THE INNER PLANETS.
Much progress has been made during the last hundred years in our
knowledge of the planets. In fact, the study of Mercury only dates from
the commencement of the nineteenth century. Our knowledge of the
vicinity of the Sun is very limited, and Mercury is difficult of
observation. So limited, in fact, is our knowledge of the Sun’s
surroundings, that it is not yet known for certain whether there is a
planet, or planets, between Mercury and the Sun. Perturbations in the
motion of the perihelion of Mercury’s orbit led Le Verrier in 1859 to
the belief that a planet of about the size of Mercury, or else a zone of
asteroids, existed between Mercury and the Sun. It was, however, obvious
that such a planet could only be seen when in transit across the Sun’s
disc, or during a total eclipse. Meanwhile a French doctor, Lescarbault,
informed Le Verrier that he had seen a round object in transit over the
Sun’s disc. Le Verrier, certain that this was the missing planet, named
it “Vulcan,” and calculated its orbit, assigning it a revolution period
of twenty days. But it was never seen again. Transits of “Vulcan” were
fixed for 1877 and 1882, but nothing was seen on these dates. During the
total eclipse of July 29, 1878, two observers—_James Watson_
(1838-1880), the well-known astronomer, and _Lewis Swift_ (born
1820)—believed themselves to have discovered two separate planets, and
ultimately claimed two planets each, which were never heard of again.
During the total eclipse of 1883 an active watch for “suspicious
objects” was kept, but with no result. At the eclipses of 1900 and 1901
respectively, photographs were exposed by the American astronomers, W.
H. Pickering and _Charles Dillon Perrine_ (born 1867), but on none of
these plates could any trace of “Vulcan” be found. At the total eclipse
of August 30, 1905, plates were again exposed, but no announcement has
been made of an intra-Mercurial planet; and the prevalent opinion among
astronomers is that no planet comparable with Mercury in size exists
between that planet and the Sun.
The study of the physical appearance of Mercury was inaugurated by
Schröter, who in 1800 noticed that the southern horn of the crescent
presented a blunted appearance, which he attributed to the existence of
a mountain eleven miles in height. From observations of this mountain he
came to the conclusion that the planet rotated in 24 hours 4 minutes.
This was afterwards reduced by _Friedrich Wilhelm Bessel_ (1784-1846) to
24 hours 53 seconds.
After the time of Schröter there was no astronomer who paid much
attention to either Mercury or Venus until the arrival on the scene of
the most persistent planetary observer and one of the foremost
astronomers of the nineteenth century. _Giovanni Virginio Schiaparelli_
was born at Savigliano, in Piedmont, in 1835, and graduated at Turin in
1854. Called to Milan as assistant in the Brera Observatory in 1860, he
became director in 1862, and there for thirty-eight years he studied
astronomy in all its aspects, making a great name for himself in various
branches of the science. In 1900 he retired from the post of director,
and pursues his astronomical researches in his retirement.
In 1882 Schiaparelli took up the study of Mercury in the clear air of
Milan. Instead of observing the planet through the evening haze, like
Schröter and others, he examined it by day, and was enabled to follow it
hourly instead of looking at it for a short period when near the
horizon. At length, after seven years’ observation, he announced, on
December 8, 1889, that Mercury performs only one rotation during its
revolution round the Sun—in fact, that its day and year coincide. As a
consequence, the planet keeps the same face towards the Sun, one side
having everlasting day and the other perpetual night; but owing to the
libratory movement of Mercury—the result of uniform motion on its axis
and irregular motion in its orbit—the Sun rises and sets on a small zone
of the planet’s surface. Schiaparelli’s observations indicated that
Mercury is a much spotted globe, with a moderately dense atmosphere, and
he was enabled to form a chart of its surface-markings.
Schiaparelli’s conclusions remained until 1896 unconfirmed and yet not
denied, although most astronomers were sceptical on the subject. In 1896
the subject was taken up by the American astronomer, _Percival Lowell_
(born 1855), who, in the clear air of Arizona, confirmed Schiaparelli’s
conclusions, fixing 88 days as the period of rotation. He remarked,
however, that no signs of an atmosphere or clouds were visible to him.
