William F. Denning.

Telescopic Work for Starlight Evenings online

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will merely require to know the approximate place, and this is to be
found by estimating the difference in R.A. and Dec. between a comet
and a known star. The position of the latter may be found in a good
catalogue and corrected for precession; then, allowing for the observed
differences, the comet’s place may be assigned to within very small
limits of error. A low power, embracing a field of 1° or more, is best
adapted for these observations, as it is more likely to include a
catalogued star, and will exhibit the comet, especially if a large one,
to the best effect.

The announcement of a new comet is always read with avidity by
amateurs, and their first desire is to see it for themselves. This
they may readily do by marking its place on a star-map or globe, and
noting its relative place amongst the stars near. The telescope should
then be directed towards the point indicated, and if the comet is not
presented in the field, the instrument should be moved a little so
that the surrounding region may be examined. If failure still attends
the effort, the observer should point the telescope a few degrees E.
or W. of the suspected point, and then carefully sweep over the place
of the comet. It will then be picked up, unless it is too faint for
his aperture. The first announcement of a comet generally gives the
position at discovery, and the daily rate and direction of motion. The
latter must of course be allowed for when the search is instituted.

The physical aspects of comets are as diversified as they are variable.
No two comets are exactly alike, nor does the same comet exhibit a
permanency of detail. Of course, when these objects are enormously
distant, and barely visible, many of them appear to present similar
characteristics; but under the closer and more expanded views
obtainable near perihelion the resemblance vanishes, and every comet is
seen to possess features peculiar to itself. To trace these features,
and to record them by delineation and description, forms one of the
most interesting branches in which amateurs may engage. Much has been
learnt of previous comets by successively noting their transitions of
form and brightness, and the same scrupulous attention should be given
to future comets.

The tails of comets are not always turned away from the Sun. Indeed,
the contrary effect is sometimes produced. Occasionally there is a
duple tail, the largest branch of which follows the normal direction,
while the other is turned towards the Sun. Forms of this character
require close watching from night to night. Is the sunward tail
developed suddenly? and has it a fairly durable existence? Instances of
singular curvature should also be noted. The tails are seldom perfectly
straight, especially those attached to naked-eye comets, and decided
changes affect their visible outlines at very short intervals. In large
comets the space over which the tail extends should be sketched upon a
star-map on successive evenings; its changes of position and curve will
then be manifested by comparisons, and its increasing or decreasing
length will also be apparent. Dark rifts, like shadows, often run
lengthwise through the tail, and occasion a fan-like appearance
analogous to that which distinguished the great comet of 1744 and gave
it a sextuple tail.

The light of comets sometimes fluctuates in a very extraordinary
manner, and too rapidly and irregularly to be consistent with theory.
In this respect, Pons’s Comet, at its last return in 1883-4 presented
an eccentric behaviour. Bigourdan found that during the nineteen days
from Sept. 5 to 24, 1883, the increase in the comet’s brilliancy
exceeded by thirty or forty times that resulting from reflected light
alone! This increase appears to have been due to a sudden outburst on
Sept. 22, which occurred some time within the four hours preceding
midnight. Dr. Müller, of Potsdam, witnessed a further outburst on Jan.
1, 1884, within 1-3/4 hour; and the extent of this was accurately
determined by means of a photometer. He found an augmentation of
seven tenths of a magnitude in the brightness of the comet, and an
equally sudden fall to its previous lustre. While these fluctuations
were in progress, he noticed variations in the shape of the nucleus
not less remarkable than its variations in light. Those who observe
future comets will do well, therefore, to be on the alert for similar
phenomena. The apparent brightness of the nuclei and alterations of
shape or size should be recorded on every night when observations are
feasible.

As a comet approaches the Sun its material apparently contracts, while
with increasing distance from that luminary it expands. Usually the
nucleus is extremely small and bright, and it often looks like a star
shining through nebulosity. High powers must therefore be applied
in its examination. Jets, aigrettes, luminous sectors, and other
appendages are often involved with the nucleus and outlying coma, and
they form a complicated structure well deserving further study. A good
deal of mystery still surrounds these appearances; their curious forms
and vagaries have yet to be explained.

