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The planets have been studied with optical aid for hundreds of years, and now those worlds are being scrutinised at close range by space probes. However, the mechanics of the Solar System, including Kepler’s discovery of elliptical planetary orbits, were worked out with the naked eye before the telescope was invented. What would we know about the Solar System without that device? We would probably have discovered the sunspot cycle, the solar atmosphere, the asteroid belt, the moons of Jupiter, and the seventh planet. The crescent phase of Venus might have been detected, implying that that planet was a sphere, and it might have been suspected that there was something odd about the appearance of Saturn.

The Sun
Looking at the unscreened Sun, even with the naked eye, can cause eye damage.
Sunspots
* The Greek Theophrastus, in the 4th century B.C., in his book on weather prediction (
De Signis Tempestatum), described black marks or spots on the Sun as a sign of rain. A Chinese Encyclopaedia studied by Alexander Hosie in 1877 recorded ‘black shadows’ on the Sun in 28 B.C. A black spot was seen on the Sun, March 15, A.D. 807, according to Adelmus, and remained visible for eight days. Sunspots were also seen from May 28 to August 26, A.D. 840.

* The first known drawing of sunspots appears in the Chronicles of John of Worcester. On December 8, 1128, ’two black circles’ appeared in the Sun. The drawing shows two sunspots with what appear to be umbrae and penumbrae. If such details were actually visible to the naked eye, the spots must have been exceptionally, if not unprecedentedly, large. Two enormous sunspots seen on August 30, 1839 seem to show that this might sometimes be possible. The largest was 186,000 miles long, with an area of 25,000,000,000 square miles.
See here

* E Walter Maunder wrote in 1902; “Had it occurred to the classical and medieval astronomers to watch the sun systematically...they would not only have detected the existence of spots on its disc, but have determined as certainly as we know it to-day the period of the sunspot cycle, and the value to us of that information would have been incalculable. More than that, it would have been possible for them, from a long series of observations, to have fixed the solar rotation period fairly exactly...”

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The Corona
* The corona, the outer atmosphere of the Sun, is only visible to the unaided eye during a total solar eclipse. Plutarch (c. A.D. 46-120) describes ‘a certain brightness’ round the edge of the Moon during totality, ‘which does not allow the shadow to be deep and absolute’. The Byzantine historian Leo Diaconus recorded that during the total eclipse of December 22, A.D. 968, “...it was possible to see the disc of the Sun, dull and unlit, and a dim and feeble glow like a narrow band shining in a circle round the edge of the disc.” Clavius, during the eclipse of April 9, 1567, saw the corona, but thought it was part of an imperfectly concealed Sun; Kepler showed by calculation that this was not the case, and supposed that it was part of a lunar atmosphere.

* In 1851, G. P. Bond, in the Alps, attempted to catch sight of the corona and ‘red flames’ (prominences) outside an eclipse by concealing the body of the Sun behind outcrops of a mountain. He was generally thwarted by bad weather, and saw nothing definite.
* The corona may have disappeared during the ‘Maunder Minimum’, a prolonged period of solar inactivity from about 1645 to 1715. Eclipse records of that period describe only a dull narrow, sometimes reddish, glow around the lunar limb. The corona seems to have reappeared by 1715, when, during the eclipse of May 3, when a white ring of light surrounded the Moon, together with ‘rays of much fainter light, in the form of a rectangular cross’.
See here.

Prominences
* The first known observation of ‘what could only have been eruptive prominences’ was made at Novgorod during the solar eclipse of May 1, 1185, when “the Sun became like a crescent of the Moon, from the horns of which a glow similar to that of red-hot charcoals was emanating.”

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Mercury

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* When Mercury is at its greatest east or west elongation, or the greatest apparent angular distance from the Sun as viewed from the Earth, “it shines with a rosy lustre equal to that of any star of the second magnitude, notwithstanding the strong twilight in which it is always enveloped.” (Edwin Dunkin)

* Transits of the Sun by Mercury are invisible to the naked eye.

Venus

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* Edmund Halley calculated Venus' maximum naked eye brightness in 1716, when many Londoners were alarmed by its appearance in the daytime. Venus reaches greatest brilliance, magnitude -4.4, 36 days before or after inferior conjunction, when its phase is that of a 5 day old Moon. Then Venus has an angular diameter of about 40 seconds of arc, the angular width of a circle 1 inch in diameter 430 feet away. The possibility of seeing the crescent phase of Venus with the naked eye has thus been rejected by most astronomers, although “it is just possible that an impression of elongation might be given, though this would show exceptionally keen sight.” The German mathematician Carl Friedrich Gauss is said to have claimed that his mother was able to see the crescent phase of Venus with the naked eye; on being shown the planet through an inverting telescope, she asked why the crescent was the wrong way round.