The surface of Mercury, he says, is colourless,—“a geography in black
and white.” The determination of the rotation period by Schiaparelli and
Lowell is now generally accepted, and is confirmed by the theory of
tidal friction. It is only right to add that _William Frederick Denning_
(born 1848) in 1881 suspected a rotation period of 25 hours, but this
remains unconfirmed. In April 1871 the spectrum of Mercury was examined
by _Hermann Carl Vogel_ (born 1842) at Bothkamp. He suspected traces of
an atmosphere similar to ours, but was not certain. Of more interest are
the photometric observations of Zöllner in 1874. These observations
indicated that the surface of Mercury is rugged and mountainous, and
comparable with the Moon,—a conclusion supported by Lowell’s
observations in 1896.
Venus, the nearest planet to the Earth, has been attentively studied for
three centuries, and still comparatively little is known regarding it.
This is due to its remarkable brilliancy, combined with its proximity to
the Sun. The great problem at the beginning of the nineteenth century
was the rotation of the planet. In 1779 the subject was taken up by
Schröter at Lilienthal. Nine years later, from a faint streak visible on
the disc, he concluded that rotation was performed in 23 hours 28
minutes, and in 1811 this was reduced by seven minutes; but as Herschel
was unable to observe the markings seen by Schröter, many astronomers
were inclined to be sceptical regarding the accuracy of the Lilienthal
observers results. Schröter also observed the southern horn of Venus
when in the crescent form to be blunted, and he ascribed this to the
existence of a great mountain, five or six times the elevation of
Chimborazo; while he observed irregularities along the terminator, which
he considered to be more strongly marked than those on the Moon.
Schröter’s opinion on this point, although rejected by Herschel, was
confirmed by Mädler, Zenger, Ertborn, Denning, and by the Italian
astronomer _Francesco Di Vico_ (1805-1848), director of the Observatory
of the Collegio Romano. In 1839 Di Vico attacked the problem of the
rotation, and his results were confirmatory of those of Schröter. He
estimated that the axis of Venus was inclined at an angle of 53° to the
plane of its orbit. Meanwhile a series of important observations had
been made on Venus by the Scottish astronomer and theologian, _Thomas
Dick_ (1772-1857), who suggested daylight observations on Venus to solve
the problem of the rotation.
In 1877 the question was attacked by Schiaparelli, who commenced a
series of observations on Venus at Milan in that year. The results of
his studies were summed up in 1890 in five papers contributed to the
Milan Academy. He came to the conclusion that the markings observed by
Schröter, Di Vico, and others were not really permanent, and
concentrated his attention on round white spots, which remained fixed in
position. Instead of observing Venus in the evening, Schiaparelli
followed it by day, watching it continuously on one occasion for eight
hours. But the markings remained fixed. Schiaparelli accordingly
concluded that the planet’s rotation was performed in probably 225 days,
equal to the time of revolution. One face is turned towards the Sun
continually, while the other is perpetually in darkness.
The announcement was so startling that, as Miss Clerke says, “a clamour
of contradiction was immediately raised, and a large amount of evidence
on both sides of the question has since been collected.” Perrotin at
Nice, Tacchini at Rome, Cerulli at Teramo, Mascari at Catania and Mount
Etna, and Lowell in Arizona, all in favourable climates, confirmed
Schiaparelli’s results, as also did a second series of observations by
the Milan astronomer himself in 1895. On the other hand, Neisten,
Trouvelot, _Camille_ _Flammarion_ (born 1842), and others, under less
favourable climatic conditions, arrived at a period of 24 hours.
_Aristarch Bélopolsky_ (born 1854), from spectroscopic observations at
Pulkowa, by means of Doppler’s principle, found a period of 12 hours.
Lowell, by the same principle, found, in 1901-03, a period of 225 days,
in agreement with Schiaparelli’s results. This is the last word on the
subject. Schiaparelli’s rotation period, confirmed by the theory of
tidal friction, is generally accepted.
