The York Minster astronomical clock is a memorial to the airmen operating from bases in Yorkshire, Durham, and Northumberland who were killed in action during World War II, designed by R. Atkinson, and installed in York Minster in 1955. [1]
The York Minster astronomical clock is a memorial to the airmen operating from bases in Yorkshire, Durham, and Northumberland who were killed in action during World War II, designed by R. Atkinson, and installed in York Minster in 1955. [1]
The clock was first conceived in 1944 and designed by Robert d'Escourt Atkinson, chief assistant at the Royal Greenwich Observatory. Atkinson based the design on the appearance of the sun and stars from the viewpoint of a pilot flying over York. The mechanism and two dials were constructed by A.C.S. Westcott and Royal Greenwich Observatory craftsmen, and installed in the North Transept of York Minster in 1955.
Damage to the clock's mechanism was sustained during a fire, on 9 July 1984; after 10 years' work attempting repair, vergers ceased winding it, owing to its inaccuracies of time-keeping.[ citation needed ]
The front dial shows the locations of the Sun and certain navigational stars as would be seen by a pilot flying south above York Minster. To locate his position, a fixed circular plate, about 62 cm (slightly over 2 feet) in diameter and slightly dished, carries a ground plan of the Minster and surrounding markings representative of city walls, rivers, and principal roads. Since the edge of the plate represents the effective horizon, its centre identifies the pilot's nadir.
Mounted behind the plate there is a star-planisphere, based on a north pole stereographic zenithal projection (a projection from the north pole onto a plane passing through the south pole, and perpendicular to the solar axis). This rotates once in a sidereal day on an axis passing through its south celestial pole and located some 13 cm above the centre of the horizon plate. For decoration it carries a few basic star patterns (considerably distorted owing to the projection used) and an eccentric zodiac / ecliptic / calendar ring faced with silver, and restricted in width to the distance between the solstitial points.
Associated with the ring is a rayed apparent-sun emblem whose supporting arm system moves to and fro relative to a main mean-motion arm by an amount corresponding to the equation of time, and also radially to represent changes in the sun's declination. Sun-emblem and horizon-plate therefore combine to show the times of the Sun's rising, southing, and setting throughout the year and, by means of the rays of the sun emblem, the extent of the twilight.
To complete the front dial display, two sets of curved wires are mounted in front of the planisphere. One set of two wires indicates lines of equal altitude 36° and 59.5° and supplements the edge of the horizon plate (altitude 0°); they cross a vertical north–south wire to indicate the Sun's meridian altitude at the equinoxes and solstices respectively. The second set, in addition to the north–south element, forms a complete circle and the sectors of two larger circles; all four wires intersect at the centre of the horizon plate (nadir) and a point near the top of the north–south line (zenith).
The rear planisphere, similar in size to the one in the front but strictly circular, displays the brightest stars of the main circumpolar constellations (Ursa Major, Ursa Minor, and Cassiopeia). It is centred on the north celestial pole and, with its encircling silvered ring of 24 hour intervals, rotates once a sidereal day relative to a fixed meridian-index.
The planisphere is attached to a large wheel driven by a worm gear, which is connected to a gear box containing the wheel-train. The gear wheels have ratios 45⁄29, 71⁄151, and 257⁄187, producing a solar to sidereal ratio of 821115 : 818873 = ( 45 × 71 × 257 ) : (29 × 151 × 187 ) . This produces an approximation for the mean tropical year of 365.24219447 days (compare to 365.24219265 days [2] : for 2000 CE ).
The worm gear serving the similar wheel of the mean-sun arm is connected to the input end of the gear box, and also, through a 3000:1 reduction gear, to a 1500 RPM synchronous electric motor, running off the main electricity supply.
For the equation correction, a wheel on a lever associated with the mean solar arm rolls round an annular cam of sheet brass about 61 cm in diameter. Beneath the planisphere, three small dials show hours, minutes, and seconds of Greenwich Mean Time, and three similar and balancing dials give local sidereal time in degrees, minutes, and seconds of arc. An aperture indicates the name of the day of the week.