* Venus can be a brilliant object, and its appearance has often caused interest and alarm. The planet remains one of the most popular UFOs. Napoleon Bonaparte, while at a reception in Luxembourg, is supposed to have wondered why the assembled crowds were gazing at the sky rather than himself. One of his attendants said that a bright star had appeared in broad daylight, and was believed to be the star of the conqueror of Italy. The ‘star’ was of course Venus. Another story concerns an American train driver, who, when on a single-track line, reversed into a siding to allow Venus to pass. During World War II, the U.S. warship Houston fired 250 rounds into the sky in an attempt to shoot down Venus.

* James Bruce, in the Sudan, during October, 1772, said that for the whole month “the planet Venus appeared shining with undiminished light all day, in defiance of the brightest sun”.

* For transits of Venus,
see here.

Unknown Bodies
* Mysterious astronomical objects have occasionally been seen with the naked eye. Some of these may have been asteroids or comets which happened to pass close to the Earth.

* On November 19, 1762, Lichtenberg saw with the naked eye, soon after sunrise on a cold and foggy morning near Erlangen in Bavaria, a large round black spot before the Sun. The object, which was one-twelfth the apparent diameter of the Sun, moved from north to south, and took about 3 hours to cross the Sun. During the nineteenth century there were numerous telescopic observations of dark circular bodies crossing the Sun. These were attributed to a hypothetical planet within the orbit of Mercury, named by the famous French astronomer Leverrier ‘Vulcan”. Vulcan remained elusive, and was eventually deemed not to exist. What the observers actually observed remains a mystery, especially in a few instances when the objects were seen, as here, with the naked eye.

* At sunset on August 7, 1921, six observers at Mount Hamilton, California, including the Director of the Lick Observatory, Prof. W.W. Campbell, and Capt. Eddie V. Rickenbacker, the famous aviator, were watching as the Sun touched the horizon. They then saw, with unaided eyes, a “star-like object certainly brighter than Venus three degrees east, one degree south of (the) Sun”. The object was watched for several minutes before it set behind horizon clouds. It remained star-like when seen briefly through binoculars. Chambers said it was unquestionably a celestial object. Venus, Mercury, and the other planets were elsewhere. A few observers in Europe had earlier seen the same or a similar object, which was generally believed to be a comet “in a very narrow orbit, the whole of which might lie in daylight.” (E.E. Barnard)

The Moon
* In 1999, Dr. Philip Stooke of the University of Western Ontario said that a stone carving in a Neolithic tomb at Knowth, County Meath, Ireland, (consisting of several arcs and dots) was actually an image of the Moon, showing its markings. Plutarch quotes the poet Agesianax’s description of the Moon;
All round as fire she shines, but in her midst,/Bluer than cyanus, lo, a maiden’s eye,/her tender brow, her face in counterpart. Leonardo da Vinci drew a sketch of the Moon showing its dark areas in 1505.

* Just before and after its conjunction with the Sun (New Moon), the Moon is visible as a thin crescent. Naked-eye sightings of a crescent Moon less than 24 hours old are ‘not very common’. On 2 May, 1916, a waxing Moon 14½ hours old was observed from Scarborough in Yorkshire. An astronomer on a mountain-top in India claimed to have seen “the threadlike crescent of the waning moon” on 7 March, 1932, only six hours before New. However, the Moon was only three degrees from the Sun at that time, and a mistake of a day is suspected.

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* The naked-eye markings of the Moon, as well as forming the face of ‘the Man in the Moon’, have been variously seen as a crab, a girl reading a book, lovers kissing, a donkey, a lion, an antelope, a scorpion, and a the head of a lady.

* “It is a work of immense difficulty to draw correctly all the details that the naked eye reveals to us on the surface of our satellite....The white spots of Copernicus and Kepler are very well seen, Aristarchus with more difficulty, whilst Tycho with its brilliant surroundings, occupies an immense white surface; but...the ring itself is not seen. Guerike, Bonpland, Parry and Fra Mauro make up a bright island in the Mare Nubium.” (M.E.M. Antoniadi)

Lunar Phenomena
* ‘Transient lunar phenomena’, usually in the form of lights or vapours, have often been seen on the Moon by observers using telescopes, although their nature, and indeed their existence, remain undecided. Very rarely, similar phenomena have been seen with the naked eye.

