William F. Denning.

Telescopic Work for Starlight Evenings online

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+——————————————+——————+————————————————————+————————+——————+
| | h m| miles. | miles. | miles. |miles.|
|1865, April 29| 12 42| 52 | 37 | 75 |20 |
|1868, Sept. 5| 8 5| 250 | 85 | 1200 |28 |
|1869, Nov. 6| 6 50| 90 | 27 | 170 |35 |
|1872, July 22| 8 55| 77 | 37 | 88 | |
|1874, Aug. 10| 11 53| 77 | 33 | 105 |17 |
|1875, Sept. 3| 9 55| 75 | 40 | 35 |27 |
|1875, Sept. 14| 8 28| 52 | 13 | 104 |13 |
|1876, Sept. 24| 6 30| 58 | 16 | 45 |15 |
|1877, Nov. 23| 8 25| 95 | 14 | 135 |17½ |
|1878, June 7| 9 53| 65 | 37 | 160 |19 |
|1879, Feb. 23| 14 53| 60 | 7 | 102 |14½ |
|1886, Nov. 17| 7 18| 96 | 21 | 123 |17½ |
|1887, May 8| 8 22| 70 | 14 | 110 |18 |
|1888, Aug. 13| 11 33| 78 | 47 | 46 | |
|1889, May 29| 10 44| 58 | 23 | 76 | 8½ |
+——————————————+——————+————————————————————+————————+——————+
+——————————————+————————-+————————————————+
| |Radiant- | |
| Date of | Point. | |
| Apparition. | | Authority. |
| |R.A. Dec.| |
+——————————————+————————-+————————————————+
| | ° ° | |
|1865, April 29| 73 +47 |A. S. Herschel. |
|1868, Sept. 5| 14 -2 |G. von Niessl. |
|1869, Nov. 6| 62 +37 |A. S. Herschel. |
|1872, July 22|246 -11 |T. H. Waller. |
|1874, Aug. 10|325 -17 |W. H. Wood. |
|1875, Sept. 3|311 +52 |G. L. Tupman. |
|1875, Sept. 14|348 -0 |G. L. Tupman. |
|1876, Sept. 24|285 +35 |A. S. Herschel. |
|1877, Nov. 23| 62 +21 |G. L. Tupman. |
|1878, June 7|247 -25 |A. S. Herschel. |
|1879, Feb. 23|310 +55 |J. E. Clark. |
|1886, Nov. 17| 34 +19 |W. F. Denning. |
|1887, May 8|191 -5 |W. F. Denning. |
|1888, Aug. 13| 43 +56 |W. F. Denning. |
|1889, May 29|216 7 |D. Booth. |
+——————————————+————————-+————————————————+

[Illustration: Fig. 56.

Fireball of Nov. 23, 1877, 8^h 24^m, emerging from behind a cloud.

(Drawn by J. Plant, Salford.)]

Sometimes there is no visible explosion; the bright nucleus slowly
dies out until reduced to a faint spark before final disappearance.
Several outbursts of light are often noted; and a curious halting
motion has been observed in regard to large slow-moving meteors. I have
occasionally remarked a succession of four brilliant flashes given by
individual fireballs. These flashes, though sometimes of startling
intensity, are somewhat different to the transient vividness of
lightning; they come more softly, and remind one forcibly of moonlight
breaking suddenly from the clear intervals in passing clouds.

Fireballs differ vastly from shooting-stars in point of size; but their
origin is identical. The August meteor-shower yields the smallest
shooting-stars and the largest type of fireballs. The great display of
meteors on Nov. 27, 1885, not only presented us with large and small
members, but it also furnished us with a siderite or piece of iron,
presumably from Biela’s Comet. This fell at Mazapil, Mexico; and as
considerable interest is attached to the case, I quote a part of the
discoverer’s statement:—

“It was at about 9 o’clock on the night of November 27th, when I went
out to the corral to feed certain horses: suddenly I heard a loud
sizzing noise, exactly as though something red-hot was being plunged
into cold water; and almost instantly there followed a somewhat loud
thud. At once the corral was covered with a phosphorescent light;
while suspended in the air were small luminous sparks, as though from
a rocket.... A number of people came running towards me; and when we
had recovered from our fright we saw the light disappear, and bringing
lanterns to look for the cause found a hole in the ground, and in it a
ball of light. We retired to a distance, fearing it would explode and
harm us. Looking up to the sky, we saw from time to time exhalations of
stars, which soon went out without noise. We returned after a little,
and found in the hole a hot stone which we could barely handle; this,
on the next day, we saw looked like a piece of iron. All night it
rained stars; but we saw none fall to the ground, as they all seemed to
be extinguished while yet very high up.”

