Gears of the Astronomical Clock in Strasbourg Cathedral

Astronomical Clock

In general terms, an astronomical clock is not only a device that shows the time of day, but also a complex mechanism that presents various astronomical data. This data may include the positions of the Sun and the Moon, the current phase of the Moon, the position of the Sun along the ecliptic, and the zodiac system representing the months of the year.

Some astronomical clocks are equipped with a rotating star map that allows the tracking of the movement of stars in the sky. In addition, they can display sidereal time, which astronomers use to determine the direction in which telescopes should be pointed.

Sidereal time is based on the position of the stars and helps measure the Earth's rotation speed, allowing for a precise determination of the length of a day. Astronomical clocks are typically based on a geocentric model, meaning they represent the solar system with the Earth at the center.

Richard of Wallingford is known for having built an astronomical clock in St Albans in the 1330s. Similarly, Giovanni de Dondi created his own astronomical clock in Padua between 1348 and 1364.

Both scholars produced clocks that were considered masterpieces of their time. However, unfortunately, none of the original clocks have survived to the present day. Fortunately, their designs were carefully documented and passed on to future generations. Today, modern replicas of these clocks exist.

Richard’s clock displayed information about the Sun, the phases of the Moon, the positions of planets and stars, as well as a “wheel of fortune.” In addition, it included a mechanism that tracked the tides of the River Thames near London Bridge.

Giovanni de Dondi’s clock, known as the “Astrarium,” had a more complex structure. It consisted of 107 moving parts and featured a seven-faced construction. This clock displayed the positions of the Sun and the Moon, as well as the five known planets of the solar system, and also indicated religious feast days.

Due to technological limitations, these early astronomical clocks were not highly accurate and could not be used as reliable measuring instruments. Their primary purpose was to demonstrate technical skill and wealth.

In the 18th century, as public interest in astronomy increased, the demand for astronomical clocks also grew. Advances in technology allowed for greater accuracy, and these clocks began to be used for real astronomical calculations.

Features of a Standard Astronomical Clock

Each astronomical clock is unique in its design, but they share several common characteristics.

First of all, astronomical clocks use a 24-hour dial to display the time of day. Roman numerals are often used, with noon positioned at the top and midnight at the bottom of the dial.

At the tip of the time-indicating hand, there is usually a golden sphere or a symbol representing the Sun.

To represent the year, zodiac signs are used to correspond to the months. The zodiac circle is typically located within the clock face.

An elliptical projection is used to represent the Earth’s movement around the Sun. This system operates based on sidereal time, which means a full cycle is not 24 hours but approximately 23 hours and 56 minutes. As a result, the elliptical indicator gradually falls out of synchronization with the main clock hand.

Astronomical clocks also typically display the age of the Moon. When the Moon is new, the indicator is at zero; it reaches around 15 at the full moon and cycles between 29 and 30 days.

These clocks also show unequal hour lines, reflecting the variation in daylight throughout the year. Since the length of the day changes with the seasons, curved lines are used on the dial to visualize these differences.

One of the more fascinating elements is the so-called “dragon hand.” Approximately every 19 years, this indicator completes a full rotation along the ecliptic. At this time, the Moon, Earth, and Sun align in a straight line, resulting in an eclipse (solar or lunar).

Famous Astronomical Clocks

One of the most famous astronomical clocks is the Prague Astronomical Clock. Located in the Old Town Hall of Prague, this clock dates back to 1410 and combines both astronomical and religious elements.

The clock has undergone several restorations throughout history. According to local legend, if the clock ever stops, misfortune will befall the city.

Another notable example is the Olomouc Astronomical Clock, which is a rare heliocentric model showing the Sun as the center of the universe. Built in 1422, it has been updated approximately once every century.

During World War II, the clock was damaged but was later restored and put back into operation.

Inside the Beauvais Cathedral Astronomical Clock stands another remarkable example. Built over four years starting in 1865, this clock consists of more than 90,000 parts and includes 52 dials.

It displays the sunrise and sunset of the Sun and Moon, eclipses, lunar phases, solstices, and tides. In addition, symbolic scenes are presented at the top of each hour.

In Copenhagen, the Jens Olsen's World Clock took 50 years to complete. Construction began in 1948 and was finished in 1955.

The creator of this clock was Jens Olsen, a master clockmaker and astronomer.