William F. Denning.

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especially when the altitude is low. Reliable observations are seldom
made at a time far removed from the date of opposition. When the planet
was badly placed, in July 1882, an observer secured some observations
of position,, and published them, thinking he had seen Wells’s Comet,
which happened to be in the same quarter of the sky!

Mars, in nearer degree than any other member of our system, shows a
configuration which may be likened to that of the Earth as regards its
permanency; and in some of its outlines a general resemblance also
exists, though in detail there is evidently much that is dissimilar.
It is fortunate that the atmosphere of Mars is so rarefied that
observers can look upon his real surface-lineaments with satisfactory
perspicuity. For more than 250 years now, the telescope has been
engaged in perfecting our knowledge of Martian features, and these have
exhibited no mobility of form or place (apart from that due to rotation
or varying inclination of the planet) so far as may be judged from a
comparison of drawings. Plenty of differences exist in the latter, it
is true, though similar objects are represented; but the explanation
obviously lies in the inaccuracies of amateur artists, and has little
if anything to do with physical changes on the planet.

When the spots were discovered in 1636 by Fontana they were, of
course, very dimly glimpsed in the incompetent appliances available
at that time. Huygens, in 1659, saw them better by means of his long
telescopes, but still very imperfectly. Cassini, in 1666, effected a
further advance in the same field, and gathered data from which he
was able to announce the period of rotation. His value has proved
remarkably correct, considering the means he employed to obtain it
and the very short interval over which his inquiries were conducted.
Huygens had previously, in 1659, witnessed the returns of a certain
spot to the same approximate place on the planet, and was led to infer
rotation in either 12^h or 24^h. But this was little better than a
guess, and not nearly of the same precision as that which marked
Cassini’s subsequent determination.

[Illustration: Fig. 31.

Mars, 1836, April 13, 9^h 50^m; long. 332°.
(10-inch reflector; power 252.)
]

Near the poles of Mars are intensely bright patches, which have been
considered to be vast areas of snow-crowned surface or fields of ice.
These “polar snows” are not situated exactly at the poles, nor are
they opposite to each other. Changes affect their aspect. Occasionally
these or other bright markings, when on the limb, appear to protrude
beyond the disk, and this curious effect of irradiation distorts the
limb in a striking manner.

_Charts and Nomenclature of Mars._—It is not desirable to trace
with any detail the successive labours of those who have chiefly
contributed to our knowledge of areographic features. Maraldi, W.
Herschel, Schröter, Mädler, Schmidt, and Dawes were foremost amongst
the observers of the past; while Schiaparelli and Green are the most
successful observers of to-day. As telescopes improved in effectiveness
the true forms and characteristics of the markings were discerned, and
at the present time some thousands of delineations of this planet must
be in existence. Charts of the leading and best-assured features have
been formed, and the regions of light and shade (supposed to represent
land and sea) have received proper names to distinguish them. Thus
there is “Fontana Land,” “Maraldi Sea,” “Herschel Continent,” and
others of similar import. Schiaparelli has framed a chart in which the
spots are furnished with Latin names taken from classical geography.
Mädler’s plan was to designate the markings by capital letters of the
alphabet, and to divide these by small letters in necessary cases.
But the charts of Proctor, Green, and others, in which the names of
past and present astronomers are applied, seem to find most favour,
though it is admitted that this method of nomenclature is not free
from objections. In some instances the names have not been wisely
selected. A few years ago, when christening celestial formations was
more in fashion than it is now, a man simply had to use a telescope
for an evening or two on Mars or the Moon, and spice the relation of
his seeings with something in the way of novelty, when his name would
be pretty certainly attached to an object and hung in the heavens for
all time! A writer in the ‘Astronomical Register’ for January 1879
humorously suggested that “the matter should be put into the hands of
an advertizing agent” and “made the means of raising a revenue for
astronomical purposes.” Some men would not object to pay handsomely
for the distinction of having their names applied to the seas and
continents of Mars or to the craters on the Moon. But it is all very
well to disparage a system: can a better one be found? Probably not;
but the lavish use of undeserving names is calculated to bring any
system into contempt.

[Illustration: Fig. 32.

Orbits of the Satellites of Mars.]

