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Prague Astronomical Clock - structure and time reading

The Astrolabe

It is the cornerstone of every medieval astronomical clock. It is an astronomical instrument whose origin dates to Classical Antiquity. It was used to determine the positions of Sun, Moon and stars. It was important to astrologers and mariners. It also served to determine local time and was used in navigation.

Astrolabe itself is a large brass or copper circle comprised of two circular discs held together in the centre by a pin. The Sky section, or the upper blue part of the astrolabe sphere, represents day time. The sun pointer travels around it during the day. The lower part of the astrolabe sphere represents night, with part of dawn (Aurora) and sunrise (Ortus) painted red on the left, and sunset (Occasus) and nightfall depicted on the right. The Earth is depicted in the center of the astrolabe. Additionally, there are two concentric circles there: the Tropic of Capricorn and the Tropic of Cancer. There is an unmarked equator between them.

The astrolabe holds three time scales: Old Bohemian Time is shown using Gothical numerals on the moving Outer Ring, Central European Time is shown using Latin numerals at the outer edge of the astrolabe and Babylonian Time is shown using Arabic numerals on the Sky section of the astrolabe.

The Zodiacal Ring

The ring with signs of the Zodiac depicts the path of the Sun and Moon through the sky. The center of the Zodiac is offset from the center of the astrolabe and the ring's perimeter touches the Tropics of Cancer and Capricorn.

The Sun

The Sun is located on the same pointer as the Golden Hand showing the time. The Sun hand travels around the astrolabe once every 24 hours. The position of the Sun, or the intersection of the solar pointer and the ecliptic Circuit, shows Babylonian Time.

The Moon (Moon Hand or Pointer)

The Moon is a hollow ball with a helix and weight inside. As the hand with the Moon travels around the dial, the weight forces the ball to rotate around its axis. One half of its surface is silver, the other black. The Moon thereby waxes and then wanes just like it does on the sky. It revolves all the way around the hand in twenty-nine and a half days, i.e., from full moon to full moon. Portraying the phase is the second feature of the Moon. The first is to depict the Moon in the correct position on the ecliptic.

Old Bohemian Time

Old Bohemian Time, also called Italian Hours, is displayed in 24-hour cycle on the Outer Ring of the Dial. New day starts with sunset according to this Time, so the Outer Ring has to move during the year in order to place beginning of the day at the top of the Dial according to actual sunset time. Time is then indicated by the Golden Hand moving around the Outer Ring in 24-hours cycle.

Central European Time

Central European Time is displayed just inside the Outer Ring on the 2x12-hours dial with Latin numerals. This appearance is a remnant from the medieval times when this dial was showing Bohemian Time organized into one 24-hours cycle.

Star Time

Also called sidereal time, it is shown on the Roman numeral dial. It is derived from the apparent movement of the stars caused by the Earth's rotation. A sidereal day is shorter than a solar day.

Babylonian Time

Also called unequal or Arabic time, it was calculated only from sunrise to sunset. It is indicated by the simple black Arabic numerals 1-12. The intersection of the solar pointer and the ecliptic circuit shows Babylonian Time. The fanned golden line on the astrolabe's blue sphere comprise the Babylonian Time clock-face. The hour number is shown next to each line.