This is the first observed instance in which a meteorite has actually
reached the Earth’s surface during the progress of a star-shower.
If its identity with the meteors of Biela’s Comet is admitted, then
all classes of meteoric phenomena would appear to have a community of
origin.

_Differences of Motion._—Great differences are observed in the velocity
of meteors. An observer may notice all varieties on the same night
of observation. Some will move very slowly, others shoot quickly
across the sky. These differences are occasioned by the astronomical
conditions affecting the position of the meteor-orbit relatively to
the motion of the Earth. Thus the meteors of Nov. 13 move with great
velocity (44 miles per second), because they come directly from that
part of the heavens towards which the Earth is moving; hence the
orbital speed of the Earth (18½ miles per second) and meteors (26 miles
per second) is combined in the observed effects. But in the case of
the meteor-shower of Nov. 27 the motions are extremely slow (about 10
miles per second), as the Earth and the meteors are travelling nearly
parallel in the same direction, and the latter have to overtake the
Earth.

_Nomenclature of Meteor-Systems._—It is customary to name the showers
after the constellation from which the meteors appear to diverge. Thus
the meteors of April 20 are called _Lyrids_, the radiant being in Lyra;
the meteors of August 10 are termed _Perseids_, the point of emanation
being in Perseus. The two great streams of November are known as the
_Leonids_ (13th) and _Andromedes_ (27th). Several showers are often
visible in the same constellation; and when it is desired to name these
according to the above system, it is necessary to add the approximate
star to distinguish them. Thus, in August there are showers of μ
Perseids, ε Perseids, and α Perseids, in addition to the well-known
Perseids of August 10.

_Meteor-Storms._—On Nov. 12, 1799, Humboldt, at Cumana, in South
America, saw “thousands of bolides and falling stars succeed each other
during four hours.” On Nov. 12, 1833, this shower recurred, and was
witnessed with magnificent effect in America. One observer stated that
between 4 and 6 A.M. (Nov. 13) about 1000 meteors per minute might
have been counted! Another display occurred on Nov. 13, 1866, and on
this occasion 8485 meteors were enumerated by several observers at
Greenwich. A different system gave us a brilliant exhibition on Nov.
27, 1872, when 33,000 meteors were counted by Denza and his assistants
at Moncalieri, in Italy, between the hours of 5^h 50^m and 10^h 30^m
P.M. A repetition of this phenomenon occurred on Nov. 27, 1885, when
the same observers counted nearly 40,000 meteors between 6^h and 10^h
P.M.

[Illustration: Fig. 57.

VERTICAL.

Flight of Telescopic Meteors seen by W. R. Brooks, Nov. 28, 1883.]

_Telescopic Meteors._—Observers who are engaged in seeking for comets
or studying variable stars employ low powers and large fields, and
during the progress of their work notice a considerable number of
small meteors. At some periods these bodies are more plentiful than
at others, and appear in such rapid succession that the observer’s
attention is distracted from the special work he is pursuing to
watch them more narrowly and record their numbers. Schmidt saw 146
telescopic meteors during ten years. They ranged between the 7th and
11th mags. Winnecke in the year 1854 noticed 105 of these objects
on thirty-two evenings of observation with a 3-inch finder, power
15, and field of 3°. I have also remarked many of these objects when
using the comet-eyepieces of my 10-inch reflector[45], and find they
are apparently more numerous than the ordinary naked-eye meteors
in the proportion of 22 to 1. It would be supposed from the great
rapidity with which the latter shoot across the firmament that the
smaller telescopic meteors are scarcely distinguishable by their
motion, as they must dart through the field instantaneously and only
be perceptible as lines of light. But this impression is altogether
inconsistent with the appearances observed. They possess no such
velocity, but usually move with extreme slowness, and not unfrequently
the whole of the path is comprised within the same field of view.
The eye is enabled to follow them as they leisurely traverse their
courses, and to note peculiarities of aspect. Of course, there are
considerable differences of speed observed, but as a rule the rate is
decidedly slow and far less than that shown by naked-eye meteors. I
believe that telescopic meteors are situated at great heights in the
atmosphere, and that their diminutive size and slowness of movement
are due to their remoteness. This conclusion will hardly be avoided
by anyone who attentively studies the several classes of meteors in
their various aspects. Unfortunately no attempt appears to have been
hitherto made to determine the actual heights of telescopic meteors,
owing to the difficulty of obtaining two reliable observations of the
same object. The only way of securing such data would be for several
observers to watch certain selected regions by prearrangement either
with a low-power telescope or field-glass, and record the exact times
and paths of the meteors seen. On a comparison of the results a good
double observation of the same object might be found, in which case the
real path could be readily computed.

