« НазадПродовжити »
Art. XLII.—Meteors of November 14th, 1868; by H. A.
Newton. (With two Plates.)
In continuation of the account given in the January number of this Journal (pp. 118–126), I propose to notice certain meteors observed at two or more places, with sufficient exactness to afford a parallax.
The most remarkable meteor of the night appeared at 1h 16", New Haven mean time. It was observed at New Haven, Poughkeepsie, Palisades, Williamstown, Haverford, Wilkesbarre, and probably at Washington. The discussion of these observations will afford an occasion of explaining a method of computation of the true altitude and path from the observed tracks of a meteor.
At New Haven.—I did not see the body itself, but the bright streak remaining after its disappearance was nearly vertical, and about 2° north of Jupiter. This distance from Jupiter was carefully estimated, by Mr. Harger and myself, and we are both confident that it is within half of a degree of the truth. Mr. Harger thinks the estimate is, if anything, a trifle too great.
No note was made at the time, of the length of the bright cloud. My impression, confirmed by that of Mr. Harger, is, that Jupiter lay about the middle of the persistent streak, also that the streak was not less than five degrees long, and probably 2 or 3o longer than that. From the first it had an appreciable breadth.
At 1h 15m 40s, N. Y. time, the trail had assumed the usual curved form, and I drew a rough sketch of it, and of Jupiter, as in the figure, plate I. The upper part had apparently moved northward, and the lower part southward.
At 19 minutes past one I drew the second sketch as in the figure. The special object I had in view was to locate its several parts with reference to Jupiter.
At 30; minutes after one the cloud lay horizontally just above Jupiter and extending south eight degrees from the planet, as estimated at the time. The third sketch, plate I, gives its center an altitude of one degree or more above Jupiter, and an extent northward four degrees from it.
Ať 44} minutes past one the cloud, which was growing fainter, was horizontal, and extended 12° southward from Jupiter, as noted at the time. My fourth sketch implies that it extended 3° or 4° north of Jupiter, and had its lower edge just upon the planet at this time.
At fifty-seven minutes past one the cloud had become so faint and confused with the mists of the horizon, that in order to confirm or correct my impression that I still saw it, I asked the party whether it was actually visible. Nearly if not quite all of them (about a dozen persons) asserted that it was still visible. I think that it continued in sight till the clock struck one, if not longer, although other objects prevented us from giving it special attention and recording its appearance.
At Palisades, Fern Lodge Observatory, Rockland Co., N. Y., Mr. W. S. Gilman, Jr., saw the train of the meteor, and made four sketches of its appearance at successive times, together with the following notes.
Time of appearance, 1b 12m 18+ N. Y. mean time, 1st magnitude ; white color ; duration of flight f sec. The meteor was seen by Mr. Thomas P. Gilman, who obtained a glimpse of it only. The glare of the meteor was very brilliant ; length of path 50° or 60°; direction perpendicular; disappeared near and north of Jupiter ; explosion not witnessed. Left train of smoke of a greenish blue color.
At 1h 14m 10s the train was very distinct, but stars were seen through it. The appearance was as in the figure, plate I.
At 1h 18m 10s the train had become still more curved, and the figure represents its appearance as it was seen with the naked eye. The train viewed through a four inch glass, with power 40, appeared to move sluggishly, and to dissipate slowly. It was comparable for color and texture to the appearance of the nebula in Orion in a 12 inch glass.
At 1h 21m 30s the train is still a striking object. It is represented in the figure. Very faint at lh 24m 30s.
At 1h 28m it has still more changed its form, as in the fourth sketch.
Smoke faintly visible at 1h 34m 30s. At 1b 39m smoky train very much diffused and very faint. Not watched subsequently.
At Vassar College, Poughkeepsie, N. Y., the meteor was seen by Miss Colby of the Senior Class, and the following report is furnished by Miss Mitchell.
“At 1h 12m 30s, a meteor started from the zenith, moved toward the west, burst south of Jupiter, like a rocket, assuming a variety of colors, and disappeared, leaving a train, which at once began to wind, taking first the form of an irregular s and then of a z, then moving southward toward two stars of Cetus, whirled around completely, broke up into two pieces, the southern portion surrounding the upper star, and both fading in the mists of the horizon. The train was seen tor 44 minutes."
At Williamstown, Mass.—The following notes of observations by Messrs. B. I. Gilman, W. D. Granger and F. B. Wilder, are furnished by Mr. W. S. Gilman, Jr.
“At 1b 15m the largest meteor of the night fell near Jupiter. The motion was slow and we could have read very easily by its light. The path made an angle of 60°+ with the horizon. The train lasted an almost incredible time, and assumed different shapes." A sketch of the path accompanied the notes.
At Haverford, Pa., at 1h 9m 28s a meteor passed from near the north star in the direction of Cassiopeia vanishing near B Cassiopeia. The light was exceedingly brilliant, and the train continued visible about 44 minutes. The train was at first straight, having a direction nearly parallel to the line joining B and * Cassiopeiæ, and about 150 in length. Almost immediately the line commenced to spread and the form changed to that of an inverted s (or 2). Then it assumed an elliptic form with a hollow ellipse at each of the foci, and soon afterward the 2 form again. During this time it slowly drifted toward a Andromedæ, being last seen near 8 Andromedæ. Then the train had an elliptic form, the transverse axis being nearly parallel to the line joining a and ß Andromedæ.