* On St. Martin’s Eve, November 11, in the year 577 A.D., Gregory of Tours saw “a bright star…shining in the very centre of the Moon”. The Moon was full, and surrounded by a halo. The planet Jupiter was about 5 degrees away.

* The most well-known naked-eye lunar event is the ‘flaming torch’ which sprang up from the waxing crescent Moon on June 18, 1178, seen by five men near Canterbury in Kent and described by the chronicler Gervase of Canterbury. Debate continues as to whether this could have been a giant lunar meteorite impact or an effect in the earth’s atmosphere, and what the actual date (June or July) was.

* On November 26, 1540, a ’star’ was seen for two nights on the dark part of the Moon. By the second night, the ’star’ had split into two comet-like ‘flames’.

* In Harrison’s
Chronologie, it is recorded for the year 1587 that “A Sterre is sene in the body of the mone upon the (blank) of Marche, whereat many men merureiled (marvelled), & not without cause, for it stode directly between the pointes of her hornes, the mone being changed, not passing 5 or 6 daies before.” The Moon was new on February 27, 1587.

* In his book
An Account of Two Voyages to New-England (1674), the author Josselyn recorded, “In November (1668) appeared a Star between the horns of the Moon in the midst.” Joseph Ashbrook (Sky and Telescope, December 1964) said that both these events were probably close conjunctions of Venus with the Moon. The astronomical program Redshift calculates that on 30 November 1668, Venus was 0.8 of a degree from the Moon, and on 8 March 1587, Venus was 6.9 degrees from the Moon (this seems too far for the conjunction theory to be feasible.)

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Mars
* In August, 1719 Mars was simultaneously at its closest to Earth and almost at perihelion (closest to the Sun). The planet’s appearance “was so brilliant as to fill the minds of the vulgar with alarm, being mistaken for a new luminosity in the skies.”

* On August 5, 1918, at 10,000 feet, the crew of a flying boat over the North Sea pursued a red speck near the horizon for 30 minutes, believing it to be the exhaust of a Zeppelin. Eventually they decided that they were trying to fly to Mars.

* On August 27, 2003, Mars made its closest approach to Earth since 57617 B.C. As seen from London it was about 20 degrees above the southern horizon at midnight. To the naked eye it approached Venus in brightness, but was not noticeably coloured, although a hint of yellow was suspected. On the next night, Mars seemed brighter, and was distinctly yellow.

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Asteroids
* Ceres, the largest asteroid, can reach a magnitude of +6.7, just visible to the naked eye of a keen-sighted person in a dark sky.
Vesta, the brightest asteroid, can reach a magnitude of +5.4, visible to the naked eye in a dark sky.
Reports that the asteroid Iris (maximum magnitude +6.7) has been seen with the naked eye ‘are unverified’.
On April 13, 2029, during a very close approach to Earth, the asteroid Apophis is predicted to reach magnitude 3.4, easily visible to the naked eye.

Jupiter

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* The four largest satellites of Jupiter, discovered telescopically by Galileo, are all bright enough to be seen with the naked eye, (about mag. 5), but for the brilliance of Jupiter, which is about 800 times as bright. Lieutenant Elliot Brownlow of the Bengal Engineers, taking part in the Trigonometrical Survey of India in the mid 19th century “was able with the unaided eye to make a sketch of the positions of the moons of Jupiter which was immediately verified by another observer using a telescope.” Helen Sawyer Hogg said, “This is a real challenge for people with exceptional acuity of vision. It is by no means a trivial point. For if a person can really see the moons without a telescope, then the moons may very well have actually been discovered centuries or millennia ago, although probably the observers did not then realise what they were seeing.” Wrangel, a traveller in Russia, said that when in Siberia he once met a hunter who said, pointing to Jupiter, “I have just seen that large star swallow a small one and vomit it shortly afterwards.” Wrangel believed this to be a reference to the immersion and emersion of the 3rd satellite. E. W. Maunder thought this “a beautiful specimen of the traveller's tale.” as it takes that body over a week to pass from one elongation to another. But perhaps the hunter saw one satellite immerse and another emerge.

* During the appearance of a crimson aurora at Devizes (Wiltshire),on April 21, 1859, two satellites of Jupiter were seen with the naked eye.

Saturn

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* The rings of Saturn are ‘utterly beyond the unaided sight’, although it has been claimed that the planet sometimes appears elongated to the naked eye. At maximum brightness, when the rings are fully open as seen from Earth, Saturn has a magnitude of -0.3, brighter than any star except Sirius and Canopus.