_Discovery of Satellites and of Canal-shaped markings._—The interest
in this planet has been accentuated in recent years by several
circumstances. The discovery of two satellites in 1877 by Prof. Hall,
with the 25·8-inch Washington refractor, caused the directors of
large instruments to test their capacity upon these minute objects.
Schiaparelli’s observations of the canal-shaped markings have afforded
another attractive feature in connection with this planet. He detected
a network of dark straight lines stretching generally from N. to S.
across the planet; and in the winter of 1881 found these objects
duplicated, _i.e._ the lines ran in pairs so close together that
they were separated with difficulty. The study of the topography of
Mars had never previously revealed structures like these; yet the
Italian astronomer appears to have observed them with “comparative
ease whenever the air was still.” Other observers have not wholly
confirmed the appearances alluded to, but no favourable opposition has
occurred since 1877, and no surprise need be felt that the delicate
features visible in the pellucid sky of Italy should elude detection
in less genial climes. In 1886 M. Perrotin, at Nice, using a 15-inch
equatoreal, saw a number of the “canals,” and some of them were double.
In 1888 the observers having charge of the 36-inch refractor at Mount
Hamilton re-observed the “canals” as broad bands, but none of them
appeared to be duplicated. The conditions were unfavourable, the planet
being more than three months past opposition.

Prof. Schiaparelli re-observed the duple “canals” in June 1890 with
a refractor by Merz of 18 inches aperture, powers 350 and 500. His
observations are supported by Mr. A. S. Williams, of Brighton, who
informs me that he detected forty-three of the “canals,” and seven of
them were “clearly and certainly seen to be double.” Mr. Williams’s
instrument is a 6½-inch reflector by Calver, and powers of 320 and 430
were successfully employed on it; magnifiers under 300 were found of
little use.

_Summary of Observations._—From observations at Bristol I have drawn up
the following summary as to the configuration of Mars:—

1. That the “Hour-glass” or “Kaiser Sea,” and some other markings of
analogous character, present very bold, dark, and clearly defined
outlines, enabling them to be visible in very small telescopes. In
1873 I saw certain spots with a refractor of only 1-3/4-inch aperture.
Mr. Grover, in 1867, “made a set of pencil-drawings, with a 2-inch
telescope, which gave the general markings of the planet very well.”
In ‘Recreative Science’ it is mentioned that on June 7, 1860, a
semi-circular dark spot on the N.W. part of the disk of Mars was
distinctly seen with a 1½-inch telescope, power 120.

2. There is an intricate mass of surface-markings on the planet, which,
in its main features, is capable of being satisfactorily delineated,
and which in its general aspect is similar to the canals depicted by
Schiaparelli, though not nearly so pronounced, straight, and uniform as
he has shown in his charts.

3. The detail is visible in the form of irregular streaks,
condensations, and veins of shading, very faint and delicate in some
parts. The veins apparently connect many of the larger spots, and
here and there show condensations, which have sometimes been drawn as
isolated spots. A night of good definition, however, reveals the feeble
ligaments of shade connecting them.

4. That there exists on the immediate borders of many of the darker
patches and veins a remarkable brightness or shimmering, which reminds
one of the bright spots merging out of the dark belts on Jupiter.
Just contiguous to the “Kaiser Sea,” and on its eastern limits, this
brightness was so striking in March 1886 as to compare with that
exhibited by the N. polar cap. In drawings by many observers these
regions of special luminosity have no place, but there is little doubt
they occupy a leading position in the physical configuration of Mars.

5. That there is no trace of a dense atmosphere on Mars, as some of
the text-books infer. The pronounced aspect of the chief markings,
their durableness and continuity of form, the ease with which they
may be traced up to the limb, the absence of phenomena indicating
dense cloud-bearing air-strata, and other observed facts verify the
conclusion that the planet’s surface is comparatively free of vapours,
and in a totally different condition to that of Jupiter and Saturn.

