Quelltext der Seite History of Astronomy Forbes 1909 13 The Moon and Planets

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|Titel=History of Astronomy
|Autor=Forbes-George
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HISTORY OF ASTRONOMY

:BY
:GEORGE FORBES,
:M.A., F.R.S., M. INST. C. E.,
:(FORMERLY PROFESSOR OF NATURAL PHILOSOPHY, ANDERSON'S COLLEGE, GLASGOW)

:AUTHOR OF &quot;THE TRANSIT OF VENUS,&quot; RENDU'S &quot;THEORY OF THE :GLACIERS OF SAVOY,&quot; ETC., ETC.

:1909




==13. THE MOON AND PLANETS.==


_The Moon_.--Telescopic discoveries about the moon commence with
Galileo's discovery that her surface has mountains and valleys, like
the earth. He also found that, while she always turns the same face to
us, there is periodically a slight twist to let us see a little round
the eastern or western edge. This was called _libration_, and the
explanation was clear when it was understood that in showing always
the same face to us she makes one revolution a month on her axis
_uniformly_, and that her revolution round the earth is not
uniform.

Galileo said that the mountains on the moon showed greater differences
of level than those on the earth.  Shröter supported this
opinion. W. Herschel opposed it. But Beer and Mädler measured the
heights of lunar mountains by their shadows, and found four of them
over 20,000 feet above the surrounding plains.

Langrenus [1] was the first to do serious work on selenography, and
named the lunar features after eminent men. Riccioli also made lunar
charts. In 1692 Cassini made a chart of the full moon. Since then we
have the charts of Schröter, Beer and Mädler (1837), and of Schmidt,
of Athens (1878); and, above all, the photographic atlas by Loewy and
Puiseux.

The details of the moon's surface require for their discussion a whole
book, like that of Neison or the one by Nasmyth and Carpenter. Here a
few words must suffice. Mountain ranges like our Andes or Himalayas
are rare. Instead of that, we see an immense number of circular
cavities, with rugged edges and flat interior, often with a cone in
the centre, reminding one of instantaneous photographs of the splash
of a drop of water falling into a pool. Many of these are fifty or
sixty miles across, some more. They are generally spoken of as
resembling craters of volcanoes, active or extinct, on the earth. But
some of those who have most fully studied the shapes of craters deny
altogether their resemblance to the circular objects on the moon.
These so-called craters, in many parts, are seen to be closely
grouped, especially in the snow-white parts of the moon. But there are
great smooth dark spaces, like the clear black ice on a pond, more
free from craters, to which the equally inappropriate name of seas has
been given. The most conspicuous crater, _Tycho_, is near the south
pole. At full moon there are seen to radiate from Tycho numerous
streaks of light, or &quot;rays,&quot; cutting through all the mountain
formations, and extending over fully half the lunar disc, like the
star-shaped cracks made on a sheet of ice by a blow.  Similar cracks
radiate from other large craters. It must be mentioned that these
white rays are well seen only in full light of the sun at full moon,
just as the white snow in the crevasses of a glacier is seen bright
from a distance only when the sun is high, and disappears at
sunset. Then there are deep, narrow, crooked &quot;rills&quot; which may have
been water-courses; also &quot;clefts&quot; about half a mile wide, and often
hundreds of miles long, like deep cracks in the surface going straight
through mountain and valley.

The moon shares with the sun the advantage of being a good subject for
photography, though the planets are not. This is owing to her larger
apparent size, and the abundance of illumination. The consequence is
that the finest details of the moon, as seen in the largest telescope
in the world, may be reproduced at a cost within the reach of all.

No certain changes have ever been observed; but several suspicions
have been expressed, especially as to the small crater _Linné_, in the
_Mare Serenitatis_. It is now generally agreed that no certainty can
be expected from drawings, and that for real evidence we must await
the verdict of photography.

No trace of water or of an atmosphere has been found on the moon. It
is possible that the temperature is too low. In any case, no
displacement of a star by atmospheric refraction at occultation has
been surely recorded. The moon seems to be dead.

The distance of the moon from the earth is just now the subject of
re-measurement. The base line is from Greenwich to Cape of Good Hope,
and the new feature introduced is the selection of a definite point on
a crater (Mösting A), instead of the moon's edge, as the point whose
distance is to be measured.

