The Astrolabe Rete
The astrolabe rete rotates to simulate the daily movement of the stars in the sky. Rete (pronounced "reetee" in Latin, but pronounced "reet" by most people) is the Latin word for "net". It is also called the "spider" in other languages.
The rete has two main components: star pointers and the stereographic projection of the ecliptic. The star pointers are placed at the position of their stereographic projection. The form of the pointers varied from simple indicators to elaborate constructions of vines and animals on the more elaborate Islamic and Persian instruments. The number of star pointers varied from 10 or 12 on early and simple astrolabes to as many as 50 or 60 on later large instruments.
The offset circle on the rete is the stereographic projection of the ecliptic. The ecliptic circle on the rete is offset so it just touches the Tropic of Capricorn at the summer solstice and touches the Tropic of Cancer at the winter solstice. In antiquity, the ecliptic was defined as the path of the Sun in the sky as seen from the Earth (which is a perfectly valid way of looking at it). The Sun makes one complete revolution of the ecliptic in a year. The movement of the Sun along the ecliptic is not uniform but speeds up and slows down depending on the season. If you know the date, you know where the Sun is on the ecliptic. The Sun's position on the ecliptic is its longitude.
In ancient times, the ecliptic was divided into 30 degree sections of longitude called the zodiac. The ecliptic on old astrolabes was always divided by the signs of the zodiac. The Sun's position on the ecliptic (longitude) is found from scales on the back of the astrolabe. The rule was then rotated until it crossed the ecliptic at the correct longitude. The point where the rule crosses the ecliptic is the Sun's location for the day. The rule and rete are then rotated together to find the time or solve problems involving the Sun's location.
The outer edge of the rete might contain indexes for solving specific types of problems. The "First Point of Aries" was often marked for working with sidereal time and right ascensions. Many instruments had an index at Capricorn 0° called the "muri" that was used for problems involving differences in angles.
The rete's supporting framework was made in many styles including Gothic, Y, Trefoil and Quatrefoil designs. The rete in the picture is Late Gothic (photo courtesy Adler Planetarium and Astronomy Museum).
The rete on the The Personal Astrolabe is printed on transparent material and includes 150 stars arranged in familiar constellations. The ecliptic on the Modern Edition of the Personal Astrolabe is divided directly by the calendar to make it easier to use.