SOUTHERN ASTRONOMICAL DELIGHTS
V E N U S
Part 5 : MOUNTAINS of VENUS ARTICLE
|
Johann Schröter, William Herschel
and the Mountains of Venus: Overview
By Andrew James
[ The original paper produced here, first appeared in the Royal
Astronomical Society of New Zealand (RASNZ) journal in March 2003
(“Southern
Stars” 42, 1, March (2003).
It covers the period when questionable observational phenomena was
observed on Venus by several astronomers of the time, much of which,
was focused on the observations made by German astronomer, Johann
Schröter. It is reproduced with kind permission of the RASNZ,
which I requested after a number of enquires from some noted
planetary observers who did not have general access to the original
published article. ]
[Andrew James : April 2005.]
A B S T R A C T
Since the invention of the telescope the observations
of atmospheric phenomena regarding Venus have always proved
difficult. Some early observers reported variations in the curvature
of the cusps and so-called terminator anomalies. It was these
particular anomalies and their interpretation that lead to a
somewhat passionate debate, mainly between Friedrich Wilhelm (later
William) Herschel (1738-1822) and his fellow compatriot
Johann Heironymous Schröter (1748-1816). Schröter
assumed that the direct cause of the irregularities, especially near
the southern cusp, was a mountain or a mountain range — a
Venusian Himalayas, while Herschel concluded the “alleged”
cause of the Schröter’s Venusian
anomalies could possibly be attributed to Schröter’s tarnished speculum mirror. This paper
also discusses the life and observations of Schröter, and
attempts to explain the basis of his reputation as a poor and
unreliable observer.
I N T R O D U C T I O N
Two factors influence this problem — the brightness of
Venus and its proximity to the Sun as seen from the Earth. In the
early days of observational astronomy the early evening and morning
twilight hours were considered the best times since its apparent
brilliance was reduced but with the disadvantage of the planet being
close to the horizon. In addition, daylight observations were
limited by speculum metal mirrors that were impossible to use during
the day due to their thermal expansion. Early observations made with
refractors were not very common — likely limited by light
scattering and inadequate baffling.
Since the 1870s, the advent of glass mirrors has made it possible
for useful observations of Venus during full daylight. Today
observations are often made during the daytime.
Origin of the Mountains of Venus
Observations of
“spots“
visible on the Venusian cloud tops were first described in 1643 by
Franceso Fontana (1602-1656) from Naples, Italy. Two years
after this discovery, Fontana again reported some darker spots, that
were later confirmed in 1666 and 1667 by the brilliant French
observer Giovanni Domenico Cassini I (Jean Dominique Cassini)
(1625-1712).
The origin of any discussion on possible mountains on Venus is
from the interpretations of various “limb projections” seen by these early observers. According
to Baum (2001), the first recorded interpretation of these effects
was reported by Phillipe de la Hire in 1700 to the Paris Academy,
who declared; “Montage beaucoup plus grandes que celles
de la Lune, qui le font plus â proportion que celles de la
Terre.”
“Many mountains far
higher than those of the Moon, and greater in proportion than those
of the Earth“ (Baum, 2001)
Later Nicolas Camille Flammarion (1842-1925) came to the
same conclusion. [1] Both attributed the
visibility of these spots as telescope imperfections. Some sources
since have all attribute these phenomena as poor atmospheric
seeing.
Other types of features were also seen on Venus. In Paris,
Phillipe de la Hire, 1640-1718 (1700) he also noted dark spots
in August 1700, but also noticed that there were;
“…greater
inequalities in the termination of the light of Venus, than the
Moon.” (Schröter, 1795),
(Baum, 1973)
These terminator inequalities, also known as the Venus
Contour Anomaly, were first seen by Fontana (1643) and Burratini
(1666), who described the terminator as “raggedness.” (Baum, 2001) Johann Schröter, also
noted historically that this terminator phenomenon was again reported
by Bianchini in 1726. For many years these observations of Fontana,
Burratini, de Le Hire, Cassini and Bianchini were considered to be
oddities.
The question about the cause of these deviations in
Venus’ terminator shape escalated with
the famous clash between the two great observational titans —
the planetary discoverer of Uranus, William Herschel
and the German lunar and planetary specialist Johann
Schröter.
At first using a 3-ft long achromatic refractor, between 1779 and
1793 Schröter admitted he saw little in regards any evidence of
structured features except that the terminator gradually decreased in
brightness. Herschel said he saw nothing.