That Venus has an atmosphere was one of the conclusions reached by
Schröter in 1792; and in this at least he was correct, as the atmosphere
of Venus, illuminated by the solar rays, has been seen extending round
the entire disc of the planet. Spectroscopic observations by Tacchini,
Ricco, and Young, during the transits of 1874 and 1882, indicated the
existence of water-vapour in the planet’s atmosphere. Very little has
been discovered regarding the “geography” of Venus. White patches at the
supposed “poles” of the planet were observed in 1813 by _Franz von
Gruithuisen_, and in 1878 by the French astronomer _Trouvelot_
(1827-1895). The secondary light of Venus, similar to the “old Moon in
the new Moon’s arms,” was repeatedly observed since the time of Schröter
by Vogel, Lohse, Zenger, and others. Vogel attributed it to twilight,
and Lamp, a German observer, to electrical processes analogous to our
auroræ. In 1887 a Belgian astronomer, _Paul Stroobant_, submitted to a
searching examination all the supposed observations of a satellite of
Venus, and was enabled to explain nearly all the supposed satellites as
small stars which happened to lie near the planet’s path in the sky at
the time of observation.
The study of our own planet can hardly be said to belong to the realm of
astronomy. Nevertheless, it is through astronomical observation that the
motion of the North Pole has been discovered. For many years it has been
a problem whether there is a variation of latitude resulting from the
motion of the pole. Euler had declared, from theoretical investigation,
that, were there such a motion, the period must be 10 months. The
question was revived in 1885 by the observations of _Seth Carlo
Chandler_ (born 1846) at Cambridge, Mass., with his newly-invented
instrument, the “almucantar,” which indicated an appreciable variation
of latitude. This was confirmed by _Friedrich Küstner_ (born 1856), now
director of the Observatory at Bonn. The idea now occurred to Chandler
to search through the older records to discover if there was any trace
of the variation of latitude, with the result that he brought out a
period of 14 months instead of 10. This aroused much interest, and many
prominent astronomers denied Chandler’s results, which were announced in
1891. As a well-known astronomer has expressed it, “Euler’s work had
shown what period the motion must have, and any appearance of another
period must be due to some error in the observations. Chandler replied
to the effect that he did not care for Euler’s mathematics: the
observations plainly showed 14 months, and if Euler said 10, _he_ must
have made the mistake. I do not exaggerate the situation in the least;
it was a deadlock: Chandler and observation against the whole weight of
observation and theory.” It was now shown by Newcomb that Euler had
assumed the Earth to be an absolutely rigid body, while modern
investigations show that it is not so. Chandler’s discovery is now
accepted, and proves that the North Pole is not fixed in position, but
has a small periodic motion, though never twelve yards from its mean
position. That the small resulting variation in the position of the
stars has been noticed at all is a striking illustration of the accuracy
of astronomical observation.
Of all the planets Mars has been most studied during the nineteenth
century. Many illustrious astronomers have devoted years to the study of
the red planet, with the result that more is known of the surface of
Mars than of any other celestial body, with the exception of the Moon.
After the time of Herschel, the leading students of Mars were Beer and
Mädler, who carefully studied the planet from 1828 to 1839. They
identified at each opposition the same dark spots, frequently obscured
by mists, and they also made the most accurate determination of the
rotation period, which they fixed at 24 hours 37 minutes 23 seconds.
This estimate was confirmed in 1862 by _Friedrich Kaiser_ (1808-1872) of
Leyden, in 1869 by _Richard Anthony Proctor_ (1837-1888), and in 1892 by
_Henricius Gerardus van de Sande Bakhuyzen_ (born 1838), director of the
Leyden Observatory. In 1862 Lockyer identified the various markings seen
by Beer and Madler in 1830. The other great names in Martian study prior
to 1877 are Angelo Secchi and _William Rutter Dawes_ (1799-1868), who
studied Mars from 1852 to 1865 and secured a very valuable series of
drawings. These drawings were used by Proctor for the construction of
the first reliable map of Mars, which was published in 1870 in his work,
‘Other Worlds than Ours.’ Proctor gave names to the various Martian
features, the reddish-ochre portions of the disc being named continents
and the bluish-green portions seas; and Proctor’s views on Mars found
favour for many years. In 1877, however, Schiaparelli opened a new era
in the study of Mars. In September of that year, during the very
favourable opposition of the planet, Schiaparelli, while executing a
trigonometrical survey of the disc, discovered that the continents were
cut up by numerous long dark streaks, which he called _canali_. In 1879,
to his surprise, he found that some of the canals had become double; and
he confirmed this in 1881 and at subsequent oppositions. Meanwhile, as
Schiaparelli was the only observer who had hitherto seen the canals,
there was much scepticism as to their reality. In 1886, however, they
were seen at the Nice Observatory by _Henri Perrotin_ (1845-1904), who
also observed their duplication. Since 1886 they have been observed by
many astronomers, including Camille Flammarion in France, _William
Frederick Denning_ (born 1848) in England, _Vincenzo Cerulli_ (born
1859) in Italy, Percival Lowell and W. H. Pickering in the United
States. In 1892 W. H. Pickering successfully observed the canals, and
discovered at the junctions of two or more canals round black spots, to
which he gave the name of “lakes,” in keeping with the view that the
dark regions of the planet were seas.