The sketch furnished by Prof. Samuel J. Gummere, who himself saw the meteor, though confined to the house, shows the position of the cloud at the time it assumed the elliptic form above described.
Mr. B. V. Marsh reports that the meteor was seen by Mr. Dennis of Wilkesbarre, Pa., and that it appeared to pass near Orionis.
A person in Delaware Co., N. Y., describes it as west of south, and about 20° high.
Undoubtedly the meteor described by Prof. Eastman in “ November Meteors of 1868, U. S. Naval Observatory, ” p. 8, in the following terms, is identical with the one witnessed at New Haven, Palisades, &c.
“At thirty minutes past midnight the most remarkable meteor of the shower appeared near & Ursæ Majoris, and moved westerly across B Ursæ Minoris. The meteor and its train were colored like the one already described. [At first its color was deep orange or red, afterward changing to green, and finally to light blue. The edges of the train were colored deep orange or red, and the center was at first green, changing to light blue. The colors lasted about one second.) The meteor was much brighter than Jupiter and moved very rapidly.
After the bright colors of the train had vanished it remained perfectly straight for about one minute when it began to assume slowly the form of a serpent in motion, gradually decreasing the distance between its extremities until it became apparently a mass of light-colored cloud about 7° in length and 4° in breadth.
“The westerly end of this condensed train at one time became divided, but after the lapse of about four minutes the extremities of the separated portions united, leaving a nearly circular aperture 30' in diameter, 2o from the western end of the train. After this aperture closed up I could distinctly see a small star through the vapory mass. This cloud-like object remained visible, with slight changes in its outlines, for thirty minutes."
The latitudes and longitudes of the several places of observation of this meteor are as follows.
73° 15' Poughkeesie,
41 40 50"
73 53 30" New Haven,
41 18 33
72 55 25 Wilkesbarre,
75 54 Palisades,
73 54 7 Haverford,
40 0 36
75 18 16 Washington,
38 53 39
77 2 48 The place of Jupiter was R. A., Oh 19m 37®, and North Dec., 0° 27' 23". The relative positions of the places are represented in a skeleton map, upon plate II. The corresponding positions of the paths upon the heavens are presented in an adjoining diagram, upon which are also exhibited the radiant, the pole star, and Jupiter.
Several methods of computing the actual position of a track from the observed paths have been published. The most complete is by Bessel n the Astronomische Nachrichten, vol. xvi, p. 322. Others are by Brandes, Olbers, Quetelet, Secchi, Newcomb, &c. I have, however, found the following the most convenient method of treating observations. With a sixteen inch globe it is sufficiently accurate for any ordinary data.
First, I rectify the celestial globe for the mean latitude of the places of observation, and for the mean local time of the appearance of the meteor. Then upon the globe I paste narrow strips of paper to represent the several observed paths. In a similar manner are indicated the zeniths of the places, and those points of the celestial sphere which are at the given instant in the production of the several lines joining the places of observation. These last points will, in the case of only two observations, be in the horizon of the rectified globe, and in all cases will be very near that horizon. When the meteor belongs to a group having a well defined radiant, that point, or region, is marked in like manner.
There are certain geometrical conditions which the observations, if correct and complete, must now fulfil. Thus, through
two places of observation and any point of the meteor's actual path, as the end, for example, we may pass a plane. This plane, produced, intersects the celestial sphere in a great circle. Upon this great circle should evidently be located the two observed places of extinction of the meteor, and the points where the line joining the places of observation cuts the heavens.
If then we draw a great circle through the two points last named, and through either end of the track as observed at one place, this circle should pass through the corresponding point of the other observed path. Any deviation from this circle is due to some inaccuracy of observation. It is necessary to assume that one track is to be lengthened, or the other shortened, or that one or the other is to be changed in direction, or in position, until the two observed paths satisfy these conditions.
Again, three or more observed paths if correct must, if produced backward, pass through one and the same point on the globe, to wit, that point from which the meteor was moving. This point must, moreover, be above the horizon. Again, if some of the observations by being conformable to the usual radiant, show that the meteor belongs to a group with a well marked direction, it is often safe to assume that the unconformable paths are the ones that are in error.
By recognizing these, and other necessary geometrical conditions, we may criticise and reject observations. It will often be necessary to assume that one or the other observer saw only the latter part of a path, or has mistaken a sta of reference, or has erred in recording upon the chart the direction of the meteor.
I am well aware that this method of treating the observations throws special and undesirable responsibility upon the computer. But in any method of observing commonly practised, good observations have large probable er ors, amounting often to several degrees. The computer must assume the duty of judging the observations, and be held responsible for its faithiul discharge.
The computer is, moreover, now in a condition to use to advantage any special points in the observations which he knows to be well determined, as the passage of the meteor near, or over, a particular star. Or the peculiar position of one of the observers may far more than counterbalance any inaccuracy in the location of the path.
A chart, constructed for the purpose by means of the tables of the Coast Survey, upon which are plotted the locations of the observers, is used in measuring distances and directions whenever it gives sufficient accuracy. The figure on plate II. is a reduction of this chart.