Using his larger 7-ft long Herschelian telescope in perfect
seeing conditions at 5pm on the 28 December 1789, Schröter
reported seeing a bright circular point of light within the darkened
disk of Venus. Increasing the magnification to 370×, this
illuminated point became clearly separated from the southern
terminator of the planet’s disk. Again
on 25 December 1791 he saw this same brilliant starry point near the
southern cusp. Upon some serious pondering, Schröter convinced
himself this was no optical aberration and started to postulate
about the true cause of the phenomena. By the beginning of 1792 he
had reasoned that the only logical explanation was the reflection of
light from a tall mountain range or peak — arguing from the
similarity to the circumstance that we sometimes see on the surface
of the Moon. These hypothetical mountains affected the southern cusp
of Venus, and according to Florens Fredrick Chaladni
(1756-1827), Schröter’s argument
roughly followed the lines, that if the Venusian surface was level,
then the horns of the crescent would be even. In Schröter’s own words, he said of his observations in
March and April 1790 ; “…one cusp frequently appears
pointed and the other blunt, owing to the shadow of some mountain
darkening the extremity of the latter…”
Another quote of Schröter’s,
according to Baum (2001) was also announced to the Royal Society in
London in 1792;
“… [mountains] of such
enormous height, as to exceed 4, 5, or even 6 times the perpendicular
elevation of Cimborano, the highest off our
mountains.” (Baum 2001)
On hearing of these presumed Venusian mountains, Herschel in 1792
began criticising Schröter’s
interpretation of their origin. Perhaps Herschel was correct in this
view, but in these times, Schröter himself was to be seen as the
leading authority in observational astronomy regarding any Venusian
atmospheric phenomenon. We should also remember such speculative
views were more liberally accepted among the astronomical community
of this era. In retrospect, the mountains to which he was referring,
likely originated from de la Hire statements. It was likely his
observation of the 28th December 1789 started to change his view of
the reality of de La Hire’s Academy
claims. Yet again, on 25 December 1791 Schröter reported seeing
similar limb projection phenomena. After the publication of his
views, what happened next was something likely unexpected from the
astronomical community — Schröter had to defend his
observations and interpretation. [2]
After the Royal Society announcement in 1792,
Schröter continued to sporadically see and report
these effects throughout the 1790s, backed quietly by his assistant
Karl Ludwig Harding (1765-1834) who also noted and confirmed
the phenomena. Harding was also a noted keen observer and later
became the Professor of Astronomy at the German University of
Göttingen and went on to discover the asteroid Juno during March
1804. This support proved to be an important lever against the
increasing criticism.
The Dichotomy of Venus
Another mystery associated with Venus is the so-called
Schröter Effect — based on the problem of the
observed time of the Venusian Dichotomy. In August 1793,
Schröter had discovered that the observed time of the 50% phase
did not correspond to what they predicted, and seeing some deformity
of the southern limb remaining concave for up till eight days before
or after conjunction with the Sun. He thought the average difference
in dichotomy was six days, and some references still wrongly states
eight days, though modern values — based on several dichotomies
— suggested perhaps four days early or late, depending on the
side of the particular elongation. Some even suggest that this is
merely an optical illusion, yet it is odd that the effect is neither
aperture, field orientation nor magnification dependant. A proof, in
a partly unconvincing argument of this optical effect was presented
in Sky and Telescope in August 1994; some 201 years later! A
proper explanation of the effect has yet to be determined.
Schröter’s Lunar Observations
Schröter’s vigorous interest
for some twenty-eight years was the observation of the Moon.
In that era popular beliefs of the day held that the Moon and all
other planets might be inhabited. Like Herschel, Schröter
thought this might be true. Starting serious lunar observing in
1779, Schröter discovered many lunar features and made
thousands of drawings According to some. Schröter was not a
good draughtsman, and his drawings both disorganised and crude,
based on the small part of his work which remains. King (1955)
further says of his complete ardour for the subject, he was; “…a persistent lunar and planetary
observer.” Unlike his
predecessors, he made several useful repeated observations of
different aspects of the same lunar features throughout the lunar
month. From this he could estimate the heights of lunar mountains
and the general topography of the lunar terrain.
Today, Schröter is often described as the ‘Father of Selenography’ based on his extensive studies of the Moon.
He wrote the two-volume classic book “Selenotopographische Fragmente’ (1791 and 1802), yet he never produced a
complete lunar map. [3]
Schröter’s first discoveries
were of the lunar features known as “clefts’ - to
the eye seemingly long cracks in the lunar surface. One of his most
famous discoveries was Rupes Recta, commonly called the “Straight Wall”
which lies within Mare Nubium. The “Straight Wall”
extends about twenty-six (26) kilometres in length and is 250 to 300
metres in height. Spacecraft have revealed the feature has a width of
some two-and-a-half kilometres, that slopes downward one side of the
central edge around 8 degrees. One of the very few existing
Schröter’s drawings is from 1781,
and one remaining is of this particular feature. (Figure 1)
Another concerns the discovery of the largest of the sinuous
rilles, later named Vallis Schröter
(Schröter’s Valley). This crack,
roughly two-hundred kilometre long by one kilometre deep, lies in an
island complex within the mare, Oceanus Procellarum, in the
north-western quadrant and hemisphere of the Moon. Vallis
Schröteri starts 25km north of crater Herodotus, where the rill
begins from river delta-like feature common known as the “Cobra’s
Head.” From here the arc-like rill
leads northward, narrowing to some 400 to 500 metres across. The end
of the valley slowly losses its depth and abruptly ends near another
sinuous rille. [4]
Schröter’s Early Life
Schröter’s personally was both
enthusiastic and passionate, yet often appeared reserved in
character. Throughout his life suffered frequent bouts of ill
health. Professionally a lawyer, he became the well-respected
bailiff and the community leader in the township of Lilienthal,
about 150 kilometres southeast from the North Sea coast of Germany.
