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The Gnomon - Sundial

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Bianca
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« Reply #30 on: December 16, 2007, 08:29:33 am »








U





umbra: the central, darkest portion of a shadow, i.e. the region which does not receive direct rays from any part of a distributed light source (e.g. the Sun).

umbra recta: Latin for "upright shadow", it is the label often found on the cotangent scale of altitudes < 45º on a shadow square.

umbra versa: Latin for "reverse shadow", it is the label often found on the tangent scale of altitudes > 45º on a shadow square.

Universal Time (UT): see time (types of).
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« Reply #31 on: December 16, 2007, 08:30:34 am »








V





vernal {spring} equinox: see equinoxes.

verdigris: (pron. ver-de-gree) the green patination found on weathered brass and copper.

vernier: a small moveable scale for obtaining fractional parts of the subdivisions of a fixed scale. Invented by Pierre Vernier in 1631 (published Brussels, 1638). For circular scales, a Type A vernier has a central zero. After about 1780, sextants commonly had a Type B vernier with the zero on the right of the scale. European instruments often have the very similar nonius.

vertical angle: the angle from the zenith to the horarius circle passing through the sun, measured along the prime vertical (the E-W vertical circle). It is one of the ptolematic co-ordinates.

vertical plane: see principal plane.

volvelle: (pron. vol-vel) an old device consisting of one or more movable circles surrounded by other graduated or figured circles. It is used for showing the rising and setting times of the Sun and Moon, the state of the (marine) tides etc. A ~ has sometimes been included around the rod-gnomon of equiangular dials.
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« Reply #32 on: December 16, 2007, 08:31:36 am »








W





wedging out: (or canting out) placing wedges between a dial plate and its mounting surface. (a) for a horizontal dial: to compensate for moving a dial to a different latitude from the one it was designed for, (b) for a vertical dial: to ensure that it faces a cardinal point of the compass (usually S).

West: the point on the horizon 90º (measured anti-clockwise) from the north point. The Sun appears to set at the west point at the equinoxes.

winter solstice: see solstices.
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« Reply #33 on: December 16, 2007, 08:32:38 am »








X



Y





year: the time that it takes the Earth to make one orbit of the sun. The tropical year is the interval in which the mean ecliptic longitude of the Sun increases by 360º. This is the version of the year used in normal calendars and has a length of 365.24219 days. Other versions of a year (e.g. the sidereal, anomalistic and Julian years) have differences of about a hundredth of a day to this figure.

Yule: an ancient Celtic festival held on or around the 21st of December, celebrating the winter solstice. It is one of the cross-quarter days, and has now become synonymous with Christmas.
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« Reply #34 on: December 16, 2007, 08:33:48 am »







Z





zenith: (astronomical) the point on the celestial sphere vertically above the observer. In everyday parlance, ~ usually implies the highest point. This gives rise to confusion as the mid-day Sun is often described as its ~, irrespective of the latitude.

zenith distance (or zenith angle): [z] the complement of the altitude i.e. (90º - a)

zodiac: an imaginary band, centred on the ecliptic, across the celestial sphere and about 16º wide, in which the Sun, Moon and the planets Mercury, Venus, Mars, Jupiter and Saturn are always located. The band is divided in 12 intervals of 30º , each named (the Signs of the Zodiac) after the constellation of stars which it contains. The sun's ecliptic longitude may be measured against this scale. The names (and/or signs) of the constellations are given in Appendix I and are often used in sundials, instead of the date, to specify declination lines etc. Because of the effects of precession over the period of 2,300 years since the constellations were first named, the signs of the zodiac have slipped by a whole sign, i.e. at the vernal equinox (defined as the first point of Aries), the Sun is actually in the constellation of Pisces.

Zonwvlak: a major suite of computer programs for calculating sundial lines. Written by Fer de Vries (Netherlands) it is available at www/iaehv.nl/users/ferdv. The name is short for zonnewijzer (sundial) vlak (plane).


http://www.sundialsoc.org.uk/glossary/alpha.htm
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« Reply #35 on: December 16, 2007, 10:02:18 am »











                                                                    Scaphion





From Wikipedia,
the free encyclopedia



The scaphion was a portable gnomon, developed by hellenistic astronomers.

They put a gnomon in a metallic hemisphere, which was divided inside in concentric circles.

They could use it for determination of geographical coordinates from measured solar altitudes. Using this measuring instrument Eratosthenes of Cyrene (ca. 220 BC) had measured the length of Earth's meridian, and after that they used this instrument to survey smaller regions as well.








                                                                          Scaphe











From Wikipedia,
the free encyclopedia



The scaphe (or skaphe, also scaphium or scaphion) was a sundial said to have been invented by Aristarchus
(3rd century BC).

It consisted of a hemispherical bowl which had a vertical gnomon placed inside it, with the top of the gnomon
level with the edge of the bowl. Twelve gradations inscribed perpendicular to the hemisphere indicated the hour
of the day.