_Rotation of Mars._—The diurnal period of this planet is known with
far greater certainty and precision than that of any other planet, the
Earth excepted. It will be useful to quote the values derived since
Cassini’s time:—

h m s
1666. J. D. Cassini 24 40
1704. J. P. Maraldi 24 39
1781. W. Herschel 24 39 21·7[31]
1784. W. Herschel 24 37 27
1838. J.H. Mädler 24 37 23·8
1845. O. M. Mitchell 24 37 20·6
1859. A. Secchi 24 37 35
1864. F. Kaiser 24 37 22·62
1866. R. Wolf 24 37 22·9
1869. R. A. Proctor 24 37 22·735
1873. F. Kaiser 24 37 22·591
1873. J. F. J. Schmidt 24 37 22·57
1883. A. Marth 24 37 22·626
1884. W. F. Denning 24 37 22·34[32]
1885. H. G. v. de S. Bakhuyzen 24 37 22·66

The last of these, by Prof. Bakhuyzen of Leyden, is probably the best.
It was based on a large number of observations extending over 220
years, viz. from those of Huygens in 1659 to those of Schiaparelli in
1879.

In a terrestrial day Mars rotates through 350°·8922, according to Mr.
Marth’s period. In one hour the axial motion is 14°·6, whereas on
Jupiter the horary rate of rotation is 36°·7. At intervals of 40 days
(during which Mars completes 39 rotations) the various features on the
disk are presented at very nearly the same times as before. Mr. Marth’s
ephemerides of this planet are extremely useful to those who study
the markings; and these, in combination with the charts and memoirs
of Schiaparelli, Green, Terby, and others, greatly facilitate and
encourage the renewed study of this object.

_Further Observations required._—Favourable oppositions of Mars occur
every 15 years, as in 1877 and 1892. It is at such periods that
this planet should be sedulously interrogated for new features, or
for corroboration of those already known. Rather a high power must
be employed—certainly more than 200; and if the telescope has an
aperture of at least 8 inches, the observer will be sure to discern
a considerable extent of detail. He should compare his views with the
various charts previously alluded to, and note any inconsistencies.
Fresh drawings should also be made; and if the forms are not well
assured on one night, he may confirm them by coming 37 minutes later
to his instrument on the following night. Or the collective issue of
several nights’ work may be included in the same drawing. The bright
spots on the planet should be as attentively studied as the darker
regions, and given a place in every drawing; for it is probably in
connection with these luminous objects that active changes may be
recognized. The “canals” and their duplication form the principal
markings to be looked for; though the successful elucidation of these
appearances can only be expected in a case where a powerful telescope,
a keen eye, and a good atmosphere operate together. Something of them
may be seen under ordinary conditions, and they ought to be very
generally sought for by amateurs; for it is not always that success
is found where the best conditions prevail. The great telescopes at
Mount Hamilton, Nice, and other observatories may be expected to
command some advantages of light, power, and position; but this need
not prevent competition, or induce the idea that common appliances are
practically of no avail. Everyone should strive to achieve as much as
is consistent with his means and opportunities; indeed there is all
the more need for effort and energy in the observer when his tools are
seemingly inadequate to a research, and he should endeavour to find,
in his own eye and understanding, that power which shall compensate
in a great measure for lack of instrumental capacity. Mr. Proctor, in
his ‘Old and New Astronomy,’ has justly remarked:—“The directors of
Government observatories have usually been much less successful in
studying planetary details than those zealous amateurs who take delight
in the study of the heavenly orbs and are ready to wait and watch for
favourable opportunities.”

_Changes on Mars._—Changes have been confidently reported in some of
the Martian spots. Instances have been quoted in which particular
markings, though very plain at certain times, have scarcely been
perceptible at others. Variations in outline as well as in visibility
appear to have been witnessed, and the subject is one which merits
more extended notice. It has been asserted that the origin of such
variations probably lies in the aerial envelope of Mars. In April and
May 1888 M. Perrotin, with the great refractor at Nice, failed to
re-observe the feature known as the continent “Libya” on Schiaparelli’s
chart, and stated that though this formation was plainly visible in
1886, it had ceased to exist in 1888. He suggested that the obscuration
was really produced by clouds or mists circulating in the atmosphere of
Mars. But Prof. Holden reported, from the Lick Observatory, that the
object alluded to was distinctly visible with the 36-inch refractor
there at the end of July, and in the same form in which it was drawn by
Prof. Schiaparelli in 1877-8. It is to be assumed, therefore, that if
any change occurred it was one of transient nature.