Schröter’s home on the river Worp
was named “Amthaus”, and here he built his observatory. This
location later became the central meeting place of many other
observers in the region — including the so-called “Celestial Police”. Schröter continued to be an ardent
amateur astronomer all his life and financed from his own income.
In his early years, he once worked in Hannover. According to Baum
(2003), Schröter association and friendship was with the
Herschel family and older William, and began when he met them
during several organised musical concerts. According to Berry
(1898), William Herschel’s father had
sent his son to England during the outbreak of the Seven Years’ War mainly because of his delicate health
problems displayed during the early parts of the campaign. It is
likely that Schröter understood Herschel’s problems, and continued corresponding
with him from afar once he moved to England. Schröter’s observatory at Lilienthal was initially
equipped with instruments from Herschel.
Schröter had a number of acquaintances and friends during
this time. One of the people he personally influenced was Friedrich
Wilhelm Bessel (1784-1864) when Bessel was only twenty-one
years old. Bessel later went on to became the director at
Königsberg Observatory in 1810.
Schröter’s Last Years
Schröter’s final years of life
reads like a very sorry tale. In 1813, during the final years of the
Napoleonic Wars, the French occupied and sacked Bremen under the
Vandammes. The little town of Lilienthal fell into the hands of the
French and was razed to the ground. Even Schröter was captured
and imprisoned for a short time and the soldiers then burnt and
pillaged his house and observatory, destroying much of his precious
observations, manuscripts and books. Some say that the soldiers even
mistook his brass telescopes for gold and pillaged them for
themselves.
After this setback, Schröter had neither the finances,
strength, nor the inclination to start another observatory —
the reconstruction of the township was a much higher priority, and
he died three years later. [5] Fortunately, most
of his observational work has been discovered elsewhere, either
through his letters, publications or discovery of his works
posthumously. [6] [7] After
1816, the observatory fell into disrepair and finally was completely
demolished in the next decade, with the observatory instruments being
eventually passed on to Göttingen University. The observatory
site is now marked by a small ground tablet.
Schröter died in 1816 at the age of 68 years old, the same
year as Herschel was knighted by George III for his astronomical
work.
Schröter’s efforts were later
honoured by the IAU with the naming of the crater “Schröter”, on the edge of Mare Imbrium (Sea of
Rains).
Schröter’s Instruments
At thirty-years-old, one of his earliest telescopes was a small
Dolland telescope, a gift from Herschel’s brother Johann Alexander Herschel in
1779. [8] Johann Schröter already knew the
Herschel family, and this telescope inspired him to greater things.
As his interest grew, a larger aperture was necessary. In 1784, he
bought from William Herschel a 4-foot (12cm aperture. f/13)
telescope, and in 1786 a larger 7-foot (16.5cm aperture f/12.7)
telescope as well as several much larger metal mirrors. This last
telescope was identical to the one Herschel used to discover the
planet Uranus.
Later in 1792, he turned to a German physics and chemistry
professor, Johann Gottlieb Friedrich Schräder
(1763-1833), of Kiel (located near the Denmark border, some 50 km
north of Hamburg) for another 7-foot telescope. Again, in the same
year, he purchased a 13-foot (24cm aperture f/16) telescope.
Schräder’s telescopes also used a
modified method in metal mirror making by applying gaseous arsenic
and a slow cooling of the mirror, giving a more reflective surface.
[9] None of these telescopes had telescope
drives, and all had altazimuth mounts. It seems that both these
Schräder instruments ended up being of slightly lesser quality,
so Schröter eventually began constructing his own primary
mirrors and telescopes with his two Lilienthal supporters (likely
Harding and Olbers). According to King (1955 /1979), in November
1793, Schröter told Herschel of his constructing a 27-foot (47
cm aperture) Newtonian, [10] [11] stating;
“The enterprise has
been successful from first to last and with all its apparatus it is
now finished.” (Neison 1876)
Little is known about the observations from Schröter’s own telescopes, but it is fair to assume
that these instruments were built for the growing interests in the
systematic search for other planets and asteroids.
Schröter and the Minor Planets
After William Herschel’s discovery
of Uranus and the famous puzzling mathematical geometric progression
commonly known as either Bode’s
Law [12]. Schröter became the founding
member and President of the “Societas Liliatalica’ — the so-called “celestial police’. The original sextuplet of this group met
at Schröter’s Observatory in
Germany. They were Johann Schröter, Frederich Baron von Zack,
Karl Harding, Heinrich Olbers, and possibly either Ferdinand Adolf
Freiherr von Ende or Johann Gildmeister. Here the group organised a
systematic search programme of the ecliptic looking for new
planets.
