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 major planets.
Definition
The term is loosely used to refer to any clock that shows, in addition to the time of day, astronomical information. This could include the location of the Sun and Moon in the sky, the age and Lunar phases, the position of the Sun on the ecliptic and the current zodiac sign, the sidereal time, and other astronomical data such as the Moon's nodes, which can be used to indicate eclipses, or a rotating star map. The term should not be confused with an astronomical regulator, a high-precision but otherwise ordinary pendulum clock used in observatories.
Astronomical clocks usually represent the Solar System using the geocentric model. The center of the dial is often marked with a disc or sphere representing the Earth, located at the center of the Solar System. The Sun is often represented by a golden sphere (as it initially appeared in the Antikythera mechanism, back in the 2nd century BC), shown rotating around the Earth once a day around a 24-hour analog dial. This view accorded both with the daily experience and with the philosophical world view of pre-Copernican Europe.
History
The Antikythera mechanism is the oldest known analog computer and a precursor to astronomical clocks. A complex arrangement of multiple gears and gear trains could determine the positions of the sun, moon and planets, predict eclipses and other astronomical phenomena, and track the dates of Olympic Games. Research in 2011 and 2012 led an expert group of researchers to posit that European astronomical clocks are descended from the technology of the Antikythera mechanism.PBS (2013). NOVA: "Ancient Computer". Retrieved 4 April 2013.
In the 11th century, the Song dynasty Chinese horologist, mechanical engineer, and astronomer Su Song created a water-driven astronomical clock for his clock-tower of Kaifeng City. Su Song is noted for having incorporated an escapement mechanism and the earliest known endless power-transmitting chain drive for his clock-tower and armillary sphere to function. Contemporary Muslim astronomers and engineers also constructed a variety of highly accurate astronomical clocks for use in their observatories,Kasem Ajram (1992). Miracle of Islamic Science, Appendix B. Knowledge House Publishers. . (cf. ) such as the astrolabic clock by Ibn al-Shatir in the early 14th century.David A. King (1983). "The Astronomy of the Mamluks", Isis 74 (4), p. 531-555 [545–546].
The early development of mechanical clocks in Europe is not fully understood, but there is general agreement that by 1300–1330 there existed mechanical clocks (powered by weights rather than by water and using an escapement) which were intended for two main purposes: for signalling and notification (e.g. the timing of services and public events), and for modelling the solar system. The latter is an inevitable development because the astrolabe was used both by astronomers and astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar system. American historian Lynn White Jr. of Princeton University wrote:, p.122-123 The astronomical clocks developed by the English mathematician and cleric Richard of Wallingford in St Albans during the 1330s, and by medieval Italian physician and astronomer Giovanni Dondi dell'Orologio in Padua between 1348 and 1364 are masterpieces of their type. They no longer exist, but detailed descriptions of their design and construction survive, and modern reproductions have been made. Wallingford's clock may have shown the sun, moon (age, phase, and node), stars and planets, and had, in addition, a wheel of fortune and an indicator of the state of the tide at London Bridge. De Dondi's clock was a seven-faced construction with 107 moving parts, showing the positions of the sun, moon, and five planets, as well as religious feast days.
Both these clocks, and others like them, were probably less accurate than their designers intended. Although the gear ratios may have been carefully calculated, their manufacture was somewhat beyond the mechanical abilities of the time, and they never worked reliably. Furthermore, in contrast to the intricate advanced wheelwork, the timekeeping mechanism in nearly all these clocks until the 16th century was the simple verge and foliot escapement, which had errors of at least half an hour a day.
Astronomical clocks were built as demonstration or exhibition pieces, intended to impress as well as to educate or inform. The challenge of building these masterpieces meant that clockmakers would continue to produce them, to demonstrate their technical skill and their patrons' wealth. The philosophical message of an ordered, heavenly-ordained universe, which accorded with the Gothic-era view of the world, helps explain their popularity.
The growing interest in astronomy during the 18th century revived interest in astronomical clocks, less for the philosophical message, more for the accurate astronomical information that pendulum-regulated clocks could display.
Generic description Astronomical clocks vary in design, but share some common features.
Time of day
Most astronomical clocks have a 24-hour analog dial around the outside edge, numbered from to in Roman numerals, then from to again. The current time is indicated by a golden ball or an image of the Sun at the end of a pointer. Local noon is usually at the top of the dial, and midnight at the bottom. Minute hands are rarely used.
The Sun indicator or hand gives an approximate indication of both the Sun's azimuth and altitude. For azimuth (bearing from the north), the top of the dial indicates south, and the two points indicate east and west. For astronomical clocks designed for use in the Northern Hemisphere, the top represents the zenith and the two points define the horizon; this interpretation is most accurate at the equinoxes.
