Jupiter
This brilliant planet--only inferior to Venus in brightness--was often
seen by Bond (Jun.) with the naked eye in "high and clear sunshine"; also
by Denning, who has very keen eyesight. Its brightness on such occasions
is so great, that--like Venus--it casts a distinct shadow in a dark
room.[130]
The great "red spot" on Jupiter seems to have been originally discovered
by Robert Hooke on May 9, 1664, with a telescope of 2 inches aperture and
12 feet focus. It seems to have existed ever since; at least the evidence
is, according to Denning, in favour of the identity of Hooke's spot with
the red spot visible in recent years. The spot was also observed by
Cassini in the years 1665-72, and is sometimes called "Cassini's spot."
But the real discoverer was Hooke.[131]
The orbit of Jupiter is so far outside the earth's orbit that there can
be little visible in the way of "phase"--as in the case of Mars, where the
"gibbous" phase is sometimes very perceptible. Some books on astronomy
state that Jupiter shows no phase. But this is incorrect. A distinct,
although very slight, gibbous appearance is visible when the planet is
near quadrature. Webb thought it more conspicuous in twilight than in a
dark sky. With large telescopes, Jupiter's satellites II. and III. have
been seen--in consequence of Jupiter's phase--to emerge from occultation
"at a sensible distance from the limb."[132]
According to M. E. Rogovsky, the high "albedo of Jupiter, the appearance
of the clear (red) and dark spots on its surface and their continual
variation, the different velocity of rotation of the equatorial and other
zones of its surface, and particularly its small density (1·33, water as
unity), all these facts afford irrefragable proofs of the high temperature
of this planet. The dense and opaque atmosphere hides its glowing surface
from our view, and we see therefore only the external surface of its
clouds. The objective existence of this atmosphere is proved by the bands
and lines of absorption in its spectrum. The interesting photograph
obtained by Draper, September 27, 1879, in which the blue and green parts
are more brilliant for the equatorial zone than for the adjacent parts of
the surface, appears to show that _Jupiter_ emits its proper light. It is
possible that the constant red spot noticed on its surface by several
observers, as Gledhill, Lord Rosse, and Copeland (1873), Russel and
Bredikhin (1876), is the summit of a high glowing mountain. G. W. Hough
considers Jupiter to be gaseous, and A. Ritter inferred from his formulæ
that in this case the temperature at the centre would be 600,000° C."[133]
The four brighter satellites of Jupiter are usually known by numbers I.,
II., III., and IV.; I. being the nearest to the planet, and IV. the
farthest. III. is usually the brightest, and IV. the faintest, but
exceptions to this rule have been noticed.
With reference to the recently discovered sixth and seventh satellites of
Jupiter, Prof. Perrine has suggested that the large inclination of their
orbits to the plane of the planet's equator seems to indicate that neither
of these bodies was originally a member of Jupiter's family, but has been
"captured by the planet." This seems possible as the orbits of some of the
minor planets lie near the orbit of Jupiter (see "Minor Planets"). A
similar suggestion has been made by Prof. del Marmol.[134]
Many curious observations have been recorded with reference to Jupiter's
satellites; some very difficult of explanation. In 1711 Bianchini saw
satellite IV. so faint for more than an hour that it was hardly visible! A
similar observation was made by Lassell with a more powerful telescope on
June 13, 1849. Key, T. T. Smyth, and Denning have also recorded unusual
faintness.[135] A very remarkable phenomenon was seen by Admiral Smyth,
Maclear, and Pearson on June 26, 1828. Satellite II., "having fairly
entered on Jupiter, was found 12 or 13 minutes afterwards _outside the
limb_, where it remained visible for at least 4 minutes, and then suddenly
vanished." As Webb says, "Explanation is here set at defiance;
demonstrably neither in the atmosphere of the earth, nor Jupiter, where
and what could have been the cause? At present we can get no answer."[136]
When Jupiter is in opposition to the sun--that is, on the meridian at
midnight--satellite I. has been seen projected on its own shadow, the
shadow appearing as a dark ring round the satellite.
On January 28, 1848, at Cambridge (U.S.A.) satellite III. was seen in
transit lying between the shadows of I. and II. and so black that it could
not be distinguished from the shadows, "except by the place it occupied."
This seems to suggest inherent light in the planet's surface, as the
satellite was at the time illuminated by full sunshine; its apparent
blackness being due to the effect of contrast. Cassini on one occasion
failed to find the shadow of satellite I. when it should have been on the
planet's disc,[137] an observation which again points to the glowing light
of Jupiter's surface. Sadler and Trouvelot saw the shadow of satellite I.
double! an observation difficult to explain--but the same phenomenon was
again seen on the evening of September 19, 1891, by Mr. H. S. Halbert of
Detroit, Michigan (U.S.A.). He says that the satellite "was in transit
nearing egress, and it appeared as a white disc against the dark southern
equatorial belt; following it was the usual shadow, and at an equal
distance from this was a second shadow, smaller and not so dark as the
true one, and surrounded by a faint penumbra."[138]
A dark transit of satellite III. was again seen on the evening of December
19, 1891, by two observers in America. One observer noted that the
satellite, when on the disc of the planet, was intensely black. To the
other observer (Willis L. Barnes) it appeared as an ill-defined _dark_
image.[139] A similar observation was made on October 9 of the same year
by Messrs. Gale and Innes.[140]
A "black transit" of satellite IV. was seen by several observers in 1873,
and by Prof. Barnard on May 4, 1886. The same phenomenon was observed on
October 30, 1903, in America, by Miss Anne S. Young and Willis S. Barnes.