_The Inferior Planets_.--When the telescope was invented, the phases
of Venus attracted much attention; but the brightness of this planet,
and her proximity to the sun, as with Mercury also, seemed to be a bar
to the discovery of markings by which the axis and period of rotation
could be fixed. Cassini gave the rotation as twenty-three hours, by
observing a bright spot on her surface. Shröter made it 23h. 21m. 19s.
This value was supported by others. In 1890 Schiaparelli[2] announced
that Venus rotates, like our moon, once in one of her revolutions, and
always directs the same face to the sun. This property has also been
ascribed to Mercury; but in neither case has the evidence been
generally accepted. Twenty-four hours is probably about the period of
rotation for each of these planets.

Several observers have claimed to have seen a planet within the orbit
of Mercury, either in transit over the sun's surface or during an
eclipse. It has even been named _Vulcan_. These announcements would
have received little attention but for the fact that the motion of
Mercury has irregularities which have not been accounted for by known
planets; and Le Verrier[3] has stated that an intra-Mercurial planet
or ring of asteroids would account for the unexplained part of the
motion of the line of apses of Mercury's orbit amounting to 38&quot; per
century.

_Mars_.--The first study of the appearance of Mars by Miraldi led him
to believe that there were changes proceeding in the two white caps
which are seen at the planet's poles. W. Herschel attributed these
caps to ice and snow, and the dates of his observations indicated a
melting of these ice-caps in the Martian summer.

Schroter attributed the other markings on Mars to drifting clouds. But
Beer and Mädler, in 1830-39, identified the same dark spots as being
always in the same place, though sometimes blurred by mist in the
local winter. A spot sketched by Huyghens in 1672, one frequently seen
by W. Herschel in 1783, another by Arago in 1813, and nearly all the
markings recorded by Beer and Mädler in 1830, were seen and drawn by
F. Kaiser in Leyden during seventeen nights of the opposition of 1862
(_Ast. Nacht._, No. 1,468), whence he deduced the period of rotation
to be 24h. 37m. 22s.,62--or one-tenth of a second less than the period
deduced by R. A. Proctor from a drawing by Hooke in 1666.

It must be noted that, if the periods of rotation both of Mercury and
Venus be about twenty-four hours, as seems probable, all the four
planets nearest to the sun rotate in the same period, while the great
planets rotate in about ten hours (Uranus and Neptune being still
indeterminate).

The general surface of Mars is a deep yellow; but there are dark grey
or greenish patches. Sir John Herschel was the first to attribute the
ruddy colour of Mars to its soil rather than to its atmosphere.

The observations of that keen-sighted observer Dawes led to the first
good map of Mars, in 1869. In the 1877 opposition Schiaparelli revived
interest in the planet by the discovery of canals, uniformly about
sixty miles wide, running generally on great circles, some of them
being three or four thousand miles long. During the opposition of
1881-2 the same observer re-observed the canals, and in twenty of them
he found the canals duplicated,[4] the second canal being always 200
to 400 miles distant from its fellow.

The existence of these canals has been doubted.  Mr. Lowell has now
devoted years to the subject, has drawn them over and over again, and
has photographed them; and accepts the explanation that they are
artificial, and that vegetation grows on their banks.  Thus is revived
the old controversy between Whewell and Brewster as to the
habitability of the planets. The new arguments are not yet generally
accepted. Lowell believes he has, with the spectroscope, proved the
existence of water on Mars.

One of the most unexpected and interesting of all telescopic
discoveries took place in the opposition of 1877, when Mars was
unusually near to the earth. The Washington Observatory had acquired
the fine 26-inch refractor, and Asaph Hall searched for satellites,
concealing the planet's disc to avoid the glare. On August 11th he had
a suspicion of a satellite. This was confirmed on the 16th, and on the
following night a second one was added. They are exceedingly faint,
and can be seen only by the most powerful telescopes, and only at the
times of opposition. Their diameters are estimated at six or seven
miles. It was soon found that the first, Deimos, completes its orbit
in 30h. 18m.  But the other, Phobos, at first was a puzzle, owing to
its incredible velocity being unsuspected. Later it was found that the
period of revolution was only 7h. 39m. 22s. Since the Martian day is
twenty-four and a half hours, this leads to remarkable results.
Obviously the easterly motion of the satellite overwhelms the diurnal
rotation of the planet, and Phobos must appear to the inhabitants, if
they exist, to rise in the west and set in the east, showing two or
even three full moons in a day, so that, sufficiently well for the
ordinary purposes of life, the hour of the day can be told by its
phases.

The discovery of these two satellites is, perhaps, the most
interesting telescopic visual discovery made with the large telescopes
of the last half century; photography having been the means of
discovering all the other new satellites except Jupiter's fifth (in
order of discovery).