There are other questions relating to the physical aspect of this
planet which future observers should be able to answer. Do the
markings retain their distinctness right up to the limb? Is the opaque
crescent of the disk (when Mars is in quadrature) involved in any
phosphorescence or glow indicating an atmosphere? Are the bright spots
and luminous borders to the continents equally as stable as the dark
spots, and do they maintain an equable brilliancy?

The N. hemisphere of Mars needs much further study, as it is not so
familiarly known as the S. hemisphere. This is due to the circumstance
that, at favourable oppositions, the region of the S. pole is suitably
presented for observation. It is only when the planet is comparatively
distant, and small in diameter, that his N. hemisphere comes into view.

The difference of inclination under which the features are seen at
successive oppositions gives rise to many apparent changes of figure.
When the S. hemisphere is exposed to the Earth numerous objects
are seen which are quite invisible when the opposite hemisphere is
displayed to us. These altering conditions have to be considered in
their influences by every student of areography.

_Satellites of Mars._—After evading the keen and searching eyes of
Sir W. Herschel, and the power of his 40-foot telescope—after eluding
the grasp of Lord Rosse’s 6-foot speculum, and the frequent scrutiny
of Lassell with his 2-and 4-foot mirrors, the two satellites of Mars
were ultimately revealed to Prof. Hall in the 25·8-inch refractor
at Washington. These tiny orbs had been enabled to avoid previous
discovery by their minuteness and by their close proximity to Mars.
Yet as soon as they were known to exist many observers saw them, and
in certain cases success was undoubtedly attained with comparatively
small instruments. The late Dr. Erck picked up the outermost satellite
with a 7-1/3-inch objective, and Mr. Pratt saw it with an 8-1/7-inch
mirror by With. But the effect of this eye-straining may just possibly,
in one or two instances, have drawn the imagination out of its normal
repose. Mr. Pratt’s instrument shows stars in the group ε Lyræ which
are invisible in the great Washington telescope and in the 36-inch
mirror formerly used by Mr. Common; so that it may well have produced a
spectral satellite of Mars. But the satellites are certainly within the
occasional reach of moderate means; for they were repeatedly seen with
a 9½-inch refractor at the Observatory of Princeton, U.S.A., in October
and November 1879. They “were decidedly more easy to see than Mimas,”
the innermost satellite of Saturn.

Phobos, the inner satellite, revolves round the planet in 7^h 39^m, in
an orbit 6000 miles from the centre of Mars. At max. elongation the
satellite is about 12″ distant from its primary, and its opposition
magnitude is 11½. Deimos, the outer satellite, revolves in 30^h 18^m,
and its orbit is 15,000 miles distant from Mars. Its elongations extend
to 32″, and its opposition mag. is 13½. These diminutive objects are
probably not more than 10 miles in diameter. They are obviously too
faint for common instruments, nor are they objects on which ordinary
amateurs may occupy themselves with advantage. Of course it forms a
highly interesting spectacle to glimpse, just for once, it may be, the
small bodies which resisted telescopic power for more than two and a
half centuries; but for really useful observations, large aperture and
means of accurate measurement are required.

_Occultations of Mars._—The most ancient account of a planetary
occultation is probably that given by Aristotle, who refers to a lunar
obscuration of Mars that occurred on April 4, 357 B.C., according to
the calculations of Kepler. Another occultation of Mars appears to
have been recorded by the Chinese on Feb. 14, 69 B.C. Tycho Brahe
observed a repetition of the event on Dec. 30, 1595. Mr. Baily
describes a phenomenon of this kind which occurred on Feb. 18, 1837,
when “the planet appeared of a fine yellow colour both at its ingress
and egress. No projection was observed.” Mr. Snow, of Ashhurst, saw
the occultation of March 12, 1854, and he states the planet “was of
almost precisely the same colour as the Moon, and he could not help
comparing it to a spangle on the face of the sky. Whilst it was slowly
and solemnly vanishing, it gave for several seconds the notion of its
being the summit of a lunar mountain, but melted gradually away.” As
Mars emersed, “nothing whatever was to be seen of the two bodies,
clinging together, as it were, by threads of light; nothing of the
pear-shaped appearance often recorded as put on by planets under
similar circumstances.” Mr. J. Tebbutt, of Windsor, N. S. W., watched
an occultation of Mars in full daylight on Aug. 12, 1875, when “the
rapid disappearance of the planet’s disk was an exceedingly interesting
phenomenon, its extinction taking place at a considerable distance from
the Moon’s illuminated disk. The line marked by the Moon’s dark limb
across the disk was well defined.” At the reappearance clouds were
prevalent, and “the planet was observed as a small projection on the
bright limb;” but he found it difficult to fix the exact time of last
contact, owing to the ill-defined character of the planet’s gibbous
limb. An occultation of Mars was also seen by Prof. Grant at Glasgow on
June 3, 1878.