A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and set "due west". This occurs twice each year, around 20 March and 23 September.
Right ascension is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the point in question above the earth. When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system.
A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.
Sidereal time is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coordinates in the night sky. In short, sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars", or more correctly, relative to the March equinox.
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.
Diurnal motion is an astronomical term referring to the apparent motion of celestial objects around Earth, or more precisely around the two celestial poles, over the course of one day. It is caused by Earth's rotation around its axis, so almost every star appears to follow a circular arc path, called the diurnal circle, often depicted in star trail photography.
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time and mean solar time.
In observational astronomy, culmination is the passage of a celestial object across the observer's local meridian. These events were also known as meridian transits, used in timekeeping and navigation, and measured precisely using a transit telescope.
In astronomy, a planisphere is a star chart analog computing instrument in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date. It is an instrument to assist in learning how to recognize stars and constellations. The astrolabe, an instrument that has its origins in Hellenistic astronomy, is a predecessor of the modern planisphere. The term planisphere contrasts with armillary sphere, where the celestial sphere is represented by a three-dimensional framework of rings.
The Prague Astronomical Clock or Prague Orloj is a medieval astronomical clock attached to the Old Town Hall in Prague, the capital of the Czech Republic.
An astronomical clock, horologium, or orloj is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the Sun, Moon, zodiacal constellations, and sometimes major planets.
The Zytglogge is a landmark medieval tower in Bern, Switzerland. Built in the early 13th century, it has served the city as guard tower, prison, clock tower, centre of urban life and civic memorial.
Spherical astronomy, or positional astronomy, is a branch of observational astronomy used to locate astronomical objects on the celestial sphere, as seen at a particular date, time, and location on Earth. It relies on the mathematical methods of spherical geometry and the measurements of astrometry.
An astrocompass is a navigational tool for determining the direction of true north through the positions of various astronomical bodies.
A star clock is a method of using the stars to determine the time. This is accomplished by measuring the Big Dipper's position in the sky based on a standard clock, and then employing simple addition and subtraction. This method requires no tools; others use an astrolabe and a planisphere.
The meridian circle is an instrument for timing of the passage of stars across the local meridian, an event known as a culmination, while at the same time measuring their angular distance from the nadir. These are special purpose telescopes mounted so as to allow pointing only in the meridian, the great circle through the north point of the horizon, the north celestial pole, the zenith, the south point of the horizon, the south celestial pole, and the nadir. Meridian telescopes rely on the rotation of the sky to bring objects into their field of view and are mounted on a fixed, horizontal, east–west axis.
The Moon orbits Earth in the prograde direction and completes one revolution relative to the Vernal Equinox and the stars in about 27.32 days and one revolution relative to the Sun in about 29.53 days. Earth and the Moon orbit about their barycentre, which lies about 4,670 km (2,900 mi) from Earth's centre, forming a satellite system called the Earth–Moon system. On average, the distance to the Moon is about 385,000 km (239,000 mi) from Earth's centre, which corresponds to about 60 Earth radii or 1.282 light-seconds.
Rasmus Jonassen Sørnes, was a Norwegian inventor, clockmaker and radio technician, and is most famous for his advanced astronomical clocks, the most precise of which has an inaccuracy of 7 seconds during 1000 years. During his lifetime, Sørnes also designed and built a large variety of agricultural, radio-technical and mechanical devices, only a few of them patented.
An astrarium, also called a planetarium, is the mechanical representation of the cyclic nature of astronomical objects in one timepiece. It is an astronomical clock.
The Astrarium of Giovanni Dondi dell'Orologio was a complex astronomical clock built between 1348 and 1364 in Padova, Italy, by the doctor and clock-maker Giovanni Dondi dell'Orologio. The Astrarium had seven faces and 107 moving parts; it showed the positions of the sun, the moon and the five planets then known, as well as religious feast days. It was one of the first mechanical clocks to be built in Europe.
Coordinates: 53°57′43″N1°4′55″W / 53.96194°N 1.08194°W
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