In 1894 Percival Lowell erected at Flagstaff, Arizona, an observatory
for the specific purpose of observing Mars and its canals in good and
steady air. He was assisted by W. H. Pickering and by _Andrew Ellicott
Douglass_ (born 1867). During a year’s study Douglass measured the
Martian atmosphere and discovered canals crossing the dark regions of
the planet, finally disproving the idea of their aqueous character.
Lowell recognised all Schiaparelli’s canals, and discovered many more.
He also attentively studied the south polar cap of Mars, which
disappeared entirely on October 12, 1894. Lowell noticed, also, that as
the cap melted the canals became darker, as if water was being conveyed
down; and accordingly he adopted the view put forward by Schiaparelli,
that the canals are waterways lined on either side by banks of
vegetation. His observations were published in the end of 1895 in his
work ‘Mars.’ He is of opinion that the reddish-ochre regions or
“continents” are deserts, and the greenish areas marshy tracts of
vegetation. The lakes are named by him “oases,” and, as Miss Clerke
observes, he “does not shrink from the full implication of the term.” He
regards the canals as strips of vegetation fertilised by a small canal,
much too small to be seen, an idea which originated with W. H.
Pickering. The canals are believed by Lowell to be waterways down which
the water from the melting polar cap is conveyed to the various oases.
He considers, in fact, that the canals are constructed by intelligent
beings with the express purpose of fertilising the oases, regarded by
him as centres of population. He remarks that water is scarce on the
planet, owing to its small size, and as a consequence the inhabitants
are forced to utilise every drop. The canal system is the result.
Lowell’s theory has not been cordially received—although it is now
gradually gaining popularity,—and several other hypotheses have been
propounded to explain the canals. Proctor, who died some years before
Lowell’s theory was given to the world, regarded them as rivers, but
this view may now be looked upon as abandoned. It was suggested that the
canals might be cracks in the surface of Mars or meteors ploughing
tracks above it: and Professor _John Martin Schaeberle_ (born 1853) of
the Lick Observatory put forward the view that the canals were chains of
mountains running over the light and dark regions. None of these
theories, however, gained popularity, and had to give way to a more
popular theory, the “illusion” hypothesis, put forward by the Italian
astronomer Cerulli, and supported by Newcomb and Maunder. On the basis
of the illusion theory, Newcomb explains that the “canaliform”
appearance “is not to be regarded as a pure illusion on the one hand or
an exact representation of objects on the other. It grows out of the
spontaneous action of the eye in shaping slight and irregular
combinations of light and shade, too minute to be separately made out
into regular forms.” Experiments were made by Maunder in 1902, and the
results pointed to the truth of the theory that the canals were really
illusions. But the studies of Lowell at the oppositions of 1903 and 1905
have seriously weakened the hypothesis of Cerulli and Maunder, and
strongly confirm the theory of the artificial origin of the canals. In
1903 Lowell was enabled, from a study of the development of the canals,
to show the probability of their artificial nature, and his study of the
double canals showed a distinct plan in their distribution. Finally, on
May 11, 1905, several photographs of Mars were secured at the Lowell
Observatory, on which the canals appeared, not as dots of light and
shade, as on the illusion theory, but as straight dark lines. This goes
far to prove the reality of the canals,—in spite of the ridicule cast on
them and their observers,—and consequently the truth of the theory of
intelligent life in Mars.
