| Whitehurst & Son sundial | |
|---|---|
 Sundial in Derby Museum | |
| Material | bronze |
| Created | 1812 |
| Present location | Derby Museum, England |
The Whitehurst & Son sundial was produced in Derby in 1812 by the nephew of John Whitehurst. [1] It is a fine example of a precision sundial telling local apparent time with a scale to convert this to local mean time, and is accurate to the nearest minute. The sundial is now housed in the Derby Museum and Art Gallery.
The Whitehurst family was known in Derby as eminent mechanics. John Whitehurst (1713–1788) was born in Congleton, but came to Derby where he entered business as a watch and clock maker. He moved to London when appointed to the post of Inspector of Weights. His nephew continued the business under the name of Whitehurst & Son. The family business was known for their turret clocks. [1]
Sundial construction is based on understanding the geometry of the Solar System, and particularly how the Sun will cast a shadow onto a flat surface, in this case a horizontal surface. Each day over a yearly cycle the shadow will be different from the day before, and the shadow is specific to the location of the dial, particularly its latitude. The dial is designed to tell local apparent time so the longitude is not significant. By this we mean that noon will be at the point when the Sun is highest in sky and due south, standard time would be when the Sun was due south at another point such as the Royal Observatory at Greenwich. Derby is at 1° 28′ 46.2″ West of Greenwich, [2] so the Sun reaches noon approximately 5m 52.05s later. The other point to consider when telling the time is that as the Earth moves around the Sun in a slight ellipse, day length varies slightly giving a cumulative difference from the average, of up to 16 minutes in November and February, [3] this, with another correction, is known as the equation of time and was really irrelevant until people started comparing sundials with mechanical clocks, which must either ignore the consequences or be re balanced each day to make them correspond with the natural cycle. They measure average time or local mean time. Railways timetabling demanded a fixed noon and fixed day leading to the adoption of Greenwich Mean Time. The local Midland Railway had adopted Greenwich Mean Time by January 1848. [4]
This particular bronze sundial is marked "Whitehurst and Son / Derby / 1812" and is thought to have been made for George Benson Strutt (who was the younger brother of the cotton spinner William Strutt), for his home at Bridge Hill House, in Belper. [5] [6] This has the precise location of 53° 1′ 49.08″ north, and 1° 29′ 26.88″ west of Greenwich, [7] [ full citation needed ] which is slightly different from those of Derby, at 52° 55′ 00″ north, [2] a difference of 6′ 49″ . The Longitudes are almost identical, with only a 2 second time difference.
This dial is stoutly made, with a thick gnomon, one edge of gnomon is the style [8] which throws the shadow before noon, and the other edge is the style which throws the shadow after noon. The dial plate is not made up of one complete circle, but two semicircles separated by the thickness of the style. In this design of sundial, the angle of the style to the dial plate is exactly the same as latitude, which is 53° 1′ 49″ the latitude of the Bridge Hill House. The dial plate will be perfectly horizontal, slight adjustments for latitude can be made were the dial moved by shimmying up the dial plate by the change in degree, from the horizontal. [9] As the dial is now 0° 6′ 49″ , or about one-tenth of a degree south - the nose of the gnomon needs to be raised by that amount.
In the horizontal sundial (also called a garden sundial), the plane that receives the shadow is aligned horizontally, rather than being perpendicular to the style as in the equatorial dial. [10] [11] [12] Hence, the line of shadow does not rotate uniformly on the dial face; rather, the hour lines are spaced according to a calculation. [13] [14] The dial plate is precision engraved, the hour lines were calculated using the formula:
where λ is the sundial's geographical latitude (and the angle that the style rises above the northern horizon), θ is the angle between a given hour-line and the noon hour-line (which always points towards true North) on the plane, and t is the number of hours before or after noon.
For each of the hours 1 to 6, the formula is calculated. For instance, for Belper, England, at 3 hours after noon, we substitute the numbers 53°.03 (Belper's latitude) and 3 into the formula which generates these results
| One hour | 12°.08 |
| Two hours | 24°.76 |
| Three hours | 38°.62 |
| Four hours | 54°.15 |
| Five hours | 71°.46 |
| Six hours | 90°.00 |
The hours before noon are exactly the same, the dial is symmetrical, the other lines are mirror images of those above: 2 way symmetry. In the same manner, the half hours and the minute lines would be calculated. As noon lies exactly on the north–south line, and not 5′ 54″ to one side we can tell that this dial is telling Belper time and not Greenwich time. [15]
Significantly, on the plate we can see a pair of scales that help the observer make the equation of time correction: One scale gives the date in months and days while alongside of it another engraved with the minutes on that day that the watch would be running faster or slower. Here it is labeled "Watch Slower, Watch Faster. The 15th April is one day when no conversion needs to be made." [16] This dial can be used both to read solar time shown by sundials and also the mean time that is displayed by clocks, [17] with the practical purpose that observers can use the dial to calibrate their pocket watches. Watches would not always run true when the sundial was made in 1812; by 1820 watch manufacture had improved: The lever escapement was universally adopted and frequent calibration was no longer needed.
Another Whitehurst sundial dated around 1800 sold for £1850 in 2005 in Derby. [18]
A sundial is a horological device that tells the time of day when direct sunlight shines by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate and a gnomon, which casts a shadow onto the dial. As the Sun appears to move through the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude.