Future observers should note the different forms of telescopic meteors.
Safarik has divided them into four classes, viz.:—(1) Well-defined
star-like objects of very small size; (2) Large luminous bodies of
some minutes of arc in diameter; (3) Well-defined disks of a very
perceptible diameter brighter at the border than at the centre, which
gives them the aspect of hollow transparent shells; and (4) faint
diffused masses of irregular shape, considerable size, and different
colours. He has seen hundreds of meteors of every magnitude from the
2nd down to the 12th pass through the field of his 6½-inch reflector
(ordinary power 32, field 54′). On Aug. 30, 1880, 9^h to 15^h he
observed between 50 and 100 telescopic meteors, and many others were
seen on the following night. Whenever a shower of these bodies, such
as that witnessed by Brooks on Nov. 28, 1883, occurs, observers should
notice whether the objects participate in a common direction of motion;
because, if so, the radiant-point will admit of determination. The
horary rate of their apparition ought also to be ascertained. Those
who habitually search for comets should invariably make a note of
telescopic meteors, as such records would aid inquiries into the
relative frequency of these phenomena.

_Meteor Showers._—The following short list includes the principal
displays of the year:—

+———————————————-+——————————————-+————————+——————————————-+——————————+
| Name of | Duration. |Date of | Radiant- | Sun’s |
| Shower. | | Max. | Point. |Longitude.|
+———————————————-+——————————————-+————————+——————————————-+——————————+
| | | | α δ | |
| | | | ° ° | ° |
|Quadrantids |Dec. 28-Jan. 4 |Jan. 2 | 229·8 +52·5 | 281·6 |
|Lyrids |April 16-22 |April 20| 269·7 +32·5 | 31·3 |
|η Aquarids |April 30-May 6 |May 6 | 337·6 - 2·1 | 46·3 |
|δ Aquarids |July 23-Aug. 25|July 28 | 339·4 -11·6 | 125·6 |
|Perseids |July 8-Aug. 22 |Aug. 10 | 45·9 +56·9 | 138·5 |
|Orionids |Oct. 9-29 |Oct. 18 | 92·1 +15·5 | 205·9 |
|Leonids |Nov. 9-17 |Nov. 13 | 150·0 +22·9 | 231·5 |
|Andromedes |Nov. 25-30 |Nov. 27 | 25·3 +43·8 | 245·8 |
|Geminids |Dec. 1-14 |Dec. 10 | 108·1 +32·6 | 259·5 |
+———————————————-+——————————————-+————————+——————————————-+——————————+


_Notes._

_Quadrantids._ Heis was the first to determine this radiant accurately.
It was subsequently observed by Masters and Prof. Herschel (1863-4).
The radiant is circumpolar in this latitude, but low down during the
greater part of the night, hence the display is usually seen to the
best advantage on the morning of Jan. 2.

_Lyrids._ Attention was first drawn to the April meteors by Herrick in
the United States. Active displays occurred in 1863 and 1884.

η _Aquarids._ Further observations are urgently required of this
stream. The radiant is only visible for a short time before sunrise.
There is a considerable difference between my results and those secured
by Lieut.-Col. Tupman, the discoverer of this system in 1870, whose
observations place the radiant at 326½—2½ April 29-May 3. These May
Aquarids are interesting from the fact that they present an orbital
resemblance to Halley’s Comet, which makes a near approach to the Earth
on May 4, twelve days before reaching the descending node.