Stars are frequently observed through the head of a comet, which
apparently, however, exercises no influence in dimming their lustre.
But the stars are commonly seen behind the envelopes or comæ, and very
rarely through the nucleus. Nothing is better calculated to exhibit
the transparent and tenuous character of comets than observations of
this kind, and observers should seek for further opportunities of
making them. If the motion of a comet is obviously carrying it in the
direction of one of the stars in the field, the observer may determine
for himself the approximate time of conjunction by noting the distance
between the star and comet and allowing for the motion of the latter.
He will then know when to come to his telescope and witness the
phenomenon. Should it appear probable that the comet’s nucleus will
pass over the star, he should commence his watch some time before it
occurs; he may then make comparisons before the star is involved in
the outlying nebulosity, and trace the whole event from beginning to
end. Any changes in the light or aspect of either star or comet would
then be manifested. The comet of 1847 is said to have passed centrally
over a 5th mag. star, but the latter was unaffected. Encke’s Comet on
one occasion interposed itself directly over one of a pair of 10th
mag. stars, but their relatively equal brilliancy suffered no change.
Encke’s Comet, however, has no stellar nucleus. The latter feature
is so bright and compact as displayed in many other comets, that its
transit over a small star must have some effect either in obliterating
it altogether, or in detracting from its lustre.

Visible evidences of rotation seem to have been suspected in certain
comets, but this has never been substantiated on sufficient grounds.
The circumstance is one, however, which should be remembered. During a
series of observations the observer who notes the details of structure
with particular regard to position may discover similar traces, and
possibly learn something of the cause. The nucleus of a bright comet
should always be examined with a moderately high power, so that any
variations or peculiarities of form may be detected.

_Nomenclature of Comets._—It must be confessed that no perfectly
satisfactory method has yet been devised as regards the naming of
comets. The plan of affixing Roman numbers progressively for each
year, according to date of perihelion passage, answers pretty well,
though a little confusion is sometimes caused by prematurely affixing
the number, especially when two comets are discovered successively,
the first of which is a long time before perihelion, and the second
considerably after it. Until a comet can be safely assigned its
catalogue place, it is preferable to refer to it by the name of the
discoverer and date of discovery. This is more distinctive than the
common method of lettering comets according to the epochs of their
detection. As to periodical comets, it is not difficult to find some
inconsistencies in their names. In the case of Halley’s Comet (1682,
discovered by Flamsteed) and Encke’s Comet (1819 I., discovered by
Pons), it was most fitting that they should be known to posterity by
the names of the two able computers whose investigations first revealed
to us comets of long and short period respectively. Under ordinary
circumstances the name of the discoverer is applied to a comet as a
means of convenient reference, and perhaps as a suitable recognition
of the patient labours of the man who first announced it to the world.
The plan seems to have been to name comets after those fortunate
persons who sighted them at the particular apparition during which
periodicity was determined. Thus Tuttle’s Comet (1858 I.) had been seen
as long before as 1790 by Mechain, and Biela’s (1826 I.) was previously
observed in 1772 by Montaigne, and in 1805 by Pons. It is, however,
strange that a comet found by Pons in 1819 (III. of that year), and
which Encke showed to be revolving in an ellipse with a periodic time
of 5½ years, should be called after Winnecke, who rediscovered it in
1858. To Pons the real priority belongs, though Winnecke deserves
much praise for redetecting and identifying this body after it had
effected six unobserved returns to perihelion. It is also curious to
find that the comet of short period discovered by Swift in 1880 is
called “Tempel’s 3rd Comet” in Galle’s catalogue (1885), from the fact
that Tempel found it at a previous return (1869), when, however, its
period was not ascertained. There is little doubt that the title justly
belongs to Swift. Tempel himself called it “Swift’s Comet.” One plan
should in fairness to observers be consistently adhered to. If comets
are to be called after their original discoverers, then Biela’s Comet
should be known as “Montaigne’s,” Tuttle’s as “Mechain’s,” &c.

