William F. Denning.

Telescopic Work for Starlight Evenings online

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duration, and details of appearance should be registered in a tabular
form, with the R.A. and Dec. of the beginning-point and probable
radiant. The end-point and length of path may be left until next day,
in order to save valuable time. The wand is a great assistance to the
eye in retaining the approximate directions and noting the places.
If a meteor belongs to the slow, trained class, or if it belongs to
the swift, streak-leaving order, the path may be very accurately
noted, for the wand can be adjusted to its direction before the meteor
or its visible offcome has died away. In the case of short, quick
meteors, devoid of either streaks or trains, and generally shooting
from radiants at high altitudes, they are more difficult to secure, as
they vanish before one may turn, and the observer must rely upon the
mere impression he received. But even these succumb to experience, and
will be found to resolve themselves into a number of sharply defined
radiants scarcely less certain than the positions derived from the
streaked or trained meteors.

These positions are only to be fixed by the exercise of much cautious
discrimination on the part of the observer, for the direction of the
flight is not sufficient, alone, to indicate it. The visible aspect of
the meteor has to be equally considered, for the place of its radiant
imparts certain peculiarities to it which are rarely to be mistaken.
First, _the astronomical position_ of the radiant. If the radiant is
at, or within 50° of, the Earth’s apex (a point 90° preceding the Sun
along the ecliptic, and towards which the Earth’s motion is directed)
the meteors generally leave streaks, especially the brighter ones, and
move with great speed. They are usually white, exhibiting a high degree
of incandescence. If the radiant is near the anti-apex or anywhere
in the anti-apex half-sphere the meteors are streakless, they travel
slowly or very slowly, and often leave trails of yellowish sparks.
Bearing these facts in mind the region may be assigned in which any
radiant is situated, if not the exact position of the radiant itself.
If, say, on Aug. 10, at midnight a swift, streaked meteor is seen
shooting from the Pleiades towards Aldebaran, just risen, the radiant
is either in Musca, Triangulum, or Andromeda. But if the meteor is
slow, with a train, then we must go further back in the direction of
its flight, and seek the radiant in the S. or S.W. sky. If the motion
is very slow, the radiant may be as far away as Aquila. Second, _the
sensible position of the radiant_. A low radiant yields long-pathed
meteors, characterized by slowness of speed and a flaky appearance
either of the streaks or trains. A radiant near the zenith gives
short, darting meteors, with rather dense streaks or trains. These
nearly vertical meteors have a less extensive range of atmosphere to
penetrate than the horizontal meteors, which are sometimes abnormally
long. In the case of brilliant meteors, however, the paths occasionally
extend over considerable arcs though the radiant may be high. Third,
_the position of the radiant relatively to the path of a meteor_. If a
meteor is close to its radiant its track is usually slow, and appears
greatly foreshortened by the effects of perspective. It is travelling
(approaching) nearly in the line of sight, and the streak or offcome
of sparks is especially dense because it is seen through its entire
depth; and the nucleus in such a case has a brushy diffused appearance.
Such meteors often traverse sinuous, or curved paths of 2°, 3°, or 4°,
and they are readily distinguishable from other meteors far from the
radiants to which they belong.

A good method of tabulating meteor-tracks is that adopted by
Lieut.-Col. Tupman in his catalogue published by the British
Association in 1874. I have adopted the same form, and herewith append
a copy of my register of a few isolated bright meteors observed in the
autumn of 1890:—