Four months later, Giuseppe Piazzi, the head astronomer of
Sicily’s principal observatory,
Palermo Observatory, was working on some observations for his star
catalogue and discovered the first of the minor planets, Ceres, on
1st January, 1801. [12a]
At the Society’s first meeting,
Piazzi’s nomination was a natural
choice for such a project because of the much better observing
conditions in Sicily. It must have been disappointing to all that the
introductory letter about the group had been in transit to Piazzi at
his time of discovery. Once Ceres was found, it soon became lost in
the solar glare, with the subsequent conjunction. At first Piazzi
thought the body was merely a comet, but his uncertainty diminished
as Ceres closed in on the Sun. The minor planet was later recovered
by one of “The Police” after the conjunction, based on the
intuitive orbital calculations of Karl Gauss (1777-1855), who
predicted Ceres to an accuracy that placed it within a medium powered
eyepiece. He was first to calculate the orbit of Ceres, so it could
be recovered after its next conjunction with the Sun, and did all the
positional reductions. Gauss was a frequent visitor of
Schröter’s. The number in this
group reached twenty-four after Ceres’ discovery. Of all the observational
planetary devices, Schröter main astronomical invention was the
lucid disk micrometer, that was used to measured the physical
diameters of planetary bodies. William Herschel had also made a
similar device, which he first used to measured Ceres’ diameter in 1802. [13]
After Heinrich Wilhelm Matthäus Olbers suggested in
1803 that Ceres may be only one of many small bodies, these observers
earnestly searched for more, resulting in the discovery of Juno and
Vesta in 1804 and 1807, respectively.
Later, Schröter sent a letter to Heinrich Olbers stated that
his observations of both Ceres and Pallas seemed unusually variable
in brightness. [14] Olbers suggested ;
“…these asteroids are
irregular rather than round figures.” (Grosser, 1982)
At the beginning of the 17th Century, Schröter’s work was soon quenched with the outbreak
of the European Napoleonic Wars. [15] [16]
Schröter’s Observations of
Mercury
Schröter had also observed similar irregularities in the
appearance of Mercury, with respect to both surface features and of
the horns of the planet’s cusps.
During observations on the 26th March 1800 both Schröter and
Harding reported seeing some features. Again, Herschel replied he
could not see any surface details on Mercury. One of
Schröter’s last observations was
of Mercury in 1808, where he saw a phenomenon similar to what he was
seeing on Venus — a smallish speck of light projected away
from the southern limb. He also calculated that Mercury’s mountain was about 18.3 kilometres in
height and stated it was larger than any of the mountains on the
Earth! He also estimated the axial tilt of Mercury was 20°, and
suspected that he saw “cloud-like
patterns” hiding the surface. A few
years later Franz von P. Gruithuisen (1774-1852) reported
seeing a similar phenomenon on 17th March 1814, in which, the
southern cusp was distinctly blunt.
Schröter also once reported seeing what is now called the
ashen light, although he was not the first to report this
phenomenon. [18] This phenomenon consists of
faint illumination on the darkened limb of the planet’s atmosphere or surface. It is today
sometimes seen on both Venus and Mercury. In an article that appeared
in Philosophical Transactions of 1795, Schröter concluded
the cause of this odd brightened light might be something akin to
earthshine that is sometimes seen during the times of the thin
crescent Moon. This ashen light was also later seen by Herschel,
Harding (1806) and by Schröter over a short period of five weeks
around inferior conjunction. [19]
Schröter frequently speculated on the origins of Venus’ sometimes uneven and jagged terminator.
These particular changes in the shape of the terminator suggested to
him information on Venus’s atmospheric
rotation, and hence the surface rotation and he proposed the diurnal
period of 23h 21m 05s. [20], [21], [22] Other astronomical
events he observed, include the transits of Mercury on 3rd May 1786
and again on 5th November 1789. Several observers including
Schröter, observed the effects of the aureole — the
telescopic view of the planetary atmospheric haze surrounding a
planet’s disk. Often this effect occurs
during planetary transits during the disk’s ingress or egress against the limb of the
Sun. In the case of Mercury, this was erroneously thought to be an
indication that Mercury had a tenuous atmosphere.
Schröter vs. Herschel
Once Sir William Herschel, the most credible of the then known
prominent observers, had heard of the alleged Venusian Mountains, he
wanted them seriously and properly investigated (Herschel, 1790).
Herschel boldly requested all the available observations of this
“…extravagance.” This outburst seems very unusual against
what is known and accepted about Herschel’s general character, as he was normally
considered quite gentle and placid. Between 1792 and 1793, Herschel
observationally again saw nothing, and in his detailed observation
books, he kept obstinately repeating each time, and with little
variation, ”No Mountains
Visible.” Among the many
observations he also quotes;
“As the mountains of
Venus, I may venture to say that no eye, which is not considerably
better than mine, or assisted by much better instruments, will ever
get a sight of them; though from [the] analogy that obtains between
the only two planetary globes that we compare, there is little doubt
but this planet has inequalities on its surface, which maybe, for
what we can say to the [contrary], very considerable.”
Another highly relevant journal extract by Herschel says;
“You will find that I
have never been able to see any mountains on Venus; that to me the
horns were always sharp.”
It is possible that Herschel was only quelling the sudden rise of
his more inexperienced and younger rival, but there maybe other
motivations to his remarks.
In January 1783, Schröter congratulates Herschel
in his discovery of Sidus Georgium; He wrote;
“…it is hoped
that His Majesty has presented you entirely to Astronomy and provide
you with a handsome pension. Will all lovers of astronomy I wish this
with my whole heart.”
“…the newspaper
have it, to the new planet, which in Germany from the first has been
called Uranus.”