_Curiosities of Comets._—The comet of 1729, which was hardly visible to
the naked eye, has far the greatest perihelion distance (4·0435) of any
comet known. Barnard’s Comet (1885 II.) comes next with a perihelion
distance of 2·5068.

Pons’s Comet at its return in 1883-4 remained visible for nine months.
When last seen, on June 2, 1884, it was 470 millions of miles from the
Earth, and more remote in the depths of space than any other observed
comet since that of 1729. Barnard’s Comet (1889 I.), though never
visible to the naked eye, was followed from Sept. 2, 1888, to Aug. 18,
1890. Its distance from the Sun was then 6·25 (Earth’s distance = 1),
or about 580 millions of miles, which is greater than that of many of
the short-period comets at aphelia. The most prolonged visibility of
any previous comet was that of 1811 I. (510 days). But this comet of
Barnard has been retained in view 715 days.

The great comet of 1882 was watched right up to the Sun’s limb by
Messrs. Finlay and Elkin at the Cape of Good Hope on Sept. 16, 1882.
The comet was actually seen to disappear at the margin, and not a
vestige of it could be traced during its transit across the solar
disk. The nucleus of the comet was 4” in diameter, and before transit
it looked as bright as a part of the Sun’s surface; but it was quite
invisible when projected on the disk. The alleged observations by
Pastorff and Stark, which were construed into visible transits of
comets, are therefore thoroughly disproved, and will require another
interpretation.

At the time of the total solar eclipse of May 17, 1882, a bright comet
was observed near the Sun. It was a striking object visible to the
naked eye. In the photographs which were taken of the eclipse the comet
is well shown, but this body escaped subsequent observation, so that
its orbit could not be determined.

_Naked-eye Comets._—Arago mentions that twelve comets were visible to
the naked eye during the period from 1800 to 1853, but there appear
to have been certainly thirty comets fulfilling this condition, and I
believe a careful search amongst cometary records would further augment
the number. During the ten years from 1880 to 1889 inclusive there
were no less than sixteen comets perceptible to unaided vision, and a
considerable proportion of these were fine comets. It is very rarely
that two naked-eye comets are to be seen at the same epoch, as in
August 1881 and at the end of April 1886.

_Comet-seeking._—For a long time after the invention of the telescope
comet-seeking does not appear to have been undertaken in a methodical
way, and to have formed the habitual work of certain observers. But
the expected return of Halley’s Comet in 1759 roused observers to
take the initiative in a branch of practical research which in after
years was destined to prove remarkably productive. Messier, Palitzch,
and others began a system of sweeping the heavens for the predicted
comet; and it had a successful issue, for Palitzch, who did not relax
his labours even on Christmas day, alighted upon the coveted prize on
Dec. 25, 1758. Since that time a regular search after comets has been
maintained. Messier pursued it with indomitable energy through a long
period of years, and achieved many successes. It is said of him that
on one occasion he was anticipated in a discovery by Montaigne, and
he appears to have deplored the loss of the comet more than the loss
of his wife, who was lying dead at the time. A friend visited him,
and spoke a few words of sympathy in reference to his bereavement,
but Messier, in despair about the comet, exclaimed: “I had discovered
twelve—alas! that I should be robbed of the thirteenth by Montaigne!”
and his eyes filled with tears. Recollecting himself, and appreciating
the loss he had sustained in his wife, he added, “Ah, this poor woman!”
Messier encountered some serious obstacles to his favourite pursuit.
Breen, in his ‘Planetary Worlds,’ mentions that Messier, while walking
in President Saron’s garden, fell into an ice-house, and was disabled
for a time. Later on “the revolution deprived him of his little income
and every evening he was wont to repair to the house of Lalande to
replenish the supply of oil for his midnight lamp. The political storm
necessitated his removal to another neighbourhood, where he no longer
heard the clocks of forty-two churches sounding the hours during his
night-watchings.” Messier discovered all his comets with a small 2-foot
telescope of 2½ inches aperture magnifying 5 times and with a field of
4°.

Dr. Olbers, of Bremen, was another diligent student in this field. He
did not effect many discoveries, but, from an upper apartment of his
house, he observed nearly all the comets which appeared during half a
century.