Meanwhile the old-fashioned Martian observations have been continued in
less favourable climates than Arizona and Italy by various astronomers,
among them the famous Camille Flammarion, the American astronomers
_James Edward Keeler_ (1857-1900), _Edward Emerson Barnard_ (born 1857),
the English astronomer W. F. Denning, and others. These conscientious
and painstaking observers have done much for Martian study in increasing
the number of accurate delineations of the Martian surface.
The spectrum of Mars was first examined by Huggins in 1867. He found
distinct traces of water-vapour, and this was confirmed by Vogel in
1872, and by Maunder some years later. In 1894, however, _William
Wallace Campbell_ (born 1862), the American astronomer, observing from
the Lick Observatory, California, was unable to detect the slightest
difference between the spectra of Mars and the Moon, indicating that
Mars had no appreciable atmosphere; and from this he deduced that the
Martian polar caps could not be composed of snow and ice, but of frozen
carbonic acid gas. In 1895, however, Vogel confirmed his previous
observations, and reaffirmed the presence of water-vapour in the Martian
atmosphere.
During the opposition of 1830, Mädler undertook an extensive search for
a Martian satellite, but was unsuccessful. In 1862 the search was
resumed by _Heinrich Louis D’Arrest_ (1822-1875), the famous German
observer, who was also unsuccessful. Accordingly the red planet was
referred to by Tennyson as the “moonless Mars.” In 1877 the search was
taken up by _Asaph Hall_, the self-made American astronomer, born at
Goshen, Connecticut, in 1829, and employed from 1862 to 1891 at the
Naval Observatory, Washington. During the famous opposition of August
1877, favoured by the great 26-inch refractor, he succeeded in
discovering two very small satellites of Mars, to which he gave the
names of Phobos and Deimos. He determined the time of revolution of
Phobos at 7 hours 39 minutes, and that of Deimos at 30 hours 17
minutes,—Phobos revolving round Mars more than three times for one
rotation of the planet on its axis. These two satellites are very small,
not more than thirty miles in diameter. After Hall’s successful search,
photographs were exposed at the Paris Observatory for other Martian
satellites, but none was discovered. No further moons have been found
belonging to the red planet, nor is it likely that any further
satellites of Mars are in existence.
The discovery of a zone of small planets in the space between Mars and
Jupiter belongs completely to the nineteenth century, although the
existence of a planet in the vacant space was suspected three centuries
ago. In 1772 the subject was taken up by _Johann Elert Bode_
(1747-1826), afterwards director of the Berlin Observatory, who
investigated a curious numerical relationship, since known as Bode’s
Law, connecting the distances of the planets. If four is added to each
of the numbers—0, 3, 6, 12, 24, 48, 96, and 192, the resulting series
represents pretty accurately the distances of the planets from the Sun,
thus—4 (Mercury), 7 (Venus), 10 (The Earth), 16 (Mars), 28, 52,
(Jupiter), and 100 (Saturn). After the discovery of Uranus, in 1781, it
was found that it filled up the number 196. Bode, however, saw that the
number 28, between Mars and Jupiter, was vacant, and predicted the
discovery of the planet. Aided by _Franz Xavier von Zach_ (1754-1832),
he called a congress of astronomers, which assembled in 1800 at
Schröter’s observatory at Lilienthal, when, for the purpose of searching
for the missing planet, the zodiac was divided into twenty-four zones,
each of which was given to a separate astronomer. One of them was
reserved for _Giuseppe Piazzi_ (1746-1826), director of the Observatory
of Palermo.