+————————+——————+——————+————————————————————-+——————+————-+
| | | | Observed Path | | |
| Date |G.M.T.| Mag. +————————-+——————————-+Length|Dura-|
| 1890. | | | From | To | of |tion.|
| | | |R.A. Dec.| R.A. Dec | Path.| |
+————————+——————+——————+————————————————————-+——————+————-+
| | h m| | o o | o o | o | sec.|
|Oct+. 17| 10 37| >1 | 219+61 | 255+65 | 16 | 3·5 |
| 19| 10 35| 1 | 61½+26 | 44½+27½ | 15½ | 0·7 |
| 19| 12 0| ½ ☽ | 326-8 | 319-10 | 7 | 0·5 |
| 25| 17 18| >♃ | 168+34 | 180+24 | 14½ | 0·8 |
| 26| 7 33| ♃ | 329+69 | 243+51 | 42 | 4·0 |
|Nov. 1 | 7 1| >1 | 278+49 | 244+11½ | 46 | 6·0 |
| 1 | 9 17| >1 | 345+11 | 307+1½ | 39 | 4·0 |
| 5 | 10 40| >♀ | 28½-25 | 25½-29½ | 5½ | 0·7 |
| 16 | 11 15| ♃ | 274+77 | 265½+67 | 10 | 1·5 |
+————————+——————+——————+————————————————————-+——————+————-+
+————————+—————————————————————+————————+
| | | |
| Date | Appearance. |Probable|
| 1890. | |Radiant.|
| | | |
+————————+—————————————————————+————————+
| | | o o |
|Oct+. 17| V. slow, B. train. | 204+56.|
| 19| Swift, streak. | Orion. |
| 19| Swift, streak. | Orion. |
| 25 |Swift, streak. | Lynx. |
| 26| Slow. | 32+18. |
|Nov. 1 | Very slow. | 50+15. |
| 1 | Slow. | 50+15. |
| 5 | Swift, strk. 15 sec.| Taurus.|
| 16 | Not very swift. | Auriga.|
+————————+—————————————————————+————————+

The _duration of flight_ is a most important element to estimate
correctly, as it affords data wherewith the real velocity may be
computed, and enables the nature of the orbit in which the meteor is
moving to be definitely assigned. This feature is, however, one of the
most difficult of all to derive with satisfactory precision. In the
case of very slow meteors lasting several seconds, it is easy by means
of a stop-watch, or by other methods, to get the times of flight within
narrow limits of error, but the swifter class of meteors complete their
visible trajectories in the fraction of a second, and are gone before
any effort can be made to gauge their durations, so that a value has to
be attributed which is little better than a mere guess.

Every adopted radiant-point should be based on at least five paths,
unless the conditions are special, and these must show a very definite
centre, and present family resemblances. It is often possible to detect
a good centre from very few paths, when the radiant is low on the
horizon, or when it occupies an isolated position.

In recording meteors the details of their appearances should also be
appended to the paths. Foreshortened and crooked courses, fluctuations
of brightness, halting motion, spark-trains, phosphorescent streaks,
broken streaks, and other features must be invariably noted when
observed, as likely to assist in fully comprehending these bodies. A
streak will sometimes brighten up perceptibly after the head has died
out.

One of the principal aims of future observers should be to ascertain
the visible duration of meteor-showers, and the displacement or fixed
position of the radiants during the period of their continuance. The
Perseids seem to endure for forty-six nights (July 8-August 22) while
the radiant moves from 3° +49° to 76° +57°. The Lyrids also exhibit
a shifting radiant, and it is highly probable some other showers are
to be included in the same category. In investigating these, the
observations of single nights should be kept separate, and the radiant
determined from each set of paths. The positions when compared will
then exhibit the rate and direction of the displacement. As to radiants
which are apparently stationary[46] during long intervals, these should
be closely observed. Are the centres of radiation, as successively
determined, identical, allowing for the slight errors of observation?
Are they continuously in operation, or intermittent? Meteors with
motions in declination and near their radiants will be specially
valuable in settling these questions, and if observed at more than one
station will possess great significance. If it can be proved that a
radiant is fixed and continuous during a few weeks, there can be no
reason why it may not be stationary for a much more lengthy interval,
unless the circumstances are exceptional.

Though I have pointed out the urgency of noting the directions and
durations of meteors, there are other features in such observations
that must not be disregarded. If the paths are being recorded for the
particular purpose of getting duplicate observations and calculating
the heights, then it is desirable to note the beginning-and end-points
of the flights as exactly as possible, for unless this is done the
combined paths will show great discordances. Those who have acquired
a familiar knowledge of the constellations will, however, experience
little trouble in insuring accuracy in these records.