δ _Aquarids._ The meteoric epoch, July 26-30, was first pointed out
by Quetelet many years ago. Biot also found, from the oldest Chinese
observations, a general maximum between July 18 and 27 (Humboldt).
Showers of Aquarids were remarked by Schmidt, Tupman (1870), and
others; but it was not known until my observations in 1878 that the
Aquarids formed the special display of the epoch, and that there were
many early Perseids visible at the same time.

_Perseids._ Muschenbroeck, in his work on ‘Natural Philosophy,’ printed
in 1762, mentions that he observed shooting-stars to be more numerous
in August than in the other months of the year. Quetelet, in 1835, was,
however, the first to attribute a definite maximum to the 9th-10th.
This stream is remarkable for its extended duration, and for the
obvious displacement which occurs from night to night in the place of
its radiant. It furnishes an annual display of considerable strength,
and is, perhaps, the best known system of all.

_Orionids._ Profs. Schmidt and Herschel were the first to discover
the Orionids as the most brilliant display of the October period, and
accurately determined its radiant in 1863-4-5. Herrick recorded a
shower at 99° +26°, Oct. 20-26, 1839, and Zezioli in 1868 recorded many
meteors which were ascribed to a radiant at 111° +29°; but there is no
doubt that the Orionids were observed in both these cases, though the
radiant was badly assigned.

The radiant of the Orionids shows no displacement like that of the
Perseids.

_Leonids._ Observed from the earliest times. Humboldt and Bonpland saw
it well on the night of November 11-12, 1799, and the phenomenon at its
magnificent return on November 12, 1833, was ably discussed by Olmsted.
It furnished a splendid shower in 1866, November 13, and many meteors
were seen at the few subsequent returns. I observed fairly conspicuous
showers of Leonids in 1879 and 1888. There is no doubt the meteors form
a complete ellipse, for the earth encounters a few of them at every
passage through the node. Grand displays may be expected at the end of
this century.

_Andromedes._ Observed by Brandes, at Hamburg, Dec. 7, 1798. It also
recurred in 1838; the very brilliant showers of November 27, 1872 and
1885, are still fresh in the memory. It is uncertain whether this group
forms an unbroken stream; if so, the regions far removed from the
parent comet must be extremely attenuated. Some of the meteors were
seen in 1877 and 1879. The radiant is diffuse to the extent of 7° or
10°. Returns of the shower should be looked for in 1892 and 1898.

_Geminids._ Mr. Greg first called attention to the importance of this
shower. It was well observed by Prof. Herschel in 1861-3-4, and some
later years.

There are an enormous number of minor systems, but these are generally
feeble, and interesting only to the regular observer of meteors. Many
showers are so slightly manifested that they yield but one visible
meteor in 6 or 7 hours, and on the same night of observation there are
often as many as 50 or 60 different systems in operation. I gave a list
of 918 radiant-points of showers observed at Bristol in the ‘Monthly
Notices,’ May 1890, and other catalogues will be found in the ‘British
Association Reports’ for 1874 and 1878.

_Varieties of Meteors._—The amateur who systematically watches for
meteors will occasionally remark instances of anomalous character.
I have sometimes observed meteors which are apparently very near,
and move with enormous velocity. They are mere gleams of pale light,
which have little analogy to ordinary shooting-stars, and suggest an
electric origin, though I do not know whether the marvellous quickness
with which they flash upon the eye is not to be held responsible for
the impression of nearness. They are somewhat rare, and one may watch
through several entire nights without a single example, but as far as
my memory serves I must have witnessed some scores of these meteoric
flashes.

One of the most interesting class of meteors includes those which
move so slowly that the eye is enabled to note the details of their
appearance. Some of these objects are small when first seen, but
enlarge considerably under the increasing temperature, and after a
great slackening of speed (due to atmospheric resistance) their nuclei
are finally spent in thick streams of luminous dust. On Dec. 28, 1888,
I recorded a meteor which on its first apparition was tolerably bright,
small, and compact. It moved slowly, and I had an excellent view of
its passage. The nucleus quickly expanded, though with no increase of
brilliancy. Towards the end it assumed a sensible disk, and at the
last phase the mass spread or deployed itself into a wide stream of
fine ashes and disappeared. The whole phenomenon was so curious, and
observed with such distinctness, that I made the above sketch of it
directly afterwards.

[Illustration: Fig. 58.

Meteor of Dec. 28, 1888, 6^h 17^m.]