[Illustration: JUPITER.  From a drawing by E. M. Antoniadi, showing
transit of a satellite's shadow, the belts, and the &quot;great red spot&quot;
(_Monthly Notices_, R. A. S., vol. lix., pl. x.).]

_Jupiter._--Galileo's discovery of Jupiter's satellites was followed
by the discovery of his belts. Zucchi and Torricelli seem to have seen
them. Fontana, in 1633, reported three belts. In 1648 Grimaldi saw but
two, and noticed that they lay parallel to the ecliptic. Dusky spots
were also noticed as transient. Hooke[5] measured the motion of one in
1664. In 1665 Cassini, with a fine telescope, 35-feet focal length,
observed many spots moving from east to west, whence he concluded that
Jupiter rotates on an axis like the earth. He watched an unusually
permanent spot during twenty-nine rotations, and fixed the period at
9h. 56m. Later he inferred that spots near the equator rotate quicker
than those in higher latitudes (the same as Carrington found for the
sun); and W. Herschel confirmed this in 1778-9.

Jupiter's rapid rotation ought, according to Newton's theory, to be
accompanied by a great flattening at the poles. Cassini had noted an
oval form in 1691. This was confirmed by La Hire, Römer, and
Picard. Pound measured the ellipticity = 1/(13.25).

W. Herschel supposed the spots to be masses of cloud in the
atmosphere--an opinion still accepted.  Many of them were very
permanent. Cassini's great spot vanished and reappeared nine times
between 1665 and 1713. It was close to the northern margin of the
southern belt. Herschel supposed the belts to be the body of the
planet, and the lighter parts to be clouds confined to certain
latitudes.

In 1665 Cassini observed transits of the four satellites, and also saw
their shadows on the planet, and worked out a lunar theory for
Jupiter. Mathematical astronomers have taken great interest in the
perturbations of the satellites, because their relative periods
introduce peculiar effects. Airy, in his delightful book,
_Gravitation_, has reduced these investigations to simple
geometrical explanations.

In 1707 and 1713 Miraldi noticed that the fourth satellite varies much
in brightness. W. Herschel found this variation to depend upon its
position in its orbit, and concluded that in the positions of
feebleness it is always presenting to us a portion of its surface,
which does not well reflect the sun's light; proving that it always
turns the same face to Jupiter, as is the case with our moon. This
fact had also been established for Saturn's fifth satellite, and may
be true for all satellites.

In 1826 Struve measured the diameters of the four satellites, and
found them to be 2,429, 2,180, 3,561, and 3,046 miles.

In modern times much interest has been taken in watching a rival to
Cassini's famous spot. The &quot;great red spot&quot; was first observed by
Niesten, Pritchett, and Tempel, in 1878, as a rosy cloud attached to a
whitish zone beneath the dark southern equatorial band, shaped like
the new war balloons, 30,000 miles long and 7,000 miles across. The
next year it was brick-red. A white spot beside it completed a
rotation in less time by 51/2 minutes than the red spot--a difference
of 260 miles an hour. Thus they came together again every six weeks,
but the motions did not continue uniform.  The spot was feeble in
1882-4, brightened in 1886, and, after many changes, is still visible.

Galileo's great discovery of Jupiter's four moons was the last word in
this connection until September 9th, 1892, when Barnard, using the
36-inch refractor of the Lick Observatory, detected a tiny spot of
light closely following the planet. This proved to be a new satellite
(fifth), nearer to the planet than any other, and revolving round it
in 11h. 57m. 23s. Between its rising and setting there must be an
interval of 21/2 Jovian days, and two or three full moons. The sixth
and seventh satellites were found by the examination of photographic
plates at the Lick Observatory in 1905, since which time they have
been continuously photographed, and their orbits traced, at Greenwich.
On examining these plates in 1908 Mr. Melotte detected the eighth
satellite, which seems to be revolving in a retrograde orbit three
times as far from its planet as the next one (seventh), in these two
points agreeing with the outermost of Saturn's satellites (Phoebe).

_Saturn._--This planet, with its marvellous ring, was perhaps the most
wonderful object of those first examined by Galileo's telescope. He
was followed by Dominique Cassini, who detected bands like Jupiter's
belts. Herschel established the rotation of the planet in 1775-94.
From observations during one hundred rotations he found the period to
be 10h. 16m. 0s., 44. Herschel also measured the ratio of the polar to
the equatoreal diameter as 10:11.

