Let’s begin with a measurement of the smallest indivisible unit of meaningful time. This is the time it takes light, travelling at 186,000 miles per second, to travel across the smallest measurement of length which has any meaning.
This turns out to be 0.00000000000000000000000000000000001 of a centimetre, and is called a ‘Planck length’, named by quantum physicist Max Planck in the first years of the twentieth century. This tiny unit of time is called, yes, you have guessed it, ‘Planck time’. This turns out to be 0.00000000000000000000000000000000000000000001 of a second, or 10 to the power of minus 43. This is a tenth of a millionth of a millionth of a millionth of a millionth of a millionth of a millionth of a millionth of a second.
What would we do without all these zeros?
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.
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.
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.
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.
One of the faces of the 15th-century astronomical clock on the Zytglogge tower, a landmark medieval tower in Bern, Switzerland. Built in the early 13th century, it has served the city as guard tower, prison, clock tower, centre of urban life and civic memorial.