During the first twenty-seven years of the present century, Pons
discovered the majority of the comets that were seen. He was a
door-keeper at the Observatory at Marseilles, and owing to the teaching
and encouragement he received from Thulis, the director, he achieved
phenomenal success as a comet-hunter.

Discoveries of comets have rarely been effected in England. This
is chiefly to be assigned to two circumstances. First, because the
labour involved in seeking for these bodies has never perhaps been
pursued to an equal degree and with the same tenacity as it formerly
was in France, and as it has recently been in the United States; and
second, because the cloud-laden skies of England oppose the successful
prosecution of a research in which a clear atmosphere is eminently
desirable.

Though comet-seeking does not always produce new discoveries, it is
certainly entertaining to those engaged in it; for one of the most
agreeable diversions of telescopic work is to scan the firmament with a
large-field comet-eyepiece, which exhibits the most pleasing views of
star-groups, coloured stars, nebulæ, and telescopic meteors.

The operation of sweeping for comets is attractive from other aspects,
though it undoubtedly needs close application, patience, and much
caution. The possibility of seeing a comet in the field at any time
proves a constant source of allurement to the observer, and sustains
his enthusiasm. The glimpsing of a nebulous object, and the expectation
(before it has been identified) that it _may_ prove a comet, induces
a little excitement which pleasantly relieves the monotony that might
otherwise be attached to a sedulous research of this nature; and it
is one in which amateurs may suitably engage with a fair prospect of
success. Instruments of great power, refinement, and expense are not
required. It is rather a work calling for the exercise of patience and
acute perception, and for that tireless servitude which those only who
have an inborn love for it can maintain.

_English Weather and Comet-seeking._—Only two new comets having been
discovered in England during the last forty years some people regard
our climate as in a great measure responsible for this. But the opinion
seems to be erroneous. The lack of discoveries has arisen from want
of effort as much as from want of opportunity. The best weather for
comet-seeking is when the atmosphere is very transparent, and the
stars are lucid and sparkling. Haze, fog, or cloud of any kind offers
a serious hindrance. A thoroughly good night for planetary work is
not usually good for cometary observation, because sharp definition
is not so requisite as a very clear sky. A little fog or thin cloud,
which will often improve planetary images, utterly obliterates a small
telescopic comet. The air is sometimes very pure and dark after storms,
and the stars remarkably bright; it is then that the best opportunities
are afforded for comet-hunting. Any systematic and regular work like
this may be pursued in this country with every prospect of success by
an observer who will persevere in it. From some statistics printed in
the ‘Science Observer,’ Boston, it appears that during the seven months
from May to November, 1882, Lewis Swift was comet-seeking during 300
hours. I have no English results of the same kind, but my meteoric
observations will supply a means of comparison. From June to November,
1887 (six months), I was observing during 217 hours, and for a nearly
similar period during the last half of 1877, though in each year work
was only attempted with the Moon absent. My result for 1887 averages
36 hours per month, which is little less than the average derived
from the comet-seeking records above quoted. It is therefore fair to
suppose that as much may be done here as in some regions of the United
States. Mr. W. R. Brooks wrote me in 1889, saying: “We have much cloudy
weather in this part of America. While in other portions of the country
clear weather abounds, it is not so in this section, where much of
my work has been done. This is a most fertile section—the beautiful
lake region of N.Y.,—but it is for this reason a cloudy belt. It is
far different in Colorado and California. In the latter place, at the
Lick Observatory, I hear they have 300 clear nights in the year—a
paradise for the astronomical observer! My former site, the Red House
Observatory, Phelps, N.Y., is only six miles from Geneva, and hence
in the same cloudy region.” Prof. Swift also referred to the subject
of weather in a letter to me dated July 30, 1889, where he says: “I
arrived home, after a five weeks’ visit to the Lick Observatory, on
March 1, and have not had half a dozen first-class nights since—not
in thirty years have I seen such prolonged rainy and cloudy weather.”
Now Mr. Brooks has discovered 13 comets in 7 years, Prof. Swift has
found 8 comets (1862-1890), and in addition to these has detected
more than 700 new nebulæ, all of the latter since 1883. From this it
appears conclusively that if such extensive and valuable results can
be obtained, notwithstanding frequently bad weather, then English
observers may prove equally successful, the important factor being that
similar energy and ability direct their labours.