Thus, clearly Schröter held some admiration for Herschel, and
at this time the relationship was more like a pupil to his teacher.
After 1783, both Herschel and Schröter embarked along different
paths in regards their observational programmes. Herschel went on to
explore the realm of the stars and deep-sky, while Schröter
continued on with his lunar and planetary observations.
The controversy of Herschel’s
attacks began in early 1791, and after this time, Schröter’s written text is far more subdued and
reflective. Whatever the reason, for sometime Herschel view of
Schröter diminished. He once tersely explained the cause of the
“alleged’
observations to be attributed to Schröter’s tarnished speculum mirror. [23]
Herschel’s pursuit of
Schröter’s observations and
explanations was both vigorous and relentless. These general attacks
eventually became supported by other prominent astronomers of the
day, mainly because of the weight and substantial reputation of
Herschel. Eventually the astronomical community’s perception was to start, especially after
his death, to discredit Schröter — then the most prominent
and respected visual observer in observational astronomy. His peers
generally concluded that he had deceived himself into believing that
Venus was both rugged and mountainous. Yet to his approbation,
Schröter stayed silent but resolute, beginning several new
series of micrometric measures to his observations. With time, he was
unable to support his theory of the Venusian terrain with his actual
observations — the essence of Herschel’s attack.
To some, Schröter’s biggest
impediments were an overactive imagination and an inability to
refrain from speculations that degraded the value of his
observations. Modern day critics continue to harp on this flaw,
especially with his drawings and observations, however, the records
of his observations remaining so scant, that most are only contained
in letters sent to other observers.
The attacks and comments of these discoveries continued after the
death of both Herschel and Schröter. [24]
For example, in 1847, Dominique François Arago
(1786-1853) continued the debate as;
“…une critique fort vive,
et, en apparence du moins, quelque peu passionnée.”
“…one strong and lively
criticism appears somewhat less than passionate.”
Despite all this, Schröter today remains a respected amateur.
Schröter’s legacy should also be
viewed well beyond his assumed limitations as an observer.
Venusian Himalayas — Out of Vogue
After Schröter’s death, the
idea of real mountains of Venus fell from popular acceptance. One of
the few discussions in the literature about observations of Venus
during the early 19th century were the observations in 1841 by
Francesco De Vico (1805-1848) (and Palomba) from the Roman
College Observatory, who believed they saw a “notch” along
the terminator (Baum, 2001). Later, in 1862 W. Lassell saw an effect
that was “crater-like” on the terminator and in 1873, F. Denning
reported a “…indentation near
the northern cusp.” Even the
renowned visual Mars observers Schiaparelli, Lowell, Slipher, and
Holden all related their own observations of these luminous
protrusions. They also vigorously promoted among the general public
the real possibility of a mountain range or one solitary peak.
Clearly the atmospheric observation of Venus along the terminator
was still a problem — and even continues today.
This, combined with a rise in an interest in Venus, prompted
popular writers to speculate on the cause of the various phenomena.
Some like Webb (1962), for example, continues the theme about the
Venusian Mountains. Here he quotes that Schröter had calculated
the height of these peaks as;
“…supposed to
be 27 to 28 miles high [43km.], but of course with great
uncertainty.” (Schröter,
1792)
Other debates ensued offering other explanations of some causes of
the limb projections. Another quote is by Ellen M. Clerke in 1893.
(Baum, 2001, p.160) Here she says;
“There is, however,
another possibility… they may not be solid rock structures but
cloud masses piled up to an abnormal height. perhaps at the meeting
point of cold and warm air currents.” [25]
Venus’ Terminator Phenomena —
Back in Vogue
Schröter’s phenomena became a
popular topic of discussion during the 1880’s, only second in some respects, to
Lowell’s infamous canals on Mars.
Furthermore, it is also likely that Lowell’s suggestion that Venus might have similar
“canals’
produced on an avalanche of interest in the planet towards the end of
the 19th century.
Other observers had already drawn up battle lines against
Lowell’s fantastic ’canal’ claims,
and they started to scrutinise Schröter’s claims as well. The English astronomical
historian, Arthur Berry (1898) unfairly notes with respect to
Schröter: (Berry, 1898, 1961)
“…his results are not
always reliable.” &
“Almost the only
astronomer of the period whose work deserves a mention besides
Herschel’s, though very inferior to it
both in extent and in originally was [Schröter].”
After this, some suggested once more that these inequalities were
caused by peak(s) rising above the Venusian plains. Three clues held
favour;
1) Shape of the cusps and the terminator
towards the poles.
2) Shape of the cusp on the southern portion
of the planet was more curved that the northern one.
3) The observed, but infrequent appearances of
points of light along cusps’ ends, or
within the un-illuminated disk. Other more positive views
about Schröter’s observational
prowess have appeared scattered in the literature. For example. Webb
(1962) pg.65, says; “…
Schröter’s observations at the end
of the last century are far more trustworthy.”