A diptych is any object with two flat plates which form a pair, often attached by hinge. For example, the standard notebook and school exercise book of the ancient world was a diptych consisting of a pair of such plates that contained a recessed space filled with wax. Writing was accomplished by scratching the wax surface with a stylus. When the notes were no longer needed, the wax could be slightly heated and then smoothed to allow reuse. Ordinary versions had wooden frames, but more luxurious diptychs were crafted with more expensive materials.
A gnomon is the part of a sundial that casts a shadow. The term is used for a variety of purposes in mathematics and other fields.
The scaphe was a sundial said to have been invented by Aristarchus of Samos. There are no original works still in existence by Aristarchus, but the adjacent picture is an image of what it might have looked like; only his would have been made of stone. 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.
The equation of time describes the discrepancy between two kinds of solar time. The word equation is used in the medieval sense of "reconciliation of a difference". The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion along the celestial equator. Apparent solar time can be obtained by measurement of the current position of the Sun, as indicated by a sundial. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would have a mean of zero.
Analemmatic sundials are a type of horizontal sundial that has a vertical gnomon and hour markers positioned in an elliptical pattern. The gnomon is not fixed and must change position daily to accurately indicate time of day. Hence there are no hour lines on the dial and the time of day is read only on the ellipse. As with most sundials, analemmatic sundials mark solar time rather than clock time.
François-Lamathe Dom Bédos de Celles de Salelles was a Benedictine monk best known for being a master pipe organ builder.
The ancient Egyptians were one of the first cultures to widely divide days into generally agreed-upon equal parts, using early timekeeping devices such as sundials, shadow clocks, and merkhets . Obelisks were also used by reading the shadow that they make. The clock was split into daytime and nighttime, and then into smaller hours.
Astronomical rings, also known as Gemma's rings, are an early astronomical instrument. The instrument consists of three rings, representing the celestial equator, declination, and the meridian.
A sundial is a device that indicates time by using a light spot or shadow cast by the position of the Sun on a reference scale. As the Earth turns on its polar axis, the sun appears to cross the sky from east to west, rising at sun-rise from beneath the horizon to a zenith at mid-day and falling again behind the horizon at sunset. Both the azimuth (direction) and the altitude (height) can be used to create time measuring devices. Sundials have been invented independently in every major culture and became more accurate and sophisticated as the culture developed.
The Noon mark is a type of sundial that at its simplest is a vertical line on a south facing wall or a north-south line on a horizontal pavement. When the shadow of a point crosses the line it will be midday. Noon in local standard time is defined as when the sun is overhead, however clocks and watches use mean time which varies from standard time by a few minutes each day. The difference is calculated using the equation of time and this can be shown on the noon mark by drawing an analemma, or using a correction table.
A bifilar dial is a type of sundial invented by the German mathematician Hugo Michnik in 1922. It has two non-touching threads parallel to the dial. Usually the second thread is orthogonal-(perpendicular) to the first. The intersection of the two threads' shadows gives the local apparent time.
Dialing scales are used to lay out the face of a sundial geometrically. They were proposed by Samuel Foster in 1638, and produced by George Serle and Anthony Thompson in 1658 on a ruler. There are two scales: the latitude scale and the hour scale. They can be used to draw all gnomonic dials – and reverse engineer existing dials to discover their original intended location.
A London dial in the broadest sense can mean any sundial that is set for 51°30′ N, but more specifically refers to a engraved brass horizontal sundial with a distinctive design. London dials were originally engraved by scientific instrument makers. The trade was heavily protected by the system of craft guilds.
A tide dial, also known as a Mass or scratch dial, is a sundial marked with the canonical hours rather than or in addition to the standard hours of daylight. Such sundials were particularly common between the 7th and 14th centuries in Europe, at which point they began to be replaced by mechanical clocks. There are more than 3,000 surviving tide dials in England and at least 1,500 in France.
A schema for horizontal dials is a set of instructions used to construct horizontal sundials using compass and straightedge construction techniques, which were widely used in Europe from the late fifteenth century to the late nineteenth century. The common horizontal sundial is a geometric projection of an equatorial sundial onto a horizontal plane.
The Sunquest Sundial is a sundial designed by Richard L. Schmoyer in the 1950s. Adjustable for latitude and longitude, the Sunquest's gnomon automatically corrects for the equation of time allowing it to tell clock time.
Vertical declining dials are sundials that indicate local apparent time. Vertical south dials are a special case: as are vertical north, vertical east and vertical west dials. The word declining means that the wall is offset from one of these 4 cardinal points. There are dials that are not vertical, and these are called reclining dials.
The Sundial in Taganrog was set in 1833. It is situated in Grecheskaya Street, close to the Depaldo Stairs. It is officially declared an object of cultural heritage of Russia of Regional significance.
A Butterfield dial is a portable horizontal sundial designed to be folded flat and used in latitudes between 35° and 60°. It was named after the English gnomonist Michael Butterfield, who was active in Paris around 1690.
Derby Whitehurst
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| Obsolete standards | ||
| Time in physics | ||
| Horology | ||
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| Other units of time | ||
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