact (when her disk is tangent interiorly to the sun's), and during this time the appearance of the two disks will be as in the figure:
This figure shows Venus coming on to the sun's disk, and it shows the two cusps at a and b. It will be easily seen that, if we know the length of the line a b, and the time at which it has this length, we can calculate the time of contact from these data. So that a number of measures of the cusps is the same as a number of first contacts. The reverse phenomenon occurs when Venus passes off the sun's disk.
To measure these distances, the equatorials must be provided with filar micrometers, i. e., with a contrivance by means of which two spider-lines in the focus of the telescope may be moved toward or away from each other. One of these lines is to be placed at a, and the other at b; the time is to be noted, and the number of turns and parts of a turn of the screw which moves the lines is to be noted from the head of the screw, which is finely divided.
3. The photographic method. This consists in photographing the planet Venus on the disk of the sun, and noting the time of each photograph. The negatives are carefully preserved, and are measured subsequently by a fine measuring engine. It will be seen that this method is like the preceding, except that the measuring may be done at leisure, and without the hurry and anxiety which attach to any observation of this nature.
This method requires apparatus of a special kind. The American plan is to throw the image of the sun, with the planet on its disk, into a stationary photographic telescope where the negative is taken. This is taken out, and at once developed by the photographers, into whose dark room the telescope penetrates. This method is due to Prof. Winlock, of Harvard College Observatory. The other method consists in making the photographic telescope follow the sun in its motion by means of clock-work, and in taking the negatives in the same way. The dark room, however, is some distance off, and it appears that too much dependence must be placed on the steadiness of the clock-work motion.
4. The heliometric method. This consists in measuring the cusps with a heliometer, which is merely a large telescope which has two object-glasses (or one object-glass cut into halves by a diametral cut) which slide past each other. Each half produces a complete image, and, by means of an observation of a tangency of images, the distance of the cusps may be had.
5. The spectroscopic method. In brief, we may explain this as follows: It is known that there is a thin layer of atmosphere near the sun's limb where bright lines may be seen with a powerful spectroscope, while on either side of this layer dark lines only are seen. As Venus advances, the interposition of her dark body will cut off this layer, and the instant of disappearance of the last vestige of any one of these bright lines will be truly the instant of first contact.
The ordinary method of observing first contact is open to grave uncertainties (on account of the different sensitiveness of the eyes of various observers, and for other reasons), and it is hoped that this method, as beautiful in theory as it will be difficult and delicate in practice, will obviate all these objections.
It is to be expected that the astronomers of the different nations will adopt different plans of observations, in accordance with the peculiar traditions of each school.
The Germans and Russians, among whom the use of heliometers has been hitherto confined, will (with a single exception) alone use them on the approaching transit.
The German Government will send one of these instruments to the Kerguelen Islands, or to Macdonald Island, one to the Auckland Islands, one to the Mauritius, and one to China (Chefoo). Lord Lindsay, of England (the one exception spoken of), also takes a heliometer with his very completely-equipped private expedition to the Mauritius.
Three of the twenty-seven Russian stations in Russia, Siberia, China, and Japan, will be provided with heliometers; at three, like-wise, will the photo-heliograph be used, while the remainder of the stations will be devoted to the ordinary contact observations and to measures of cusps.
At all the American stations the photo-heliograph, the contact method, and the method of cusps, will be used. The American stations will be eight in number. These will be principally in the southern latitudes, in the Indian and Pacific Oceans, except one in Siberia, and, perhaps, a photographic station in the Sandwich Islands.
Stations in Japan and China will be established also by the Americans.
Most of the English parties are to be in northern stations, though the Challenger exploring expedition is instructed to examine eligible stations in the South Pacific. Of the stations of French observers little is definitely known, although they will occupy a few posts.
Each party must be provided with instruments to observe the actual transit, and it must further have the means of determining accurately time, longitude, and latitude.
Of these quœsitœ, the latitude and the local time are most easily determined. Portable transit-instruments will suffice for the first determination, and for the second there are various adequate means.
The American parties are each to be provided with a small portable transit-instrument and zenith-telescope combined, which instruments are now making by Stackpole, of New York.
These are intended to be of the simplest possible construction and of the greatest attainable stability, and they combine several advantages. In accordance with a suggestion first proposed by Steinheil, of Munich, the tube proper of the telescope will be reduced to one-half of the usual length. A prism will be placed at the end of the tube opposite the object-glass, by which the rays which enter the telescope will be turned at right angles through the perforated axis of the pivots of the instrument, thus utilizing the necessary length of this axis by making it an integral part of the telescope.