Observers, particularly those residing in towns, must be constantly on
their guard against mistakes in identifying meteors from terrestrial
objects such as fire-balloons and the various forms of pyrotechnic
display. That such caution is necessary will be admitted when we read
the two following letters, which were published in the ‘Times’ some
years ago:—

“SIR,—

“A large meteor was seen to-night at 8.27, moving very slowly along
the northern horizon, from west to east, at an altitude of about 8
deg. It was at least three times as brilliant as Venus, remaining
visible for nearly five minutes, moving slower than any hitherto
observed. I should be glad to receive observations made at more
favourable stations....

“I remain, Sir, your obedient Servant,

“THOMAS CRUMPLEN.

“Mr. Slater’s Observatory, Euston Road, August 10th.”

“SIR,—

“The ‘large meteor’ seen by Mr. Crumplen on Monday evening at 8.27,
three times as brilliant as Venus, and moving from west to east,
was a fire-balloon sent up shortly after 8 o’clock from the Eton
and Middlesex Cricket Ground, Primrose Hill, as a _finale_ to some
athletic sports which had taken place during the afternoon.

“I am, Sir, your obedient Servant,

“B. C. C.

“St. John’s Wood, August 12th.”

In concluding this chapter I may briefly mention that an old idea
concerning meteors was that they originated gales of wind, and that, in
fact, they were the usual precursors of stormy weather. This belief is
thus expressed in Dryden’s ‘Virgil’:—

“Oft shalt thou see, ere brooding storms arise,
Star after star glide headlong down the skies,
And, where they shot, long trails of lingering light,
Sweep far behind, and gild the shades of night.”


FOOTNOTES:

[45] During the seven months from May to November 1890 I noted
ninety-five telescopic meteors while engaged in comet-seeking.

[46] A list of these was published in the ‘Monthly Notices,’ vol. 1. p.
466. See also ‘Monthly Notices,’ vol. xlv. pp. 93 _et seq._




CHAPTER XVI.

_THE STARS._

Sidereal Work.—Greek alphabet.—Learning the Names of the
Stars.—The Constellation figures.—Means of Measurement.—Dividing
power.—Number of Stars.—Magnitudes.—The Milky Way.—Scintillation
of the Stars.—Star-Disks.—Distance of the Stars.—Proper Motion of
Stars.—Double Stars and Binary Systems.—Variable Stars.—New or
Temporary Stars.—Star Colours.—Groups of Stars.—Further Observations.

“Ten thousand suns appear
Of elder beam; which ask no leave to shine
Of our terrestrial star, nor borrow light
From the proud regent of our scanty day.”
BARBAULD.


The planetary observer has to accept such opportunities as are given
him; he must use his telescope at the particular seasons when his
objects are well presented. These are limited in number, and months may
pass without one of them coming under favourable review. In stellar
work no such irregularities can affect the progress of observations.
The student of sidereal astronomy has a vast field to explore, and a
diversity of objects of infinite extent. They are so various in their
lustre, in their grouping, and in their colours, that the observer’s
interest is actively retained in his work, and we often find him
pursuing it with unflagging diligence through many years. No doubt
there would be many others employing their energies in this rich field
of labour but for the uninteresting character of star-disks, which
are mere points of light, and therefore incapable of displaying any
detail. Those who study the Sun, Moon, or planets have a large amount
of surface-configuration to examine and delineate, and this is ever
undergoing real or apparent changes. But this is wholly wanting in
the telescopic images of stars, which exhibit a sameness and lack
of detail that is not satisfying to the tastes of every observer.
True there are some beautiful contrasts of colour and many striking
differences of magnitude in double stars; there are also the varying
position and distance of binary systems, the curious and mysterious
fluctuations in variable stars, and some other peculiarities of stellar
phenomena which must, and ever will, attract all the attention that
such important and pleasing features deserve. And these, it must be
conceded, form adequate compensation for any other shortcomings. The
observer who is led to study the stars by comparisons of colour and
magnitude or measures of position, will not only find ample materials
for a life-long research, but will meet with many objects affording him
special entertainment. And his work, if rightly directed and accurately
performed, will certainly add something to our knowledge of a branch in
which he will certainly find much delectation.

