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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Astronomical clock | 3/7 | https://en.wikipedia.org/wiki/Astronomical_clock | reference | science, encyclopedia | 2026-05-05T09:40:22.228447+00:00 | kb-cron |
=== "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 goes up above the plane, and again 15 or so days later when it goes back down 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 the figure of a serpent or lizard (Greek: drakon) with its snout and tail-tip touching the outer dial, traditionally labelled Latin: "caput draconam" and Latin: "cauda draconam" 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, and so there is a good chance that a lunar eclipse will be visible on one side of the Earth. When the new Moon is aligned with the dragon hand there is a moderate possibility that a solar eclipse might be visible somewhere on the Earth.
== Historical examples ==
=== Su Song's Cosmic Engine === The Science Museum (London) has a scale model of the 'Cosmic Engine', which Su Song, a Chinese polymath, designed and constructed in China in 1092. This great astronomical hydromechanical clock tower was about ten metres high (about 30 feet) and featured a clock escapement and was indirectly powered by a rotating wheel either with falling water and liquid mercury, which freezes at a much lower temperature than water, allowing operation of the clock during colder weather. A full-sized working replica of Su Song's clock exists in the Republic of China (Taiwan)'s National Museum of Natural Science, Taichung city. This full-scale, fully functional replica, approximately 12 meters (39 feet) in height, was constructed from Su Song's original descriptions and mechanical drawings.
=== Astrarium of Giovanni Dondi dell'Orologio === The Astrarium of Giovanni Dondi dell'Orologio was a complex astronomical clock built between 1348 and 1364 in Padua, Italy, by the doctor and clock-maker Giovanni Dondi dell'Orologio. The Astrarium had seven faces and 107 moving gears; it showed the positions of the sun, the moon and the five planets then known, as well as religious feast days. The astrarium stood about 1 metre high, and consisted of a seven-sided brass or iron framework resting on 7 decorative paw-shaped feet. The lower section provided a 24-hour dial and a large calendar drum, showing the fixed feasts of the church, the movable feasts, and the position in the zodiac of the moon's ascending node. The upper section contained 7 dials, each about 30 cm in diameter, showing the positional data for the Primum Mobile, Venus, Mercury, the moon, Saturn, Jupiter, and Mars. Directly above the 24-hour dial is the dial of the Primum Mobile, so called because it reproduces the diurnal motion of the stars and the annual motion of the sun against the background of stars. Each of the 'planetary' dials used complex clockwork to produce reasonably accurate models of the planets' motion. These agreed reasonably well both with Ptolemaic theory and with observations. For example, Dondi's dial for Mercury uses a number of intermediate wheels, including: a wheel with 146 teeth, and a wheel with 63 internal (facing inwards) teeth that meshed with a 20 tooth pinion.
== Interior clocks and watches ==
=== The Rasmus Sørnes Clock ===
Arguably the most complicated of its kind ever constructed, the last of a total of four astronomical clocks designed and made by Norwegian Rasmus Sørnes (1893–1967), is characterized by its superior complexity compactly housed in a casing with the modest measurements of 0.70 x 0.60 x 2.10 m. Features include locations of the sun and moon in the zodiac, Julian calendar, Gregorian calendar, sidereal time, GMT, local time with daylight saving time and leap year, solar and lunar cycle corrections, eclipses, local sunset and sunrise, moon phase, tides, sunspot cycles and a planetarium including Pluto's 248-year orbit and the 25 800-year periods of the polar ecliptics (precession of the Earth's axis). All wheels are in brass and gold-plated. Dials are silver-plated. The clock has an electromechanical pendulum. Sørnes also made the necessary tools and based his work on his own astronomical observations. Having been exhibited at the Time Museum in Rockford, Illinois (since closed), and at the Chicago Museum of Science and Industry, the clock was sold in 2002 and its current location is not known. The Rasmus Sørnes Astronomical Clock No. 3, the precursor to the Chicago Clock, his tools, patents, drawings, telescope, and other items, are exhibited at the Borgarsyssel Museum in Sarpsborg, Norway.
=== Table clocks === There are many examples of astronomical table clocks, due to their popularity as showpieces. To become a master clockmaker in 17th-century Augsburg, candidates had to design and build a 'masterpiece' clock, an astronomical table-top clock of formidable complexity. Examples can be found in museums, such as London's British Museum. Currently Edmund Scientific among other retailers offers a mechanical Tellurium clock, perhaps the first mechanical astronomical clock to be mass-marketed. In Japan, Tanaka Hisashige made a Myriad year clock in 1851.