A MANUAL OF SCIENTIFIC ENQUIRY. ARTICLE I. FIRST DIVISION, SECTION 1. ASTRONOMY. By G. B. AIRY, ESQ., ASTRONOMER ROYAL, THE science of Astronomy may occasionally derive benefit from the observations of navigators, in the following respects : By contributions to Astronomy in general. By improvement of the methods of Nautical Astronomy. By accurate attention to Astronomical Geography. The remarks which follow will be arranged under these heads. General Astronomy. 1. The first point which calls for attention is the observation of the places of comets or other extraordinary bodies, especially those which can be seen only in low northern or in southern latitudes. In regard to these observations (and indeed to almost all others), one remark cannot be too strongly impressed on the observers-that a bad observation, or an observation which is given without the means of verification, is worse than no observation at all. In order to make the observations good, the following cautions must be observed: - The index-error of the sextant must be carefully ascer B tained. If it has not been found a short time before the observations, it must be found as soon as possible after them. The distance of the comet from three conspicuous stars in different directions must be measured with the sextant. The point of the comet which is observed with the sextant should be precisely described. It is desirable that the navigator should be possessed of some star-maps or starcharts, by means of which he will be able at once to give the proper names to the stars, and much confusion and loss of time will be avoided. If the time at the ship and the latitude are very well known, there will be no occasion to make further observations; but if these are not well known, some attempt must be made (by the use of Becher's horizon, or by any equivalent method) to ascertain the altitude of the stars and the comet. The lower these objects are, the greater must be the care in the determination of their altitudes. For affording means of verification, these rules should be followed : The observations of distance with the sextant should be entered in the book precisely in the manner in which they are made. The reading of the sextant, uncorrected, should be written down: in a column by the side of this should be written the correction for index-error, with a statement whether it is to be added to, or to be subtracted from, the sextant-reading: in the next column should be written a reference to the observations by which the index-error was determined and in the last column should be written the distance as corrected. For the altitudes, the height of the eye, the depression of the horizon, and the altitude corrected for depression, should also be stated. At some convenient place, either at the beginning or at the end of all, should be written out all the measures by which the indexerror was ascertained, exactly in the manner in which they were made, and so that any other person can deduce from them the value of the index-error. : The time of making every observation should be entered exactly as it is read from the chronometer or hack-watch. By the side of this should be placed the error of the chronometer or hack-watch on Greenwich time, or on time at the ship (as may be most convenient); and, after this, the corrected time. At some convenient place, either at the beginning or the end, must be written out all the observations by which the error of the chronometer is ascertained. If its error on Greenwich time is given, the longitude of the ship must also be given, and the means and observations by which that longitude has been determined must be stated at length. If a hack-watch is used, the comparison of the hackwatch with the chronometer must be given. The last observations by which the latitude was determined, and the course and rate of sailing of the ship, must also be given. All the observations should be sent in this detail to the Admiralty or other body appointed to receive them. 2. Opportunities will sometimes occur, when a ship is lying in a harbour of which the latitude and the longitude are well known, for observing eclipses of the sun. These observations are almost always valuable. It can seldom be expected that the time of the beginning of an eclipse can be observed accurately, but the time of the end of it can usually be observed with very great accuracy. And if the eclipse is total, the times of beginning and end of the totality can be observed accurately; if it is annular, the times of beginning and end of the annularity can be observed accurately. The observations should be made with the largest telescope which the navigator possesses; and any peculiarity of distortion of the sun's limb or the moon's limb, any light surrounding the moon, &c., should be carefully recorded. If the eclipse be total, attention should be paid to any coloured or other appendages projecting from the dark edge of the moon, also to the luminous corona surrounding the moon, its apparent breadth, and whether apparently concentric with the moon or with the sun near the moments of beginning and end of the total obscuration.* While the eclipse is in progress, but especially near the beginning or the end, measures of the distance between the cusps or sharp points at which the moon's limb crosses the sun's limb may be repeatedly taken. In recording these observations, the observations by which the time is determined, and the observations by which the index-error of the sextant is determined, should be written down in the fullest detail; and the unreduced observations should be given as well as the reduced observations. 3. In similar circumstances occultations of stars by the moon may frequently be observed. Eclipses of Jupiter's satellites may also be seen; and (if the navigator have a telescope somewhat better than is usually carried in ships, and steadily mounted) the passage of Jupiter's satellites, either behind the planet or in front of the planet, may be seen, and the times at which the centres of the satellites just touch the edge of the planet may be observed. All these observations will be useful: the observations must be recorded with the same fulness which has been mentioned before. 4. It may chance that the navigator is in some climates where the air is much more damp, and in others where it is much more dry, than in Europe. It is possible that in these places he may be able to make observations which will throw some light upon the influence of moisture in atmospheric refraction. It is recommended that repeated observations of the altitude of the sun's upper and lower limb be taken when the sun is very near the horizon. It [* For the details of the phenomena which have been observed in annular and total eclipses the Memoirs and Notices of the Royal Astronomical Society should be consulted beforehand, especially vol. xxi. of the Memoirs, of which the first part is exclusively devoted to accounts of the Total Solar Eclipse of July 28th, 1851. The accounts given by Mr. Airy and Mr. Baily of the phenomena seen by them in the Total Solar Eclipse of July 8th, 1842, will be found in vol. xv. of the Memoirs.-R. M.] will be necessary that the time at the ship and the latitude be very well known. The thermometer must be read, as also some hygrometrical instrument, and the barometer, if there is one on board, during the observations. The observations of every kind must be recorded with the utmost fulness. 5. It is certain that some of the stars of the southern hemisphere are variable in magnitude; the most remarkable of these is ʼn Argûs. It is desirable that, on favourable nights, the magnitude of this star should be observed and recorded. The best way of doing it will be, not to state that it looks like a star of the 2nd magnitude, or of the 3rd magnitude, &c., but to compare its brightness with that of some of the stars near it. Thus it will be easy to say that it appears pretty exactly as bright as one star, certainly brighter than a second, and certainly not so bright as a third.* 6. Much attention has been excited by the appearance, in several years, of meteors in great numbers, on or about the 9th of August and the 12th of November. It is probable that these appearances may be seen by persons at sea, when, either from the hour at which they occur, or from other causes not yet understood, they cannot be seen in Europe. It is impossible to observe them with accuracy; but very valuable information will be given by counting repeatedly how many can be seen in some fixed interval of time, as five minutes; and by remarking whether they all come from, or go to, one part of the heavens; what is that part of the heavens; whether they usually leave trains behind them; what is their usual brightness (as compared * See a list of variable stars, and some suggestions for observations of brightness of stars, in the Appendix.-(ED. SIR J. H.) Humboldt (Kosmos, i. 387) enumerates the following epochs as especially fertile in meteors, viz. April 22-25; July 17-26; Aug. 9 -11; Nov. 12-14 and 27-29; Dec. 6-12. Of all these epochs, that of August has hitherto proved to be the most regular. The star B Camelopardali has for several years been their point of divergence about that epoch. Any change in this respect should be recorded.(ED. SIR J. H.) |