_Aperture and Power required._—Opinions are divided as to the most
suitable aperture and power for this work. Any telescope of from
4-to 10-inches aperture may be employed in it. A low power (30 to
50) and large field (50′ to 90′) eyepiece are imperative; and the
instrument, to be really effective, should be mounted to facilitate
sweeping either in a vertical or horizontal direction. A reflector on
an alt-azimuth stand is a most convenient form for vertical sweeps.
The defining-capacity of the telescope need not necessarily be perfect
to be thoroughly serviceable, the purpose being to distinguish faint
nebulous bodies, and not details of form. Far more will depend upon
the observer’s aptitude and persistency than upon his instrumental
means, which ought to be regarded as a mere adjunct to his powers
and not a controlling influence in success, for the latter lies in
himself. Very large instruments are not often used, because of their
necessarily restricted fields. Moreover, a small instrument, apart
from its advantage in this respect, is worked with greater facility
and expedition. This is important, especially when the observer is
to examine the region in the immediate neighbourhood of the Sun. He
has then a very brief interval for the attainment of his purpose,
and a small telescope must be used on account of its large field,
its ready manipulation, and its general effectiveness on objects at
low altitudes. The case is somewhat different when the search is to
be conducted in regions far removed from the Sun’s place; for here
the comets are in general faint, and there is time for the work to
be deliberately and critically performed. Large instruments are to
be recommended for these districts as capable of revealing fainter
objects, though they are troublesome in several respects. They show
large numbers of nebulæ, especially if the observer is exploring the
region of Virgo, Coma Berenices, or Ursa Major; and he will have great
difficulty in identifying them and in feeling his way with certainty.
These complications are inseparable from the work, and, though
chiefly affecting large apertures, should not always be shunned; for
a telescope capable of displaying very faint nebulæ is also capable
of showing faint comets. Many comets have eluded discovery by the
inadequate reach of the instruments in the hands of comet-seekers;
and the statement recently made that there are only about one hundred
nebulæ liable to be mistaken for comets is not accurate, because comets
in certain positions are of the last degree of faintness, and there is
no identifying them from small nebulæ except by means of their motion.

Mr. Brooks says:—“Medium magnifying powers, with necessarily
moderate-sized fields, are better than very low powers and large
fields. While with the latter a large amount of the sky can be swept
over in a given time, the work is not so well done, and a faint comet
would be easily swept over and not seen. A small region, _thoroughly
worked_, is far more likely to be successful. This gives a feeling of
satisfaction with the work performed, even with negative results. In
support of this I may remark that, during all the years I have engaged
in comet-seeking, not a single comet has been discovered by another
astronomer in a region of the heavens that I had just previously
searched; so that I have never had occasion to feel that I had swept
over a comet and missed seeing it. Aside from the obvious requirement
of good eyesight, capable of detecting exceedingly faint objects,
a good telescope of at least moderate aperture, and a familiarity
attained by experience with the large number of nebulæ resembling
telescopic comets, the comet-seeker, to be successful, must possess
in a high degree the qualities of patience, perseverance, energy, and
enthusiasm. I have the highest admiration for the man or woman who
discovers a comet, because I know of the hard and thorough work which
the success implies.”

Mr. Brooks’s experience and success in this branch give weight to
his suggestions, and there can be little doubt that his commendation
of moderate powers is fully justified. I believe he usually sweeps
with a power of 40 (field of 1° 20′) on the 10-1/8-inch equatoreal
of his observatory. Speaking for myself, I find powers of 32 (field
1-1/4°) and 40 (field 1°) perform very satisfactorily on my 10-inch
With-Browning reflector, having frequently tried them on faint
nebulæ and comets. Sometimes I employ a power of 60, field 50′; but
for ordinary purposes this is too high. It is a good plan to sweep



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 21 of 32)