Miss Young says--
"The ingress of the satellite took place at 8{h} 50{m} (E. standard
time) when it became invisible upon the background of the planet. An
hour later it was plainly visible as a dark round spot upon the
planet. It was decidedly darker than the equatorial belt."[141]
The rather rare phenomenon of an occultation of one of Jupiter's
satellites by another was observed by Mr. Apple, director of the Daniel
Scholl Observatory, Franklin and Marshall College, Lancaster, Pa.
(U.S.A.), on the evening of March 16, 1908. The satellites in question
were I. and II., and they were so close that they could not be separated
with the 11·5-inch telescope of the Observatory.[142] One of the present
writer's first observations with a telescope is dated May 17, 1873, and is
as follows: "Observed one of Jupiter's satellites occulted (or very nearly
so) by another. Appeared as one with power 133" (on 3-inch refractor in
the Punjab). These satellites were probably I. and II.
Jupiter has been seen on several occasions apparently without his
satellites; some being behind the disc, some eclipsed in his shadow, and
some in transit across the disc. This phenomenon was seen by Galileo,
March 15, 1611; by Molyneux, on November 12, 1681; by Sir William
Herschel, May 23, 1802; by Wallis, April 15, 1826; by Greisbach, September
27, 1843; and by several observers on four occasions in the years
1867-1895.[143] The phenomenon again occurred on October 3, 1907, No. 1
being eclipsed and occulted, No. 2 in transit, No. 3 eclipsed, and No. 4
occulted.[144] It was not, however, visible in Europe, but could have been
seen in Asia and Oceania.[144] The phenomenon will occur again on October
22, 1913.[145]
On the night of September 19, 1903, a star of magnitude 6-1/2 was occulted
by the disc of Jupiter. This curious and rare phenomenon was photographed
by M. Lucien Rudaux at the Observatory of Donville, France.[146] The star
was Lalande 45698 (= BAC 8129).[147]
Prof. Barnard, using telescopes with apertures from 5 inches up to 36
inches (Lick), has failed to see a satellite through the planet's limb (an
observation which has been claimed by other astronomers). He says, "To my
mind this has been due to either poor seeing, a poor telescope, or an
excited observer."[148] He adds--
"I think it is high time that the astronomers reject the idea that the
satellites of Jupiter can be seen through his limb at occultation.
When the seeing is bad there is a spurious limb to Jupiter that well
might give the appearance of transparency at the occultation of a
satellite. But under first-class conditions the limb of Jupiter is
perfectly opaque. It is quibbling and begging the question altogether
to say the phenomenon of transparency may be a rare one and so have
escaped my observations. Has any one said that the moon was
transparent when a star has been seen projected on it when it ought to
have been behind it?"
Prof. Barnard and Mr. Douglass have seen white polar caps on the third and
fourth satellites of Jupiter. The former says they are "exactly like those
on Mars." "Both caps of the fourth satellite have been clearly
distinguished, that at the north being sometimes exceptionally large,
covering a surface equal to one-quarter or one-third of the diameter of
the satellite."[149] This was confirmed on November 23, 1906, when Signor
J. Comas Sola observed a brilliant white spot surrounded by a dark marking
in the north polar region of the third satellite. There were other dark
markings visible, and the satellite presented the appearance of a
miniature of Mars.[150]
An eighth satellite of Jupiter has recently been discovered by Mr. Melotte
at the Greenwich Observatory by means of photography. It moves in a
retrograde direction round Jupiter in an orbit inclined about 30° to that
of the planet. The period of revolution is about two years. The orbit is
very eccentric, the eccentricity being about one-third, or greater than
that of any other satellite of the solar system. When nearest to Jupiter
it is about 9 millions of miles from the planet, and when farthest about
20 millions.[151] It has been suggested by Mr. George Forbes that this
satellite may possibly be identical with the lost comet of Lexell which at
its return in the year 1779 became entangled in Jupiter's system, and has
not been seen since. If this be the case, we should have the curious
phenomenon of a comet revolving round a planet!
According to Humboldt the four bright satellites of Jupiter were seen
almost simultaneously and quite independently by Simon Marius at Ausbach
on December 29, 1609, and by Galileo at Padua on January 7, 1610.[152] The
actual priority, therefore, seems to rest with Simon Marius, but the
publication of the discovery was first made by Galileo in his _Nuncius
Siderius_ (1610).[153] Grant, however, in his _History of Physical
Astronomy_, calls Simon Marius an "impudent pretender"! (p. 79).
M. Dupret at Algiers saw Jupiter with the naked eye on September 26, 1890,
twenty minutes before sunset.[154]
Humboldt states that he saw Jupiter with the naked eye when the sun was
from 18° to 20° above the horizon.[155] This was in the plains of South
America near the sea-level.