_Heights of Meteors._—Usually the height of meteors at their first
appearance is less than 90 miles, and at disappearance more than 40
miles. From a comparison of a large number of computations I derived
the following average values:—

Beginning height 76·4 miles (683 meteors)
End height 50·8 ” (756 ” )

But if fireballs and the smaller shooting-stars are separated I
find the usual heights at disappearance are:—fireballs, 30 miles;
shooting-stars, 54 miles. Fireballs therefore approach much nearer to
the Earth’s surface before disruption than the ordinary falling stars.

[Illustration: Fig. 59.

Large Meteor, and successive appearances of its streak, seen at Cape
Jask, in the Persian Gulf, on June 8, 1883, 7^h 51^m to 8^h 33^m.]

A very slight acquaintance with trigonometry will enable anyone to
compute the real path of a meteor if two or more observations, made at
distant stations, are available for the purpose. The observed courses
of the meteor should be marked upon a celestial globe, and extended
backwards to the point where they mutually intersect; this will be the
_radiant-point_. The globe having been set for the time and latitude,
the apparent tracks should also be prolonged in a forward direction
until they meet the horizon, this will indicate the _Earth-points_, or
azimuths of the place where the meteor would have been precipitated
on the Earth had it been enabled to continue its flight so far. The
azimuths and altitudes of the beginning and end of the path, and the
azimuths of the Earth-point should then be read off, and by means of
a reliable map and a protractor their points of intersection over the
Earth’s surface may be readily found by lines drawn from the two
places of observation. From the spot where the Earth-points intersect a
straight line should also be drawn in the direction of the radiant, and
it is along this line the meteor’s motion was directed. The coordinates
of the observed points of appearance and disappearance of the meteor,
at the two stations, would intersect this line at identical points were
the observations perfectly accurate, but this is rarely the case. The
distance between the observer’s station and the places over which the
meteor began and ended is easily derived from the map, and the height
of the object may be found by adding the logarithm of the distance to
the log. of the tangent of the altitude. Thus, if the end of a meteor
is witnessed from London in azimuth 130° W. of S. (alt. 25°), and from
Bristol in azimuth 216° W. of S. (alt. 30°) the place of intersection
on the map will be at Warwick, so that the meteor must have disappeared
when vertically over this city. London is distant from Warwick about
86 miles, and from Bristol 70 miles, and the resulting height of the
meteor is:—

London. Bristol.
86 log. 1·93450 70 log. 1·84510
25° tan 9·66867 30° tan 9·76144
———- ———-
1·60317 = 40·1 1·60654 = 40·4

so that the observations accord very closely in fixing the height at a
little exceeding 40 miles at disappearance, but a slight correction is
necessary to allow for the Earth’s curvature. There are other methods
of computing the heights, one of which is explained by Prof. A. S.
Herschel in a paper entitled “Height of a Meteor” (‘Monthly Notices,’
vol. xxv. p. 251).

_Meteoric Observations._—A large number of meteor-showers still await
discovery, and there are features even in connection with the best
known streams which remain to be elucidated. Such doubts as now exist
are only to be cleared away by assiduous observation made with the
utmost accuracy possible both of the _directions_ and _durations_ of
meteors.

This attractive field of investigation has certainly been neglected
in recent years, and the reason of this may perhaps be found in the
complications inseparable from it, in the need of great patience
and scrupulous care in observation, and the necessity of gaining
experience before the observer can feel a reliance on his work,
and draw safe conclusions. Meteors are so fugitive, so diverse and
erratic in their apparitions, as to be quite beyond the scope of
instrumental refinements. They must necessarily be observed under many
disadvantages. Positions have to be fixed from very hurried and often
imperfect impressions. But these drawbacks, formidable as they at
first appear, may be severally overcome by practice, by careful regard
for the conditions under which meteors are displayed, and the marked
differences of aspect induced by these conditions. When the observer
has acquired a practical knowledge he will proceed with confidence in
his work, and avoid many of the difficulties surrounding it.

In recording meteor-tracks for the purpose of discovering the
radiant-points, the chief feature in which precision is essential is
the _direction_ of flight. A perfectly straight wand, held in the hand
for the purpose, should be projected upon the path of every meteor
directly it is seen, and then when the eye has quickly noted the
position and slope relatively to the fixed stars near, it should be
reproduced on the chart or celestial globe. The time, mag., estimated



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 23 of 32)