Uranus
* The Reverend Thomas W. Webb, observing in the year 1863, described the naked-eye appearance of Uranus as “a very minute but sharp and steady point” of light.

Occultations
* Conjunctions of planets and occultations of planets by the Moon are well-known phenomena, but one of the rarest naked-eye events is a mutual occultation, when one planet actually passes in front of another. The only known occasion when this has been seen with the naked eye was on September 12, 1170, when the chronicler Gervase of Canterbury recorded that, “…two planets were seen in conjunction to such a degree that it appeared as though they had been one and the same star: but immediately they were separated from each other.” The planets were Mars and Jupiter. Calculations by Donald Olson, Russell Doescher and Steven Albers showed that Mars passed almost centrally across the disc of Jupiter. (This could only have been seen with a telescope.) The occultation was also observed in China. On May 28, 1737, the astronomer John Bevis saw Venus pass before Mercury; but he used a telescope.

Comets

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* Pliny the Elder, about 70 A.D., classified many types of comet, based on their naked-eye appearance, including
disciformis (disc), doliiformis erectus (an upright cask), equinus barbatus (a horse’s mane), lampadiformis (torch), barbatus (bearded), cornutus bicuspidatus (double-pointed), faculiformis lunatus (small torch), ensiformis (sword-like), hastiformis (spear-like) and monstriferus (horror-producing). A ‘bearded’ comet may refer to one with an ‘anti-tail’ pointing toward the Sun. At its brightest, the tail of McNaught’s Comet of 2007 resembled an equinus barbatus.

* Tycho Brahe, from naked-eye observations of the comet of 1577, demonstrated that it was much further away than the Moon, and in an orbit vastly different from those of the planets. The solid spheres in which the planets were supposed to move therefore could not exist. This comet was seen by Tycho before sunset.

* The German astronomer and mathematician Peter Apian noted that the tails of five comets observed by him between the years 1531 and 1539 always pointed away from the Sun as a prolongation of the radius vector (an imaginary line from the Sun’s centre to the head of the comet). Chinese astronomers had observed this with Halley’s Comet in A.D. 837.

Meteors

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* The first scientific investigation of ’shooting stars’ was made by Brandes and Benzenberg in 1798. They observed meteors from each end of a baseline 46,200 feet long. They concluded that meteors appeared at heights from 45 to over 140 miles, and moved at speeds of up to 25 miles a second.
For meteor fireballs or bolides,
see here.

Zodiacal Light
* The Zodiacal Light was well known to Arabic astronomers as the ‘tall twilight’, or ‘first dawn’. The earliest English description of the Zodiacal Light was given by Dr Childrey in his
Britannia Baconica of 1660. “...in February and for a little before and a little after that Month...about 6 in the Evening, when the Twilight has almost deserted the Horizon, you shall see a plainly discernible way of the Twilight striking up towards the Pleiades...and seeming almost to touch them.”
Rev. George Jones observed the Light in 1853-5, aboard the frigate Mississippi. He distinguished between the inner bright part (the 'Stronger Light”), and the fainter outlying part (the 'Diffuse Light'). Near the horizon he sometimes saw a region brighter than the Stronger Light, which he called the “Effulgence'. Outside the Diffuse Light he sometimes saw the sky 'slightly paled”, as if the Diffuse Light was continued in a much more attenuated degree. He found that the Moon was accompanied by a small glow resembling the Zodiacal Light.

* On exceptionally clear dark nights, the Zodiacal Light can sometimes be seen to extend in a band crossing the entire sky (the Zodiacal Band). At the point in the sky directly opposite the Sun, a brighter region forms the Gegenschein (‘Counterglow’). The gegenschein was first described by the French Jesuit astronomer Esprit Pézenas in 1730.

* The Zodiacal Light is an elusive phenomenon, and there are some curious observations regarding it. Marian said that the Light’s colour was less white than the Milky Way, and inclined somewhat ‘to yellow or red in the parts nearest the horizon’. This yellowish-red colour was also seen by Arago and others in 1843, who compared it to the tail of the great comet of that year. Derham, in 1707, also recorded a red tint. Marian and Cassini, in unconfirmed observations, recorded ‘momentary sparklings’ in the Light, and Humboldt described ‘sudden undulations which...traversed the luminous pyramid.’

THE SOLAR SYSTEM WITH THE NAKED EYE
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