If is not at the top of the dial, or if the numbers are Arabic rather than Roman, then the time may be shown in Italian hours (also called Bohemian, or Old Czech, hours). In this system, 1 o'clock occurs at sunset, and counting continues through the night and into the next afternoon, reaching 24 an hour before sunset.
In the lead photograph of the Prague astronomical clock, the time indicated by the Sun hand is about 9a.m. ( in Roman numerals), or about the 13th hour (Italian time in Arabic numerals).
Calendar and zodiac The year is usually represented by the 12 signs of the zodiac, arranged either as a concentric circle inside the 24-hour dial, or drawn onto a displaced smaller circle, which is a projection of the ecliptic, the path of the Sun and planets through the sky, and the plane of the Earth's orbit.
The ecliptic plane is projected onto the face of the clock, and, because of the Earth's tilted angle of rotation relative to its orbital plane, it is displaced from the center and appears to be distorted. The projection point for the stereographic projection is the North pole; on astrolabes the South pole is more common.
The ecliptic dial makes one complete revolution in 23 hours 56 minutes (a sidereal day), and therefore gradually moves out of phase with the hour hand during the year.
The date is found where the hour hand or Sun disk intersects the ecliptic dial: this indicates the current star sign, or the Sun's current location on the ecliptic. The intersection point slowly moves around the ecliptic dial during the year, as the Sun moves out of one astrological sign into another.
In the diagram showing the clock face on the right, the Sun's disk has recently moved into Aries (the stylized ram's horns), having left Pisces. The date is therefore late March or early April.
If the zodiac signs run around inside the hour hands, either this ring rotates to align itself with the hour hand, or another hand, revolving once per year, points to the Sun's current zodiac sign.
Moon A dial or ring indicating the numbers 1 to 29 or 30 shows the Moon's age: a new moon is 0, the Moon waxes until the full moon around day 15, and then wanes until day 29 or 30. The phase is sometimes shown by a rotating globe or black hemisphere, or a window that reveals part of a wavy black shape beneath.
Hour lines Unequal hours were the result of dividing up the period of daylight into 12 equal hours and nighttime into another 12. Daylight lasts longer in summer and shorter in winter, so each of the 12 daylight hours is longer than a night hour in summer and shorter than a night hour in winter. These unequal hours are shown by the curved lines radiating from the center. The longer daylight hours in summer can usually be seen at the outer edge of the dial, and the time in unequal hours is read by noting the intersection of the sun hand with the appropriate curved line.
Aspects Astrologers considered the arrangement and alignment of the Sun, Moon, and planets in the sky significant. If certain planets appeared at the points of a triangle, hexagon, or square, or if they were opposite or next to each other, the appropriate aspect was used to determine the event's significance. On some clocks, the common aspects – triangle, square, and hexagon – are drawn inside the central disc, with each line marked by the symbol for that aspect; the signs for conjunction and opposition may also appear. On an astrolabe, the corners of the different aspects could be lined up on any of the planets. On a clock, however, the disc containing the aspect lines cannot be rotated at will, so they usually show only the aspects of the Sun or Moon.
On the Torre dell'Orologio, Brescia clock in northern Italy, the triangle, square, and star in the centre of the dial show these aspects (the third, fourth, and sixth phases) of (presumably) the moon.
"Dragon" hand: eclipse prediction and lunar nodes The Moon's orbit is not in the same plane as the Earth's orbit around the Sun but crosses it in two places. The Moon crosses the ecliptic plane twice a month, once when it passes above the plane, and again approximately 15 days later when it passes below the ecliptic. These two locations are the ascending and descending lunar nodes. Solar and lunar eclipses will occur only when the Moon is positioned near one of these nodes because at other times the Moon is either too high or too low for an eclipse to be seen on the Earth.
Some astronomical clocks keep track of the position of the lunar nodes with a long pointer that crosses the dial, with its length extended out to both sides of the dial to pointing at two opposite points on the solar or lunar dial. This so-called "dragon" hand makes one complete rotation around the ecliptic dial every 19 years. It is sometimes decorated with a serpent or lizard () with its snout and tail-tip touching the outer dial, traditionally labelled and even if the decorative dragon is omitted (not to be confused with the similar-seeming names of the two sections of the constellation Serpens).
During the two yearly eclipse seasons the Sun pointer coincides with either the dragon's snout or tail. When the dragon hand and the full Moon coincide, the Moon is on the same plane as the Earth and Sun, so a lunar eclipse may be visible on one side of the Earth. When the new Moon is aligned with the dragon hand, a solar eclipse may be visible somewhere on the Earth.
Historical examples
Su Song's Cosmic Engine