Using this measuring instrument Eratosthenes of Cyrene (ca. 220 BC) measured the length of Earth's meridian.
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« Reply #36 on: December 16, 2007, 10:06:30 am »








                                                               Early Sundials





In antiquity, day refers primarily to the hours of available sunlight. When we refer to the day in its abstract form, we mean both the daytime and nighttime. But before our modern methods of timekeeping, the overall day was seen as two very different events. The clearest markings for each day hinged on the rising and the setting of the sun. 

Naturally, the earliest timepieces relied on the sun as their referent. Or, to be more precise, the earliest timepieces relied on the shadows caused by the sun. At dawn, with the sun rising in the east, long shadows form which fall to the west. As the sun rises to its position overhead at noon, the shadows shorten. With the sun�s descent toward dusk, shadows begin to lengthen again, this time pointing to the east. The movement and length of shadows, as the sun rises and falls, is the basis for many early sundials.

The first sundial was most likely a stick in the ground. If one were to stand facing the stick (with their back to the south), the first, long shadow of day would appear on the ground to the left. While this gnomon (Greek for to know) was good for marking the dawn, noon, and sunset, it did little to "tell time," as we are used to it today. As sundials spread from Egypt to Greece to Rome, additional refinements and markings were added, in the attempt to make a more accurate timepiece.
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« Reply #37 on: December 16, 2007, 10:17:48 am »



Sketch of early Egyptian sundial, with shadow. Note that the markings separating each "hour" grow closer together as the approach the bar of the sundial.

 

Image scanned from Jo Ellen Barnett's "Time's Pendulum."

Physical reconstruction of an Egyptian sundial. Markings are harder to see, but are clearly spaced at different intervals.






Early Egyptian Sundial





One such refinement was the marking of hours during the day. A sundial found in Egypt (circ. 1500 BC, in the reign of Thutmose III) held markings that divided the day in half (dawn to noon, noon to sunset). This early sundial was in the shape of a T, with the bar of the sundial slightly raised. Along the length of the T were six lines. By setting this sundial so that the bar of the T faces the East, a shadow would be cast onto the length of the sundial. As the sun moved to its position at noon, the shadow would move closer to the bar of the T.

In order to measure the time from noon until sunset, the sundial had to be adjusted at noon. This adjustment meant that someone would have to "turn" the sundial so that the bar of the T faced to the West. As the sun set, the procession of the shadow reversed itself, moving father away from the bar of the T.






One interesting thing to note is that the markings on this Egyptian sundial were not of equal length. As the markings get closer to the bar of the T, the space between each marking lessens. This adjustment in the space between the markings allows for the varying lengths of the sundial�s shadow as the sun makes its way across the sky. 

 

Image scanned from Eric Bruton's "The History of Clocks and Watches."
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« Reply #38 on: December 16, 2007, 10:32:42 am »








Unequal Hours



As the Greeks and Romans began to use sundials, their construction and design became more elaborate. At this point, it seems appropriate to return to the mention of abstraction in the first paragraph. To the Greeks and Romans, their day consisted of the hours that the sun was in the sky, when the sun was visible.

The difference between day and daylight caused many problems in the construction of sundials. For any given day (except the equinoxes, when the hours of day and night are equal), the amount of time that the sun is "up" varies. A full period of daylight in the summer is longer than a full period of sunlight in the wintertime.

While sundials divided the day into twelve hours, these were not hours as we know them today. Instead, the "hours" were of unequal length. Were these sundials to be accurate, some adjustment had to be made for the varying amounts of daylight on any given day.





Hemispherium / Hemicyclium



The movement from a horizontal sundial (as in the gnomon and Egyptian example) to that of a vertical sundial aided in this adjustment. In his De architectura, Vitruvius (circa 25 BC) mentions some 13 variations on sundials, while also noting the inventor of each type:

the hemicyclium of Berossos, the scaphion of Aristarchus
of Samos, the disc in plano of Aristarchus, the arachnid of
Eudoxius of Cnidus, the plinth of Scopus of Syracuse, the
prosta istoroumena of Parmenion, the pro pauclinia of
Theodosus, the pelecinon of Patrocles, the cone of
Dionysidorus, the quiver of Apollonius.

(Rohr)
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« Reply #39 on: December 16, 2007, 10:35:13 am »



The hemispherium employed a vertical gnomon that traced the path of the sun. The marks along the interior of the hemispherium mirror the sun's path. This particular sundial would be placed so that it was facing North (the top right hand corner of the screen).

Image scanned from René Rohr's "Sundials: History, Theory, and Practice."






Looking at Berossos' hemispherium, and the modifications which led to his hemicyclum, highlights this transition from a horizontal gnomon to that of a vertical one. The hemispherium was a hemisphere sunk into a block of stone, with a vertical gnomon at the bottom. The interior markings (arcs which were to represent the sun�s arcs as it traveled across the sky) "corresponded to the position of the sun at the equinox and the solstices" (Rohr). The hemispherium was also one of the first sundials which was portable, its construction meant to be as light as possible.