Born in 1746 at Ponte, in Lombardy, Giuseppe Piazzi, after entering the
Theatine Order of monks, became in 1780 Professor of Mathematics at
Palermo, where an observatory was erected in 1791; and at that
observatory Piazzi worked till his death in 1826. In 1792 he commenced a
great star-catalogue, and while making his nightly observations he
discovered, on January 1, 1801—the first night of the nineteenth
century,—what he took to be a tailless comet, but which proved to be a
small planet revolving round the sun in the vacant space. The discovery
was hailed by Bode and Von Zach with much enthusiasm, and Piazzi named
the planet Ceres. The little planet was, however, soon lost in the rays
of the sun before sufficient observations had been made; but the great
mathematician, _Friedrich Gauss_ (1777-1855), came to the rescue, and
pointed out the spot where the planet was to be rediscovered. In that
spot it was found on December 31, 1801, by Von Zach at Gotha, and on the
following evening by _Heinrich Olbers_ (1758-1840) at Bremen.
On March 28, 1802, while observing Ceres from his house at Bremen,
Olbers was struck by the presence of a strange object near the path of
the planet. At first he supposed it to be a variable star at maximum
brilliance, but a few hours showed him that it was in motion, and was
therefore another planet. He named it Pallas, and propounded the theory
that the two “Asteroids”—so named by Herschel—were fragments of a
trans-Martian planet, which, through some accident, had been shattered
to pieces in the remote past. Olbers urged the necessity of searching
for more small planets. His advice was taken. In 1804 _Karl Ludwig
Harding_ (1765-1834), Schröter’s assistant, discovered Juno, and Olbers
himself detected Vesta, March 29, 1807.
After 1816 the search was relinquished, as no more planets were
discovered. In 1830, however, a German amateur, _Karl Ludwig Hencke_
(1793-1866), ex-postmaster of Driessen, commenced a search for new
planets, which was rewarded, after fifteen years, by the discovery of
Astræa, December 8, 1845. On July 1, 1847, he made another discovery,
that of Hebe. A few weeks later, _John Russell Hind_ (1823-1895), the
English astronomer, discovered Iris. Since 1847 not a year has passed
without one or more planets being found, sometimes as many as twenty
being discovered in a single year. Some astronomers have made the search
for asteroids their chief business. The principal asteroid discoverers
have been _Christian H. F. Peters_ (1813-1890), Henri Perrotin, _Paul
Henry_ (1848-1905), _Prosper Henry_ (1849-1903), James Watson, _Robert
Luther_ (1822-1900), _Johann Palisa_ (born 1848), and _Max Wolf_ (born
1863).
In 1891 a new impulse was given to asteroid study by the application of
photography by Max Wolf to the discovery of the minor planets. It
occurred to Wolf that the asteroid would be represented on the plate by
a trail, caused by its motion during the time of exposure; and assisted
by _Arnold Schwassmann_ (born 1870), _Luigi Carnera_ (born 1875), and
others, Wolf has discovered over a hundred asteroids, and he has the
whole field of asteroid hunting to himself. Few minor planets are now
discovered by the older method. In 1901 Wolf invented his new instrument
of research, the stereo-comparator, which, on the principle of the
old-fashioned stereoscope, represents the planetary bodies as suspended
in space far in front of the stars. In this way this ingenious
astronomer has been enabled to discover asteroids at the first glance:
year by year fresh discoveries are announced from the Heidelberg
Observatory, until more than five hundred asteroids are now known.
Waning interest in the ever-increasing family of asteroids was revived
in 1898 by the discovery by _Karl Gustav Witt_ (born 1866) of a small
planet, to which he gave the name of Eros, which comes nearer to the
Earth than Mars, and which is of great assistance to astronomers in the
determination of the solar parallax. For some time prior to 1898
astronomers had considered it a waste of time to search for new
asteroids; but this idea is not now so popular, in view of the benefit
conferred on astronomy by the discovery of Eros.
Of the physical nature of the asteroids astronomers know nothing. Only
the four largest have been measured. For many years it was supposed that
Vesta, the brightest of the asteroids, was also the largest. The
measures of Barnard with the great Lick refractor in 1895, however,
showed that Ceres is the largest, with a diameter of 477 miles. Pallas
comes next, with a diameter of 304 miles; while the diameters of Vesta
and Juno are respectively 239 and 120 miles. Barnard saw no traces of
atmosphere round any of the asteroids. It should be stated that in 1872
Vogel thought he could detect an “air-line” in the spectrum of Vesta: he
admitted that the observation required confirmation, but it has not been
corroborated either by himself or any other observer.