The Debate Continues Into the 20th Century
Henry Norris Russell (1877-1957) said, later in life, that
he too had once investigated this question of mountains on Venus in
1898. Although Russell saw the uneven terminator defects himself, he
later went on to explained that his and Schröter’s observations were likely attributed to
some unknown atmospheric or deformation effect with the Venusian
cusp. Even though Russell may have considered these scenarios were
unlikely — Schröter’s
reputation as an observer was sufficient for others to assume his
conclusions had some merit. The seemingly regularity of other similar
observations of the southern (and northern) disc meant the effect
could be a real physical process. If Schröter were correct,
Russell concluded, then the height of the mountain summits would have
had to be about sixty kilometres!
Similar sporadic reports of southern cusp deviations appear
throughout the 20th Century. One of the most interesting is the
independent observations by two English amateurs, Frank Sargent and
Henry McEwen in 1913. Both observed and documented their observations
of a terminator anomaly, which appear like a notch against the
planetary limb. This effect sometimes referred as the McEwen-Sargent
Feature (Baum, 2001). Reports from other credible amateurs also shows
that these effects must have some credence. It is quite common
for modern observers to perceive the northern or southern most parts
of the limb as unequally concave or convex, especially around the
time of dichotomy. During the gibbous or crescent phases, sometimes
parts of the terminator shows familiar short or flat linear segments.
Photographic and digital evidence leads to the conclusion that these
effects are actually optical illusions resulting from several dark
markings in the atmosphere appearing close to the terminator. One
favoured explanation is that the terminator is being seen differently
by the eye in the eyepiece, such that the observers brain interprets
the southern-most portion (the top of the field in the northern
hemisphere) as being far more curved than the reverse direction. My
own personal series of observations in 1979 show such deformations,
except they are at the northern part of the terminator, and possibly
showing us the effects of some southern hemisphere bias. About 1981,
the current advice to observers was to look at the terminator
horizontally across the field of view, instead of vertically, in an
attempt to eliminate this bias.
Many observers of Venus are still encouraged to report any
phenomena. The primary collection site for observations is the
Mercury and Venus Section of the British Astronomical Association in
London under the directorship of Richard Baum. (2003)
Mountains — Exposed at Last
In 1962, Goldstein and Zohar of the Jet Propulsion Laboratory
retrieved the first radar images of Venus, showing a possible
mountain range at latitude –45°S. Goldstein and Zohar
reconfirmed this in 1964, and again in 1969. NASA’s Magellan spacecraft subsequently showed
another prominent peak from the orbital radar images at 10°E,
–80°S. This small peak is about six kilometres above the
mean height of the surface. Incidental, the tallest large feature is
the Australian-sized continent known as the Ishtar Plateau, with the
tallest peak being Maxwell Montes, eleven kilometres above ’sea level’ at
latitude +62°N. Could this feature influence the Venusian
atmosphere to produce these visual effects that have been seen so
often? [26]
C O N C L U S I O N
For all the criticisms of Schröter’s observations, and based on the evidence of
the experience and quality of his works, Schröter remains one of
the best of all known visual observers. After the destruction of most
of his observing notes during the Napoleonic Wars, for us what
remains provides an elusive and puzzling mystery. The problem of the
Venusian Cusp still remains and therefore warrants some continued
observation. In some ways, this is a general guide for all serious
visual observers — no matter what we see, always report
anything unusual. To quote the active observer, Webb (1963)
pg.19;
“And, like old Schröter,
trust nothing to memory.” [His
italics]
or as Herschel might have quipped
“Draw what you see — not
imagine what you see.”
R E F E R E N C E S
- Abbot, David (Ed.); “The
Biographical Dictionary of Scientists: Astronomers” (1984)
- Baum, Richard, M; “The Planets
— Some Myths and Realities.”
Pub. David & Charles pg.48- 83. (1973)
- Baum, Richard, M.; “The
enigmatic Ashen Light of Venus : An Overview.”, J.BAA., 110, 325 (2000)
- Baum, Richard, M.; “The
McEwen-Sargent feature and the mystery of the Venus contour
anomaly.”; J.BAA., 111, 3
(2001)
- Baum, Richard, M.; Personal communication. (2003)
- Berry, Arthur; “A History of
Astronomy” (1898) (Preface:
September 1898) & Dover; 2nd Ed. (1961)
- Crisp, D. et al.; “Near-Infrared
Observations of Venus”; 20th Annual
DDS meeting BAA.S., 20, 14.01, 833 (1998)
- Draper, Arthur and Lockwood, Marion “The Story of Astronomy.” Unwin Bros. p.89. (1940)
- Dreyer, J.L.E.; “A Short Account
of Sir William Herschel’s Life and
Works.” (1912)
- Dreyer, J.L.E.; “Scientific
Papers of Sir William Herschel.”
(1912)
- Grosser, Morton;“The Discovery
of Neptune.” Dover (1982)
- Herschel, William; Phil.Trans., 83, pg.337,
(1790)
- Herschel, William; Phil.Trans. 82, pg.201,
(1793)
- Hoskin, Michael; “The Cambridge
Illustrated History of Astronomy.”
pg.188-201, 208. Cambridge Press (1997)
- de Le Hire, Phillipe; “Mém de l’ Académie des sciences.”; pg.296-97 Paris Obs. August (1700)
- Hoyt, William G.; “Planets X and
Pluto.” Pub. Arizona Board of
Regents pg.24, 27-28, 30 (1980)
- James, Andrew; “Neat Southern
Planetaries XII”, “Universe’
(July) pg.15. (Astronomical Society of New South Wales inc. (ASNSWI)
(1998) [See also NSP 12.]