The observer will thus occupy one position, no matter to what part of the meridian his telescope is pointed, which is, in itself, a great advantage, on the score of convenience. This also will doubtless conduce to a constant personal equation, as it has been shown by the director of the Albany Observatory, and others, that personal equations vary with the altitude of the observed star.
These instruments are provided with fine spirit-levels and with micrometers, which fit them to be used as zenith-telescopes, and thus to determine two of the three important quœsitœ.
The parties of other nations will use similar methods for this purpose. The coordinate which is most difficult of exact determination is the longitude, and the problem of its determination will be attacked in various ways.
The English parties, true to the traditions of Greenwich, are to be provided with portable altitude and azimuth instruments with which to observe moon transits, both in the meridian and out of it. A long series of such moon-culminations was observed between Harvard College Observatory and Greenwich some years ago, and it is now known that the result obtained was greatly in error. Indeed, Prof. Peirce, in his discussion of the series of observations, came to the conclusion that it was impossible to derive the longitude of a place by this means, certainly, within one second of time.
The Americans and Russians intend to depend on the occultations of small stars by the moon.
Occultations are much more likely to be free from systematic errors than the moon-culminations, and, if they can be observed throughout a lunation, a compensation of errors will obtain.
The Russians intend to mask their stations of observation, and subsequently to connect by telegraph St. Petersburg with the most important of them. The transportation of chronometers to and fro between the stations whose longitude is thus determined and the minor ones will assure the longitude of the latter.
The American parties in the southern seas will be transported to their various stations in a ship-of-war which will touch at the different islands and leave the parties, and which will make chronometric expeditions between the various stations. Besides this, all existing telegraph-lines will be utilized. As each of the parties of each nation is to be led by some astronomer of eminence, it is certain that no means will be neglected to make the preliminary results of the greatest attainable accuracy.
The various assistants are now in training at Greenwich, Poltava, and Washington, with the very instruments which they will use on the expeditions.
At Washington and Poltava an apparatus for the representation of the transit is in use. A disk representing Venus is caused to travel over an illuminated space which is representative of the sun, and the circumstances of the transit are then observed.
In this way it is hoped to obtain an idea of the personal error of each observer in watching contacts, so that, in reducing the observations of the transit, all personality may be eliminated.
Most of the American parties will start in the spring of 1874, and proceed in the most expeditious way to their stations. They must take with them every thing which they can need during their stay, for in most of the stations there is no supply of any kind to draw upon.
We can hardly realize the absolute necessity of being provided with every thing that may be needed on such an expedition: but let us conceive the feelings of an astronomer on a desert island with no screw-driver, or with no ink, or matches, or soap!
There is no repairing a blunder of outfitting in these cases, and the greatest care has to be exercised in providing for all contingencies.
Arrived at its station the party will put up its observatory, a little wooden or canvas hut which has been brought from America, for no wood grows on this island. The instruments must next be mounted, and all gotten in readiness for work.
The astronomer and his assistant set up the transit, the small equatorial (five inches' aperture, and about seven feet long), and the clock, and provide safe places for their chronograph and chronometers. Suppose a chronometer-spring breaks now: there is no help nearer than New York. The two photographers put up their hut and prepare for work. From this time until the time of the transit, all is work.
Every day the methods which will be adopted on the important day are rehearsed. Each one does the very thing which he will do, takes the very steps which he must then take, and turns the very same micrometer-screws just as he will turn them in December. This is repeated until every one is sick of it, and, from a man, each becomes a machine.
During the nights the chief astronomer is looking for occultations, or taking differential measures between the moon's limb and a star, while the assistant is determining time and latitude. Sometimes their work is interchanged, to eliminate any personal peculiarities of observing. When the final day comes, they should have their latitude and longitude thoroughly well known, and their clocks and chronometers rated perfectly. The photographers, too, should know the exact strength of both, the precise time of exposure, and the right developer to make the best possible negative of the sun.
When the time of transit actually comes, the chief will be at the equatorial, and will observe the first contact, and record the time on his chronograph, and at once commence measures of the distance of cusps. The assistant astronomer will see that the heliostat which is to throw the image of the sun into the stationary photographic telescope does this properly; and within the dark room the two photographers must be taking negatives as rapidly as possible.
This continues during the transit from first to second contacts; afterward the photographs succeed each other, but not so rapidly, and finally, the last contact is marked. It is all over now, and there is nothing to do but to write down at once all notes which are to be used in the report, and to prepare for a journey home.
Six or eight months on a rocky island, vast expense, and much trouble and discomfort: but le jeu vaut la chandelle. The moral of it is, that Science expects every man to do his duty. Let us hope that Science will not be disappointed.