Time of day

Most astronomical clocks have a 24 hour analog dial around the outside edge, numbered from I to XII then from I to XII again. The current time is indicated by a golden ball or a picture of the sun at the end of a pointer. Local noon is usually at the top of the dial, and midnight at the bottom. Minute hands are rarely used.

How the 24 hour analog dial might be interpreted.

diagram showing how the zodiac is projected on to the ecliptic dial – the symbols are often drawn inside the dial.

Stereographic projection from the North pole.

The sun indicator or hand gives an approximate indication of both the sun’s azimuth and altitude. For azimuth (bearing from North), the top of the dial indicates South, and the two VI points of the dial East and West. For altitude, the top is the zenith and the two VI and VI points define the horizon. (This is for the astronomical clocks designed for use in the northern hemisphere.) This interpretation is most accurate at the equinoxes, of course.

If XII is not at the top of the dial, or if the numbers are Arabic rather than Roman, then the time may be shown in Italian hours (also called Bohemian, or Old Czech, hours). In this system, 0 o’clock occurs at sunset, and counting continues through the night and into the next afternoon, reaching 24 an hour before sunset.

In the photograph of the Prague clock shown above, the time indicated by the sun hand is about noon (XII in Roman numerals), or about the 17th hour (Italian time in Arabic numerals).

Calendar and zodiac

The year is usually represented by the 12 signs of the zodiac, arranged either as a concentric circle inside the 24 hour dial, or drawn onto a displaced smaller circle, which is a projection of the ecliptic, the path of the sun and planets through the sky, and the plane of the Earth’s orbit.

The ecliptic plane is projected onto the face of the clock, and, because of the Earth’s tilted angle of rotation relative to its orbital plane, it is displaced from the center and appears to be distorted. The projection point for the stereographic projection is the North pole; on astrolabes the South pole is more common.

The ecliptic dial makes one complete revolution in 23 hours 56 minutes (a sidereal day), and will therefore gradually get out of phase with the hour hand, drifting slowly further apart during the year.

To find the date, find the place where the hour hand or sun disk intersects the ecliptic dial: this indicates the current star sign, the sun’s current location on the ecliptic. The intersection point slowly moves round the ecliptic dial during the year, as the sun moves out of one constellation into another.

In the photograph of the Prague clock shown above, the sun’s disk has recently moved into Aries (the stylized ram’s horns), having left Pisces. The date is therefore late March or early April.

If the zodiac signs run around inside the hour hands, either this ring rotates to align itself with the hour hand, or there’s another hand, revolving once per year, which points to the sun’s current zodiac sign.

Moon

A dial or ring indicating the numbers 1 to 29 or 30 indicates the moon’s age: a new moon is 0, waxes and become full around day 15, and then wanes up to 29 or 30. The phase is sometimes shown by a rotating globe or black hemisphere, or a window that reveals part of a wavy black shape beneath.

Hour lines

Unequal hours were the result of dividing up the period of daylight into 12 equal hours, and night time into another 12. In Europe, there’s more daylight in the summer, and less night, so each of the 12 daylight hours is longer than a night hour. Similarly in winter, daylight hours are shorter, and night hours are longer. These unequal hours are shown by the curved lines radiating from the center. The longer daylight hours in summer can usually be seen at the outer edge of the dial, and the time in unequal hours is read by noting the intersection of the sun hand with the appropriate curved line.

Aspects

Astrologers placed importance on how the sun, moon, and planets were arranged and aligned in the sky. If certain planets appeared at the points of a triangle, hexagon, or square, or if they were opposite or next to each other, the appropriate aspect was used to determine the event’s significance. On some clocks you can see the common aspects – triangle, square, and hexagon – drawn inside the central disk, with each line marked by the symbol for that aspect, and you may also see the signs for conjunction and opposition. On an astrolabe, the corners of the different aspects could be lined up on any of the planets. On a clock, though, the disk containing the aspect lines can’t be rotated at will, so they usually show only the aspects of the sun or moon.

In the photograph of the Brescia clock above, the triangle, square, and star in the center of the dial show these aspects (the third, fourth, and sixth phases) of (presumably) the moon.

Dragon hand: eclipse prediction and lunar nodes

The moon’s orbit isn’t in the same plane as the Earth’s orbit around the sun, but crosses it in two places. The moon crosses the ecliptic plane twice a month, once when it goes up above the plane, and again 15 or so days later when it goes back down below the ecliptic. These two locations are the ascending and descending lunar nodes. Solar and lunar eclipses will occur only when the moon is positioned near one of these nodes, because at other times the moon is either too high or too low for an eclipse to be noticed from earth. Some astronomical clocks keep track of the position of the lunar nodes with a long pointer that crosses the dial. This so-called dragon hand makes one complete rotation every 19 years. When the dragon hand and the new moon coincide, the moon is on the same plane as the earth and sun, and so there is every chance that an eclipse will be visible from somewhere on earth.

From Wikipedia:http://en.wikipedia.org/wiki/Astronomical_clock#Generic_description