- King, Henry; “The History of the
Telescope.” (1955) Dover 2nd Ed.
(1979)
- Neison, E.; “The Moon.” In Preface (1876)
- Panneloek, A.; “A History of
Astronomy.” Dover (1961)
- Sandner, Werner; “The Planet
Mercury.” Faber and Faber
(1963)
- Seiff. A., Kirk, D.B.; “In-Situ
Measurements of Vertical Winds in the Atmosphere of Venus.”; 20th Annual DDS meeting BAA.S.,
20, 14.01, 833 (1998)
- Simion, W. et al.; “Imaging of
the Venus Dark Hemisphere at 1.74 μ.”; 20th Annual DDS meeting BAA.S.,
20, 14.01, 833 (1998)
- Schröter, J.H.; Phil. Trans. 85, 120
(1795)
- Schröter, J.H. “Sur
genauern Kenntniss der Mondflèche Vol. 1” Selenotopographische Fragmente from
Lilienthal: auf Kosten des Verfassers. (1791)
- Sheehan, William; “The Planet
Mars: A History of Observation and Discovery.” Arizona Press (1997) — esp. Ch.3
“A Situation Similar to
Ours’
- Webb, Rev. T.W.; “Celestial
Objects for Common Telescopes Vol 1.” Dover (1962)
A P P R E C I A T I O N
I would like to thank the following people for their
considerable assistance and advise in producing this paper.
Messrs. Richard Baum, Bob Evans, Nick Loveday and Grant
Searle
E N D N O T E S
[1] Debated, then denied
as real in 1880’s by several sources,
including Rev. Thomas Webb (1807- 1885) in “Celestial Objects for Common Telescopes
Vol.1“ pg.67. Dover Edition.
[2] Modern writers and
Venus observers still debate on what these early observers saw. A
summary of the various reasons appears in the literature from time to
time. Baum (2001) pg.158, for example, says;
“There can be
little doubt the early telescopists were deceived, for it is well
known that in poor seeing Venus shows features which quite disappear
from the sharp images obtained. But if they mistook observational
artefact for objective realities it must also be said that some
irregularity originates in localised darkenings and brightenings of
the terminator shading, resulting in what appears to be depressions
or projections in the line of the terminator.”
[3] Nor did William
Herschel. The first known detailed map of the Moon was produced by
Tobias Meyer in 1775. Interesting in the fragments of Schroter’s remaining is the best reproduction we have
of Meyer’s map. Incidentally, Christian
Huygens lunar drawing was published in his manuscript in 1925,
several hundred years later.
[4] Observations since
by spacecraft of this region have shown it as far more complex.
Unfortunately for observers on Earth, we can only see this feature
from a low angle, and cannot not peer into its unique depths.
[5] Housing of his
family must have been the more important priority. His finances were
a problem, as he was dismissed from his Government position. However,
King (1979) claims; “…he was
too old.”
[6] Some have also been
sympathetic to his fate, which is likely balancing some of other
ferocious criticisms levelled personally against Schröter
— especially in England. But also sometimes fate takes its
hand.
[7] Another example is
E.J. Hartung AOST1, whose written observations were completely
destroyed during the Ash Wednesday bush fires in 1983. Another, at
the time of writing, the destruction by bush fires of Australia’s Mt. Stromlo Observatory, near Canberra on
the 18th-19th January 2003. Here the historic library was also
ravished by fire. Although interesting and irreplaceable astronomical
work was lost, at least the major astronomical works that were done
there continue in the authors papers in libraries throughout the
world and on the Internet.
[8] Created by Peter
Dolland who supplied the public and the Services demands for
telescopes, by constructing telescopes during the Napoleonic Wars.
[JBAA., 61, 192ff (1951)]
[9] According to
Richard Baum in a personal communication, the change in telescope
“manufacturer” was not due to any problems with Herschel.
He says; “Not so. The answer is
more prosaic. Quite simply and by chance he had found another
supplier!”
[10] According to
Sheeham (1997) the telescope was 19¼-inch (49cm.) f/16
(8.2m).
[11] This telescope
bore the plaque with the description; “Telescopum Newtonianum XXVII pedum
constructum Lilenthal 1793.”
