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 ![Astrolabe Parts: Back](/sites/g/files/omnuum6316/files/chsi/files/astrolabe_back.jpg)

 

**Parts of the Astrolabe:  
The Back**  
  
**Mater:** The body of the astrolabe on whose back side are engraved calendar and degree scales, diverse instruments, and sometimes tables.  
  
**Zodiacal calendar:** A circular scale divided up into the signs of the Zodiac and their degrees through which the Sun moves during the year.  
  
**Civil calendar:** A circular scale divided into months and days. It is offset (eccentric) to the Zodiacal calendar.  
  
**Circle of degrees:** A divided scale along the edge of the mater used with the alidade in measuring the angle between two objects, such as the altitude of a star above the horizon or the angle between two landmarks.  
  
**Alidade:** A sighting ruler that rotates around the bolt. It has a pair of pinhole sights that stand perpendicular to it. It is used as an index to align civil dates with Zodiacal signs and degrees, and as a sighting device to observe the positions of objects and measure the angles.  
  
**Shadow square:** A surveying instrument used with the alidade to measure relative proportions of one length to another—for example the height of a tower to the distance of the observer from its base.  
  
**Horary quadrant for unequal hours:** Used with the alidade to find the time using a system of seasonal, unequal hours that divide every day and night into twelve hours.  
  
 **Inv Number: [DW0594](https://qrgo.page.link/h2K6m)  
Date: second half of 16th century?  
Maker: French  
  
Back to ["Time, Life, & Matter: Colonial Science"](/exhibitions/tlm-colonial-science).**