Astronomical clocks are complex mechanical timepieces that track astronomical events such as the positions of the Sun, Moon, planets, and Zodiac constellations. Since ancient civilizations, time measurement methods have developed in close connection with the science of astronomy, leading to the emergence of astronomical clocks in medieval Europe. These clocks not only display the time but also model the movements of celestial bodies, allowing the tracking of astronomical events. Especially found in Gothic cathedrals and city centers, astronomical clocks have survived to this day as monumental structures where engineering and art converge.
AI-generated (Source: rawpixel.com)
History
The origins of astronomical clocks date back to ancient devices such as the Antikythera Mechanism. This mechanism is considered the oldest known analog computing device used for measuring time and calculating the movements of celestial bodies. Dated to the 2nd century BCE and believed to have been developed by Greek astronomers, this device had the capacity to model the movements of the Sun, Moon, and planets using gears and mechanical systems. Additionally, it could predict eclipses and calculate the dates of important cultural events of the time, such as the Olympic Games. Research conducted in 2011 and 2012 revealed that the technological features of the Antikythera Mechanism inspired medieval astronomical clocks in Europe.
By the Middle Ages, the development of astronomical clocks was supported by significant technological advancements across different regions. In the 11th century, Chinese scientist and engineer Su Song built a clock tower in Kaifeng, developing a water-powered astronomical clock. This clock featured an escapement mechanism and an endless chain drive system, making it one of the most advanced mechanical systems of its time. During the same period, astronomical clock technology also saw significant progress in the Islamic world. Muslim engineers and astronomers developed high-precision astronomical clocks, particularly for use in observatories. A notable example of these advancements was the astrolabe-based clock designed by Ibn al-Shatir in the 14th century.
Astrolabe at the Istanbul Museum of the History of Science and Technology in Islam (Photo: j Brew, flickr.com)
The emergence of mechanical astronomical clocks in the modern sense began in the 13th century. During this period, churches and cathedrals started using clocks equipped with astronomical indicators, providing the public with information about celestial events.
In the 14th and 15th centuries, astronomical clocks became symbols of major European cities, growing increasingly sophisticated. One of the most significant clocks built during this period was the Prague Astronomical Clock, completed in 1410. With the Renaissance, the functions of astronomical clocks in engineering and astronomy were further developed, making it possible to track calendar calculations, lunar phases, and planetary movements through these clocks.
By the late 15th century, the Industrial Revolution led to the widespread adoption of modern timekeeping systems, reducing the functional importance of astronomical clocks. However, these clocks remained culturally and historically significant. Today, historic astronomical clocks continue to be major attractions for both tourism and scientific study.
Mechanical Structure and Working Principle
Timekeeping Mechanism
Most astronomical clocks feature a 24-hour analog dial. The outer edge of this dial typically contains hour markers, numbered from I to XII, then repeating I to XII. The time is usually indicated by a marker or a Sun symbol moving along the dial. In traditional astronomical clocks, local noon is typically at the top of the dial, while midnight is at the bottom. Minute hands, common in modern mechanical clocks, are rarely found in historical astronomical clocks.
The Sun indicator not only shows the time but also provides information on the Sun’s azimuth and altitude. The top of the dial represents South, while the two VI points represent East and West. The top point of the dial corresponds to the zenith, and the VI points indicate the horizon. This system is more relevant for astronomical clocks used in the Northern Hemisphere, providing the most accurate data especially during equinox periods.
In some astronomical clocks, the time display may use Arabic numerals instead of Roman numerals, or the number XII may not be placed at the top of the dial. In such cases, the clock may operate according to the Italian hours system (also known as Bohemian or Old Czech hours). In this system, sunset is considered hour 1, and the timekeeping continues until the next sunset, reaching 24 hours. For instance, on the Prague Astronomical Clock, the time indicated by the Sun marker can simultaneously show both IX (9:00 AM) in the standard system and 13:00 (1:00 PM) in the Italian hour system.
Prague Astronomical Clock (Photo: Xiquinho Silva, flickr.com)
Astronomical Indicators
Most astronomical clocks represent the passage of the year using the Zodiac constellations. The Zodiac signs are typically arranged either as a concentric ring within the 24-hour dial or as a distorted projection of the ecliptic plane. The ecliptic represents the apparent path of the Sun across the sky and is depicted at an angle corresponding to Earth's orbital tilt.
In these clocks, the ecliptic dial rotates according to the sidereal day (23 hours 56 minutes), gradually shifting out of sync with the hour dial over time. This drift continues throughout the stellar year, indicating the Sun’s position within the Zodiac. To determine the date, observers can find the intersection point between the Sun indicator and the ecliptic dial, revealing the current Zodiac sign. For example, if the Sun has just exited Pisces and entered Aries, the date corresponds to late March or early April.
Some astronomical clocks feature a Zodiac ring that rotates in alignment with the hour hands or includes a separate indicator that completes one full rotation per year to mark the Sun’s transition through the Zodiac signs.
Lunar Phases and Movements
Astronomical clocks often include mechanisms that track the phases of the Moon. The age of the Moon is typically displayed on a ring or dial numbered from 1 to 29 or 30. The New Moon phase is marked as "0", while the Full Moon occurs around day 15. Following this, the Moon begins to wane, reaching the closing phase around days 29–30. The lunar phases can be displayed using a rotating sphere or black-and-white shapes within a translucent window.
Hour Lines and Unequal Hours
Many astronomical clocks include curved hour lines to indicate seasonal variations in daylight and nighttime duration. The unequal hours system, used in the Middle Ages, divided daylight and nighttime into 12 equal parts. In this system, daytime hours were longer in summer and shorter in winter. The intersections of the Sun indicator with specific curved lines correspond to certain unequal hour measurements.
Planetary Positions and Astrological Use
Astronomical clocks can also be used to track planetary alignments and angular relationships in the sky. In the Middle Ages, astrologers utilized astronomical clocks and astrolabes to determine astrological significance based on planetary positions. For example, the astronomical clock at Torre dell’Orologio in Brescia features triangle, square, and star symbols at its center, which are believed to indicate specific lunar and planetary phases.
Astronomical clock at Torre dell'Orologio in Brescia (Photo: Werner Austria, flickr.com)
