The Astrolabe Back
The back of the astrolabe has scales for finding the Sun's longitude for a date and an alidade and scales for finding the altitude of the Sun or a star.
One of the primary uses of the astrolabe is to find the time from the altitude of the Sun or a bright star. The back of the astrolabe has a rotating alidade and scale of degrees for making altitude measurements. To make an altitude measurement, the astrolabe is suspended above eye level and oriented toward the Sun or star. The alidade is rotated until the star is visible through the sights on the alidade or the Sun's shadow falls directly down the length of the alidade. The altitude is then read from a scale along the outer rim of the back.
The Sun's position on the ecliptic, its longitude, must be known in order to set the rete on the front of the astrolabe to a specific time and date. The back of the astrolabe has scales to find the Sun's longitude for each day of the year. The alidade is rotated to a date and the Sun's longitude is read from a scale that was always divided by the signs of the zodiac on all old instruments. The Sun's longitude does not change by a fixed amount each day. Two methods were used to find the Sun's varying longitude. On many instruments, the calendar scale was slightly off-center using the eccentric model of the Sun's motion from Ptolemaic astronomy. This allowed the calendar scale to be divided in equal length day divisions. Some astrolabes use a concentric calendar scale with variable width days. The eccentric calendar is easier to make but harder to design. The concentric calendar is easy to design, but the variable-width days are more difficult to engrave accurately.
The shadow square was used for simple surveying such as finding heights, depths and distances. The object under investigation (such as the top of a tower) was sighted using the alidade. The needed ratio of height or distance (the tangent of the angle) was read on the shadow scale.
The Unequal Hour Diagram in the figure is used to find the unequal hour of the day from the Sun's noon altitude and its current altitude. The diagram is very old, dating from 9th century Baghdad, and is used in various forms on a variety of instruments including astrolabes and quadrants.
Old astrolabes had many other scales on the back depending on where and when an instrument was made. Some Islamic instruments had scales for finding the direction to Mecca (qibla) and mathematical scales for solving problems using sines and cosines. Some instruments had special scales for finding prayer times. European astrolabes had no need for the elaborate Islamic scales and often had only a scale for converting between equal and unequal hours or no additional scales at all.
Click on the figure to see an astrolabe back with an alidade. The alidade is set at an altitude of 40º. In this position, it also shows the Sun is at Taurus 10° on April 25, and is at Scorpio 10 ° on November 1.
The astrolabe in the picture is the back of the Classic Edition of the The Personal Astrolabe, which recreates the scales on the back of a 16th century European astrolabe. The Modern Edition of the Personal Astrolabe is designed for modern timekeeping conventions. On the Modern Edition, the ecliptic is divided directly by the calendar to make it easier to set, all mention of the zodiac is removed and the equation of time is shown on the back to make it easy to convert between apparent solar time and civil time.