[12] Several
observers in the 18th Century had noted the relationship between the
mean planetary distances before Bode. Even Johannes Kepler
(1571-1630) in 1596 “Misterium
Comographicum” had noted the
inordinately large gap between Mars and Jupiter, but this was merely
commented. The first was the Oxford Professor David Gregory
(1659-1708) in his work “Astronomae
elementa” (1702), followed by the
German popularist Christian Freiherr von Wolff in 1741. The
latter’s claim was examined by Johann
Daniel Titus (1729-96), who was first to notice the
planetary”gap” missing in the orbital relationships. This
was first published in”Comptemplation
de la nature” in 1766. Here Titus
modified Gregory’s relationship to
produce the relationship; d= [ 4 + 3 × ( 2n ) ] / 10 ,
where; d = Distance in Astronomical Units and n = Orbit of Number of
the Orbital Position of the Planet. In this relationship, the
distance value “4+24” does not have any planet corresponding to
the orbit. This fact was further extended by the investigations of
Johann Elert Bode (1747-1826), who strongly objected to
Gregory’s “nonsense”
that the missing object might be just a moon of Mars. In 1776, Bode
soon became convinced of another planet existing between the orbits
of Mars and Jupiter. Later, after Uranus was discovered by Herschel
in August 1781, its value equaled almost exactly matched it
geometrical sequenced position of“4+192”, and
this also drove further speculations into other possible
trans-Uranian planets. These speculations came very popular in both
the astronomical community and among the general public. The
formation of the“Celestial Police” was a positive indication of the serious
search for minor planetary bodies. von Zack had attempted to
predicted a theoretical orbit in 1785 and make some crude searches
in 1787 — without success. In 1799, von Zack requested the
meeting of some of his prominent German colleagues that lead to the
formation of the Society of ”Celestial
police”.
[13] Ceres was
named after the Sicilian Goddess of harvest and grain, and is
alleged to be the virgin represented as the constellation Virgo.
Piazzi named it Ceres Ferdinandea, after the goddess and Sicilian
monarch and his patron at the time!
[13] Herschel states
that Ceres diameter was “…at
under 100 miles” (<161km). Although
this was really no better than guesswork, and this value changed
very little for many decades. This concluded that Ceres was
certainly not ‘planet-sized’ that was known after about 1803. This was
derived not from orbital calculations, but from the apparent
magnitude of the minor planet. Ceres present estimated diameter is
930km (1994) and was not really determined or approximated until the
20th Century.
[14] The often given
but incorrectly formulator of the famous cosmological question; Why
is the sky dark at night? He was a German doctor, and a boyhood
friend of Schröter. He was interested mainly in comets and the
weather.
[15] During the time
of both Schröter and Herschel prominence, Germany was then just
a collection of small independent states. In 1814, there were
thirty-seven politically independent states all arranged under an
agreement with the First Paris Peace Treaty. Historically, the French
Revolution, and the subsequent scamble for dominance, rocked the
entire European theatre. With the execution for treason of Louis XVI
and Marie Antoinette, and the unprovoked attempt of the Prussian
invasion of France in September 1792 in trying to isolate Paris from
the French revolutionaries, made these times particularly uncertain.
Overall the political events throughout most of the European
Continent must have layed heavily on Schröter’s and the population’s mind — but also likely in the minds
of Herschel and the English people.
[16] Herschel is
likely not to have personally favoured the French, as France had
declared war on both England and Holland in February 1793. Yet
Herschel’s popularity remained great
among the nobility and the Royal Houses throughout Europe —
especially after the discovery of Uranus. He visited Paris on
invitation in 1801, meeting Pierre Laplace (1749-1827) and even
Napoleon Bonaparte.
[17] Such ideas
persisted even in the beginning of the 20th Century. The most
significant supporter of these claims was the Turkish-born French
observer Eugène Michel Antoniadi (1870-1944). As observations
of the planet were so difficult, he became the leading expert on the
planet Mercury. In 1934, Antoniadi published “La Planète Mecure” in which he claimed to have seen notable
surface features and evidence of a significantly rarefied
atmosphere. Such beliefs were held until the time of the Mariner 10
spacecraft, which encountered the planet three times; in March and
September 1974 and March 1975.
[18] The first likely
description of the ashen light was first seen in 1715 by William
Derham, the cannon of Windsor, when Venus was near inferior
conjunction. According to Baum (2000), which I have not seen
elsewhere, perhaps Giovanni Battista Riccioli in 1643 saw the same
phenomena. However, the cryptic and ambiguous statements by
Riccioli, leads Baum summary of the observation as; “…of such an extraordinary
character, that is perhaps more correct to credit William
Durham.”
[19] The true cause,
is still being investigated by modern observers and is a regular
program of the Mercury and Venus Section of the B.A.A.
[20] Rev. T.W.Webb
(1962) pg.65
[21] In hindsight, the
rotational period is the usual geocentric view of the world of these
times.
[22] Cloud features
have been observed in infrared light at 1.74 μ with an average size of 400 kilometres. The
present upper atmospheric rotation, like the planet, is retrograde.
The upper air around sixty kilometres moves c.100 m.s-1
(Sinion, et al. (1998) and Seiff (1998)) The mean atmospheric
rotation encircles the once planet every 4.0 to 4.4 days (Compared
to the Earth’s average of about 12
days.)
[23] However, Herschel
could also be the one questioned here, as it has been suggested that
his eyesight had suffered permanent damage arising from an occasion
when he pointed a small refractor directly towards the Sun.
[24] Herschel, W.;
Phil.Trans. LXXXII, pg.xxx? (1793)
[25] This is an
interesting postulate, but recent evidence has shown by ground-based
and spacecraft near-infrared and infrared observations. I.e. Seiff
(1998) On Venus the largest measured updraft is some 2.7
m.s-1 at a height of sixty kilometers. The velocity for
this to occur is far too small to be an acceptable or plausible
explanation.
[26] I speculate
— could this southern peak possibly be affecting this
atmospheric phenomena?
Last Update : 15th April 2015
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