FOOTNOTES:

[30] This was believed by Sir J. Herschel to be due to “an ochrey tinge
in the general soil, like what the Red-Sandstone districts on the Earth
may possibly offer to the inhabitants of Mars, only more decided.”

[31] Herschel’s earlier observations were made in 1777-79, and his
period, like that of his predecessors, is about 2 min. in excess of
the correct value; but Mädler pointed out that, by giving Mars an
additional rotation on his axis, Herschel’s value will agree within
2 sec. of his own. Herschel appears to have adopted 768 rotations
instead of 769, and may have been led to this by the excessive periods
of Cassini and Maraldi and by the want of intermediate data between
his own observations in April 1777 and May-June 1779. His second
determination, made in 1784, is more correct.

[32] Deduced from observations extending over 15 years only, at
Bristol.




CHAPTER X.

_THE PLANETOIDS._

Number.—History of their Discovery.—Dimensions and
Brightness.—Occultation of Vesta.


_Number._—These bodies, also called minor planets, and, formerly,
asteroids, comprise a very numerous class, and they are extremely
small, being quite invisible to the naked eye except in one or two
special cases. They all revolve in orbits situated between Mars and
Jupiter. The total number discovered is about 300, of which Prof. J.
Palisa of Vienna has found more than 70, and the late Dr. C. H. F.
Peters of Clinton, N.Y., 49. I have not given exact numbers in the two
former cases, because these discoveries are still rapidly progressing.

_History of their Discovery._—The first known planetoid (Ceres) was
sighted by Piazzi on Jan. 1, 1801. The following year, on March 28,
Olbers found another (Pallas). In 1804, on Sept. 1, Harding discovered
a third (Juno); and in 1807, March 29, Olbers was a second time
successful (Vesta). Then for thirty-eight years no additions were made
to the number. The host of planetoids circulating between Mars and
Jupiter preserved their incognito without disturbance from the prying
and wakeful eyes of astronomers.

But in 1845 Hencke, of Driessen, after years of watching, at length
broke the spell of tranquillity by finding another small planet; and
his example was emulated by many other observers in subsequent years.
Hind, De Gasparis, and Goldschmidt were amongst the earliest and
most successful of those who gathered new planets from amongst the
stars of the zodiacal constellations. In later years Luther, Watson,
and Borrelly further extended the list; but Palisa and Peters have
distanced all competitors, and shown a zeal in the work which has
yielded an astonishing aggregate of discoveries. Charlois, at Nice, has
latterly earned distinction in the same field.

Since 1845 new planetoids have been found at the rate of more than
six per annum, and a rich harvest yet remains to be gathered by
the planet-seekers of the future. A very large proportion of those
already detected are between the tenth and twelfth magnitudes, and are
therefore only to be discerned in good instruments. They present no
distinction from small star-like points, and are to be identified by
their motions alone. The mythological dictionary has furnished names
for them, and they are numbered in the order of their discovery as well.

_Dimensions and Brightness._—Vesta is the largest and brightest of
the group, while Ceres and Pallas rank as second and third in the
same respect. Vesta is about 214 miles in diameter; but the more
insignificant members of this family are probably not more than about
15 or 20 miles in diameter. Pallas has the most inclined orbit of all,
the inclination amounting to 30° 44′; so that its position is by no
means confined to the planet-zone of the ecliptic. Vesta is sometimes
brighter than a 6th mag. star; while Ceres, Pallas, and Juno vary
between about the 7th and 8th magnitudes, according to their distances
from the Earth. A real variation of light has been assumed to occur,
but this is not fully proved.

In March 1887 Mr. Backhouse, of Sunderland, saw an apparently new,
yellowish-white star near 103 Piscium, and it was just visible to the



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 14 of 32)