The ring was a complete puzzle to Galileo, most of all when the planet
reached a position where the plane of the ring was in line with the
earth, and the ring disappeared (December 4th, 1612). It was not until
1656 that Huyghens, in his small pamphlet _De Saturni Luna Observatio
Nova_, was able to suggest in a cypher the ring form; and in 1659, in
his Systema Saturnium, he gave his reasons and translated the cypher:
&quot;The planet is surrounded by a slender flat ring, everywhere distinct
from its surface, and inclined to the ecliptic.&quot; This theory explained
all the phases of the ring which had puzzled others. This ring was
then, and has remained ever since, a unique structure.  We in this age
have got accustomed to it. But Huyghens's discovery was received with
amazement.

In 1675 Cassini found the ring to be double, the concentric rings
being separated by a black band--a fact which was placed beyond
dispute by Herschel, who also found that the thickness of the ring
subtends an angle less than 0&quot;.3. Shröter estimated its thickness at
500 miles.

Many speculations have been advanced to explain the origin and
constitution of the ring. De Sejour said [6] that it was thrown off
from Saturn's equator as a liquid ring, and afterwards solidified. He
noticed that the outside would have a greater velocity, and be less
attracted to the planet, than the inner parts, and that equilibrium
would be impossible; so he supposed it to have solidified into a
number of concentric rings, the exterior ones having the least
velocity.

Clerk Maxwell, in the Adams prize essay, gave a physico-mathematical
demonstration that the rings must be composed of meteoritic matter
like gravel. Even so, there must be collisions absorbing the energy of
rotation, and tending to make the rings eventually fall into the
planet. The slower motion of the external parts has been proved by the
spectroscope in Keeler's hands, 1895.

Saturn has perhaps received more than its share of attention owing to
these rings. This led to other discoveries. Huyghens in 1655, and
J. D. Cassini in 1671, discovered the sixth and eighth satellites
(Titan and Japetus). Cassini lost his satellite, and in searching for
it found Rhea (the fifth) in 1672, besides his old friend, whom he
lost again. He added the third and fourth in 1684 (Tethys and
Dione). The first and second (Mimas and Encelades) were added by
Herschel in 1789, and the seventh (Hyperion) simultaneously by Lassel
and Bond in 1848. The ninth (Phoebe) was found on photographs, by
Pickering in 1898, with retrograde motion; and he has lately added a
tenth.

The occasional disappearance of Cassini's Japetus was found on
investigation to be due to the same causes as that of Jupiter's fourth
satellite, and proves that it always turns the same face to the
planet.

_Uranus and Neptune_.--The splendid discoveries of Uranus and two
satellites by Sir William Herschel in 1787, and of Neptune by Adams
and Le Verrier in 1846, have been already described. Lassel added two
more satellites to Uranus in 1851, and found Neptune's satellite in
1846. All of the satellites of Uranus have retrograde motion, and
their orbits are inclined about 80° to the ecliptic.

The spectroscope has shown the existence of an absorbing atmosphere on
Jupiter and Saturn, and there are suspicions that they partake
something of the character of the sun, and emit some light besides
reflecting solar light. On both planets some absorption lines seem to
agree with the aqueous vapour lines of our own atmosphere; while one,
which is a strong band in the red common to both planets, seems to
agree with a line in the spectrum of some reddish stars.

Uranus and Neptune are difficult to observe spectroscopically, but
appear to have peculiar spectra agreeing together. Sometimes Uranus
shows Frauenhofer lines, indicating reflected solar light. But
generally these are not seen, and six broad bands of absorption
appear.  One is the F. of hydrogen; another is the red-star line of
Jupiter and Saturn. Neptune is a very difficult object for the
spectroscope.

Quite lately [7] P. Lowell has announced that V. M.  Slipher, at
Flagstaff Observatory, succeeded in 1907 in rendering some plates
sensitive far into the red. A reproduction is given of photographed
spectra of the four outermost planets, showing (1) a great number of
new lines and bands; (2) intensification of hydrogen F.  and C. lines;
(3) a steady increase of effects (1) and (2) as we pass from Jupiter
and Saturn to Uranus, and a still greater increase in Neptune.