_Greek Alphabet._—The amateur must, at the outset of his career,
thoroughly master the Greek alphabet. This will prevent many
time-wasting references afterwards, and avoid the doubt and confusion
that must otherwise result. The naked-eye stars in each constellation
have Greek letters affixed to them on our celestial globes and
star-maps.

α Alpha
β Beta
γ Gamma
δ Delta
ε Epsīlon
ζ Zēta
η Eta
θ Theta
ι Iota
κ Kappa
λ Lambda
μ Mu
ν Nu
ξ Xi
ο Omīcron
π Pi
ρ Rho
σ Sigma
τ Tau
υ Upsīlon
φ Phi
χ Chi
ψ Psi
ω Omĕga.

The letters are applied progressively to the stars (generally according
to brightness) in each constellation. The 1st-mag. stars frequently
have a duplicate name. Thus α Leonis is also known as Regulus, and α
Canis Majoris as Sirius, the Dog-star.

_Learning the Names of the Stars._—A knowledge of the stars as they
are presented in the nocturnal sky may be regarded as the entrance to
the more advanced and difficult branches of the science, and forms
the young observer’s introductory lesson. When he has learnt a few of
the principal constellations, and can point them out to his friends,
he already begins to feel more at home with the subject, and regards
it with a different eye to what he did before when the names and
configurations of the stars were alike unknown to him. He no longer
views the heavens as a mysterious assemblage of confusing objects, for
here and there he espies certain well-known groups always preserving
the same relative positions to each other. The unconscious gaze he
formerly directed to the sky has given way to the intelligent look of
recognition with which he now surveys the firmament.

An acquaintance with the leading constellations, and with the names
or the letters of the brighter stars in each, becomes very important
in some departments of observation, and various methods have been
suggested as likely to impress the positions and names on the memory.
The beginner must first be content to get familiar with a few of the
brighter stars, and make these the base for extending his knowledge.
The objects are so numerous that it is impossible his primary attempts
can be anything like complete. He must advance step by step in
his survey, and feel his way cautiously, setting out from certain
conspicuous stars with which he has already become conversant. A
lantern and a series of star-maps are the only aids required, and
with these he ought to make satisfactory progress. The stars as they
are seen in the sky may be compared with those figured in the maps,
and their names and the constellations in which they lie may then be
identified. It is an excellent plan as conducing to fix the positions
indelibly in the memory to construct maps from personal observation,
and to compare these afterwards with the published maps for
identification of the constituent stars. This plan, if repeated several
times, has the effect of impressing the positions of the leading stars
forcibly upon the observer’s mind.

It is not intended to give, in this place, any details as to the
places or distribution of the stars. Without diagrams, such a
description could not be made readily intelligible. To those, however,
who are commencing their studies, I would recommend the northern sky as
the most suitable region to aid their initiatory efforts. For

“He who would scan the figured sky
Its brightest gems to tell,
Must first direct his mind’s eye north
And learn the Bear’s stars well.”

The seven bright stars of Ursa Major are familiar to nearly everyone.
Two of them, called the Pointers, serve to direct the eye to the Polar
star, which, though not a brilliant one, stands out prominently in a
region comparatively bare of large stars. It is important to know the
Polar star, as it is situated near the centre of the apparent motion
of the firmament. When the student has assured himself as to the
northern stars he will turn his attention southwards, and recognize the
beautiful Orion and the curious groups in Taurus. He will also observe,
much further east, the well-known sickle of Leo, and in time become
acquainted with the many other constellations that make the winter sky
so attractive.

[Illustration: Fig. 60.

The constellation Orion.]

