About
A modern astronomical clock inspired by 600 years of timekeeping
The Prague Orloj
The Prague Astronomical Clock (Czech: Pražský orloj) is a medieval astronomical clock located in the Old Town Square of Prague, Czech Republic. First installed in 1410, it is the third-oldest astronomical clock in the world and the oldest still operating.
The Orloj is mounted on the southern wall of Old Town Hall. Its three main components are:
- The astronomical dial, representing the position of the Sun and Moon in the sky
- The calendar dial, featuring medallions representing the months
- The "Walk of the Apostles", a clockwork hourly show of figures
The astronomical dial is an astrolabe—a form of mechanical computer that shows the state of the cosmos. It displays Bohemian (Old Czech) time, Central European time, hours of daylight and darkness, the position of the Sun in the zodiac, and the current phase of the Moon.
A model of the universe
The Orloj was designed when people believed Earth was the center of the universe. Yet it remains accurate today because it shows what we actually see: the Sun's apparent motion through our sky. The model works because it captures the relative motions, not the absolute structure, of our solar system.
Why I Built This
Modern timekeeping has become completely divorced from astronomy. We measure time in arbitrary units, synchronized to atomic clocks, displayed on screens that never show us where the Sun is or what phase the Moon is in.
For most of human history, time was astronomical. Dawn, noon, and dusk structured the day. The Moon's phases marked weeks. The Sun's journey through the zodiac marked the year. People were connected to cosmic rhythms in ways we've largely forgotten.
This project is an attempt to reconnect with that older understanding of time—not as nostalgia, but because there's something genuinely valuable in knowing where you stand in relation to the Sun, Moon, and stars.
The Prague Orloj has been showing this cosmic perspective for over 600 years. This clock brings it to your screen, calculated for your exact location, for any moment in history.
Technical Details
The clock calculates astronomical positions using established algorithms. The calculations include:
- Solar position (ecliptic longitude, right ascension, declination)
- Lunar position and phase angle
- Subsolar and sublunar points (geographic coordinates where Sun/Moon are at zenith)
- Day/night terminator calculation
- Unequal hours based on sunrise/sunset times
The Earth map uses a north polar azimuthal equidistant projection, which preserves true distances from the center point (North Pole) and shows the day/night terminator as a clean circle.
Accuracy note
The algorithms used are simplified for browser performance. They are accurate to within a few arc-minutes for dates within a few centuries of the present, which is more than sufficient for understanding astronomical relationships. For precise astronomical calculations, use specialized ephemeris software.
Data Sources
Astronomical calculations are based on algorithms from:
- "Astronomical Algorithms" by Jean Meeus
- The VSOP87 planetary theory for solar position
- Standard lunar phase calculations
Geographic data for the Earth map (coastlines, land polygons) is derived from Natural Earth, a public domain map dataset.
Built With
This is a passion project. If you find it valuable, consider supporting its development.
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