The hemicyclium, a streamlined version of the hemispherium. Note that the vertical gnomon has been replaced by a horizontal one. In order to make the sundial more portable, much of the "extra" area has been cut away from the original hemispherium.

Image scanned from René Rohr's "Sundials: History, Theory, and Practice."


http://omega.cohums.ohio-state.edu/classes/1999-2000/AU/cla506.F99.mlm/student_projects/jung/sun.htm
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« Reply #40 on: December 16, 2007, 04:48:23 pm »



Replica

Anglo-Saxon portable sundial





Possibly owned by a monk at Canterbury Cathedral, it would have been used to recognise the time for the Morning, Midday and Evening prayers.

The original sundial is kept at Canterbury Catherdral.

It looks small, and weird, but is like an old type of wristwatch!

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« Reply #41 on: December 16, 2007, 04:58:11 pm »



Pocket Saxon Sundial







This watch is one of the only Benedictine sundials of its size in known existence. This Anglo-Saxon portable sundial is the oldest watch in the English speaking world. It's a pendant, a fob, a personal timepiece, a talisman and a real collector's treasure.

It’s thought to have been made by St. Dunstan who was a silversmith based at the Cathedral. It was found in the cloisters of the Cathedral in 1948 during excavations, but how it got there is a mystery as St. Dunstan lived in a totally different part of the old cathedral, which burnt down.

Presenter Jonathan Foyle says: "This is an object that totally changed the way we keep track of time. It’s extremely rare, one of the earliest examples, and very delicate...."

Where can it be found? At Canterbury Cathedral


http://www.bbc.co.uk/history/programmes/peoplesmuseum/week1_15.shtml
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« Reply #42 on: December 16, 2007, 05:23:53 pm »




"Navicula de Venetiis" - 1524.






                                Ship-shaped sundial, once called "Navicula de Venetiis" - 1524.




Oronce Fine
(Sixteenth century)

Donated by Piero Portaluppi, 1978 
 
 
This is the most prestigious item in the collection donated to the museum by Pier Portaluppi.

Only four examples of this curiously shaped type of sundial are known; among these, this sundial is the better documented, the most perfectly conserved, and the only one in ivory instead of brass.

The function of this object was not only to measure time, but it was also used as an astronomical instrument and to determine latitude.

The shaft is fixed between the two plates that compose the hull in order to guarantee a certain degree of mobility, which in turn allows for the shaft to be positioned according to the season.

The latitude scale is placed on the shaft, and was read by means of a plumb-line attached to the cursor, now missing. The solar hours table is inscribed on the hull, in which the central line (number 6) corresponds to midday. Also on the hull are a shadow-dial and a zodiac scale.

In a horizontal section perpendicular to the shaft, we find the symbols of the zodiac and the date in which the sun enters each of them, as well as the signature of the maker Oronce Fine, also famous for writing a literary work on sundials.

The small silver cursor on the shaft bears on the back the three lilies of the French royal family, and on the front the salamander.

This was the personal stem of Francis I, king of France from 1515 to 1547, and it is therefore likely to indicate a prestigious commission of this extraordinary object from the king himself.

The name 'Navicula de Venetiis' highlights the likeness of this sundial with the shape of a kind of sailing ship characteristic of the Venetian fleet that dominated the Mediterranean in the 16th century. Although this might suggest that this object was created within the area of Venetian dominance, it is known that ship-shaped sundials were used as early as the 13th century.

Its particular and striking appearance is the result of the combination between scientific functionality and a creative character with aesthetic and artistic aims.


http://www.museopoldipezzoli.it/PP_inglese/museo/collezioni/orologi/orologi.htm
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« Reply #43 on: December 16, 2007, 05:57:43 pm »











                                    T H E   A N G L O - S A X O N   S U N D I A L





Anglo-Saxon ~ {sometimes just Saxon ~}: a sundial from the Anglo-Saxon period (c 650 – 1050 AD); designed to show unequal hours, or the basic tides, with a horizontal gnomon. Similar to the mass dials which superseded it, a Saxon ~ shows much higher levels of craftsmanship and is often finely decorated. Also, it is invariably engraved in a separate (circular or rectangular) stone, not into a pre-existing wall. Saxon dials are often taken to be the precursors to the later scientific dials. In the early part of the period the semicircle was divided by five lines into four segments. During the latter part of the period it was subdivided into eight or twelve segments and the dial sometimes carried an inscription in Old English. Throughout the period the principal lines had a cross bar near the perimeter giving the appearance of Latin crosses. See Appendix II for the basic time periods shown on the dial.
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« Reply #44 on: December 16, 2007, 05:59:48 pm »








Clynnog Fawr, St Beuno's Church

Anglo-Saxon Sundial





N 53° 01.316 W 004° 21.962
30U E 408373 N 5875582

Quick Description: Old Anglo-Saxon sundial in cemetery of
St Beuno's church, Clynnog fawr

Location: United Kingdom
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