_The Constellation Figures._—The observer will soon realize that the
creatures after which the constellations have been named bear no
resemblance to the configuration of the stars they represent. If we
look for a Bear amongst the stars of Ursa, for a Bull amid the stars of
Taurus, or for a flying Swan in the stars of Cygnus we shall utterly
fail to find it. The names appear to have been originally given, not
because of individual likenesses between them and the star-groups
to which they are applied, but simply on account of the necessity
of dividing the sky into parts, and giving each a distinguishing
appellation, so that it might be conveniently referred to. There were
pressing needs for a system of stellar nomenclature, and the plan of
grouping the stars into imaginary figures was the one adopted to avoid
the confusion of looking upon the sky as a whole. There are some who
object to the method of the Chaldean shepherds because the series
of grotesque figures on our star-maps and globes bear no natural
analogies. But it would be unwise to attempt an innovation in what has
been handed down to us from the myths of a remote antiquity, for

“Time doth consecrate,
And what is grey with age, becomes religion.”

[Illustration: Fig. 61.

Diagram illustrating the Measurement of Angles of Position.

(In measuring angles of position the larger star is always understood
as central in the field. The north point is zero, and the angles are
reckoned from this point towards the east. If a star has a faint
component lying exactly east or following it, then the angle is 90°;
if the smaller star is south, the angle is 180°; and so on.)]

_Means of Measurement._—A micrometer becomes an indispensable
instrument to those who make sidereal observations of an exact
character. Without such means it will be impossible to determine
either positions or distances except by mere estimation, and this
is not sufficiently precise for double-star work. With a reliable
micrometer[47] excellent results may be obtained, especially with
regard to the varying angles of binary systems. Frequent remeasurement
of these is desirable for comparison with the predicted places in
cases where the orbits have been computed. In this department of
astronomy the names of Herschel, South, Struve, Dawes, Dembowski,
Burnham, and others are honourably associated, and it is notable that
refracting-telescopes have accomplished nearly the whole of the work.
But reflectors are little less capable, though their powers seem to
have been but rarely employed in this field. Mr. Tarrant has lately
secured a large number of accurate measures with a 10-inch reflector
by Calver, and if care is taken to secure correct adjustment of the
mirrors, there is no reason why this form of instrument should not
be nearly as effective as its rival. Mr. Tarrant advises those who
use reflectors in observing double stars “to test the centering of
the flat at intervals during the observations, as the slightest shift
of the large mirror in its cell will frequently occasion a spurious
image which, if it by chance happens to fall where the companion is
expected to be seen, will often lead to the conclusion that it has
been observed. In addition to this, any wings or the slightest flare
around a bright star will generally completely obliterate every trace
of the companion, especially if close and of small magnitude, and such
defects will in nine cases out of ten be found to be due to defective
adjustment. Undoubtedly for very close unequal pairs the refractor
possesses great advantages over a reflector of equal aperture; in the
case of close double stars the components of which are nearly equal
there appears to be little, if any, difference between the two classes
of instruments; while for any detail connected with the colour of stars
the reflector certainly comes to the fore from its being perfectly
achromatic.” These remarks from a practical man will go far to negative
the disparaging statements sometimes made with regard to reflectors and
stellar work, and ought to encourage other amateurs possessing these
instruments to take up this branch in a systematic way.

_Dividing Power._—This mainly depends upon the aperture, and it was
made the subject of careful investigation and experiment by Dawes,
who found that the diameters of the star-disks varied inversely as
the aperture of the telescope. Adopting an inch as the standard, he
ascertained that this aperture divided stars of the sixth magnitude
4″·56 apart, and on this base he constructed the following table:—

Aperture Least
in inches. separable
distance.


1·0 4·56
1·6 2·85
2·0 2·28
2·25 2·03
2·5 1·82
2·75 1·66
3·0 1·52
3·5 1·30
3·8 1·20
4·0 1·14
4·5 1·01
5·0 0·91
5·5 0·83
6·0 0·76
6·5 0·70
7·0 0·65
7·5 0·61
8·0 0·57
8·5 0·536
9·0 0.507
9·5 0.480
10·0 0·456
12.0 0·380
15·0 0·304
20·0 0·228
25·0 0·182
30·0 0·152

Dallmeyer, the optician, confirmed these values by remarking:—“In all
the calculations I have made, I find that 4·33 divided by the aperture



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 24 of 32)