_Asteroids_.--The discovery of these new planets has been
described. At the beginning of the last century it was an immense
triumph to catch a new one. Since photography was called into the
service by Wolf, they have been caught every year in shoals. It is
like the difference between sea fishing with the line and using a
steam trawler. In the 1908 almanacs nearly seven hundred asteroids are
included. The computation of their perturbations and ephemerides by
Euler's and Lagrange's method of variable elements became so laborious
that Encke devised a special process for these, which can be applied
to many other disturbed orbits. [8]

When a photograph is taken of a region of the heavens including an
asteroid, the stars are photographed as points because the telescope
is made to follow their motion; but the asteroids, by their proper
motion, appear as short lines.

The discovery of Eros and the photographic attack upon its path have
been described in their relation to finding the sun's distance.

A group of four asteroids has lately been found, with a mean distance
and period equal to that of Jupiter. To three of these masculine names
have been given--Hector, Patroclus, Achilles; the other has not yet
been named.


===FOOTNOTES:===

[1] Langrenus (van Langren), F. Selenographia sive lumina austriae
philippica; Bruxelles, 1645.

[2] _Astr. Nach._, 2,944.

[3] _Acad. des Sc._, Paris; _C.R._, lxxxiii., 1876.

[4] _Mem. Spettr. Ital._, xi., p. 28.

[5] _R. S. Phil. Trans_., No. 1.

[6] Grant's _Hist. Ph. Ast_., p. 267.

[7] _Nature_, November 12th, 1908.

[8] _Ast. Nach_., Nos. 791, 792, 814, translated by G. B. Airy.
_Naut. Alm_., Appendix, 1856.




*[[History of Astronomy Forbes 1909 14 Comets and Meteors|14. COMETS AND METEORS]]


==Contents==

:[[History_of_Astronomy_Forbes_1909|PREFACE]]

:BOOK I. THE GEOMETRICAL PERIOD

*[[History of Astronomy Forbes 1909 01 Primitive Astronomy and Astrology|1. PRIMITIVE ASTRONOMY AND ASTROLOGY]]

*[[History of Astronomy Forbes 1909 02 Ancient Astronomy - Chinese and Chaldaens|2. ANCIENT ASTRONOMY--CHINESE AND CHALDÆANS]]

*[[History of Astronomy Forbes 1909 03 Ancient Greek Astronomy|3. ANCIENT GREEK ASTRONOMY]]

*[[History of Astronomy Forbes 1909 04 Reign of Epicycles - Ptolemy to Copernicus|4. THE REIGN OF EPICYCLES--FROM PTOLEMY TO COPERNICUS]]


:BOOK II. THE DYNAMICAL PERIOD

*[[History of Astronomy Forbes 1909 05 Discovery of the true solar system - Brahae to Kepler|5. DISCOVERY OF THE TRUE SOLAR SYSTEM--TYCHO BRAHE--KEPLER]]

*[[History of Astronomy Forbes 1909 06 Galileo and the telescope - Notions of gravity by Horrocks|6. GALILEO AND THE TELESCOPE--NOTIONS OF GRAVITY BY HORROCKS, ETC.]]

*[[History of Astronomy Forbes 1909 07 Newton Law of Universal Gravitation|7. SIR ISAAC NEWTON--LAW OF UNIVERSAL GRAVITATION]]

*[[History of Astronomy Forbes 1909 08 Newton successors Halley Euler Lagrange Laplace|8. NEWTON'S SUCCESSORS--HALLEY, EULER, LAGRANGE, LAPLACE, ETC.]]

*[[History of Astronomy Forbes 1909 09 Discovery of new planets Herschel Piazzi Adams Le Verrier|9. DISCOVERY OF NEW PLANETS--HERSCHEL, PIAZZI, ADAMS, AND LE VERRIER]]


:BOOK III. OBSERVATION

*[[History of Astronomy Forbes 1909 10 Instruments of precision - size of the solar system|10. INSTRUMENTS OF PRECISION--SIZE OF THE SOLAR SYSTEM]]

*[[History of Astronomy Forbes 1909 11 History of the telescope spectroscope|11. HISTORY OF THE TELESCOPE--SPECTROSCOPE]]

:BOOK IV. THE PHYSICAL PERIOD

*[[History of Astronomy Forbes 1909 12 The Sun|12. THE SUN]]

*[[History of Astronomy Forbes 1909 13 The Moon and Planets|13. THE MOON AND PLANETS]]

*[[History of Astronomy Forbes 1909 14 Comets and Meteors|14. COMETS AND METEORS]]

*[[History of Astronomy Forbes 1909 15 Stars and Nebulae|15. THE STARS AND NEBULÆ]]

*[[History of Astronomy Forbes 1909 16 Index|INDEX]]





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