Editing Astronomical clock (section)

=== 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 time|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 around the ecliptic dial during the year, as the Sun moves out of one [[astrological sign]] into another.

In the diagram showing the clock face on the right, 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.