Prague Astronomical Clock

Live Web Cam of the Real Astronomical Clock in Prague

Legend about clockmaster Hanus

The origin of the Astronomical Clock was misrepresented for centuries. It was believed, that the author was clockmaster Hanus, also called Jan of Ruze, who lived in the 15 th century. The story said that the clock was admired by many foreigners, but Hanus refused to show construction plans to anybody. When Prague Councillors found out that he was going to make another, even better clock, they became jealous and blinded him so he could not finish it. Later he allegedly damaged the astronomical clock in revenge, and nobody was able to repair it.

Real history of the Astronomical Clock

The real author of the clock was discovered in 1961 in an old document, which describes the astronomical dial and says it was made by Nicolaus of Kadan in 1410.

He probably cooperated with the astronomer and Charles University professor Johannes Sindel. Born in 1375 in Hradec Krolove (Bohemia), he was the rector of the school at St. Nicholas in the Small Town in Prague from 1406, later he was a teacher of mathematics in Wien, where he studied on the Faculty of Medicine.

After his return to Prague he became professor of astronomy on Charles University. Later, in 1410, he became a doctor of medicine, the rector of Prague University and private physician of the king Wenceslas IV. According to testimony of Tycho Brahe, Sindel also performed valuable astronomical observations.

Sindel's eclipse instrument (or rather a nomogram for demonstration), which is described in treatise, was derived from Wallingford's albion (cf. 'Richard of Wallingford.' An edition of his writings with introductions, English translation and commentary by J.D. North. I-III, Oxford, Clarendon Press 1976).

The Astronomical Clock was repaired and improved by Jan Taborsky in the 16 th century.

The blue part of the dial represents the sky above the horizon, the brown part the sky below it. There are Latin words ORTVS (east) and OCCASVS (west) written above the horizon, and AVRORA (dawn) and CREPVSCVLVM (twilight) below.

There are three circles on the dial, showing different time: the outer circle with Schwabacher numerals shows the Old Czech Time ("Italian Time"), the circle with Roman numbers shows the Central European Time and the inner circle with Arabic numerals shows the "Babylonian Time": the length of an hour differs there according to the season - it is longer in the summer, shorter in the winter. The Prague Astronomical Clock is the only one in the world able to measure it. Furthermore, the little star by the zodiac ring shows the sidereal time.

The Clockwork

In the clockwork there are three great co-axial wheels of the same diameter, driven by the same pinion, with 365, 366 and 379 cogs.

The first of these gearing the zodiac and the indicator with the asterisk rotates once a sidereal day. The second gears the indicator of the Sun and rotates once a mean solar day. The third gearing the Moon's pointer rotates accordingly with the mean apparent motion of the Moon. The ball, half silvered and half black, rotates every synodic month and displays its phase.

Moon

On the average, the moon travels around the earth every 27 days, 7 hours and 43 minutes (27 d 7.716666 h, 27.3215278 days or every 655.716667 hours) (doy-1)*24+(time in hours) - 480.9866667(when moon first at 0 RA) )= t while (t>24) t=t-655.71666667 now t = sidereal RA Moon was at 0 RA Jan 20, 23:59 UTC (doy just .25 shy of 7320 cogs) at that time the zodiac angle was 11.75 degrees (.10253809355 radians) from the top center where the moon arm should be pointing. The moon arm rotates once every 24.85245902 ( =24*379/366 or 1516/61) hours. Together with the motion of the zodiac wheel, this makes the lunar sidereal year only 27 days, rather than 27.3215278 days.