Astrolabe Parts & Plates

How to use the Astrolabe

Let’s look at the different parts, and then we will get into how to use it. Below are the parts for the Single Plate version.

Front

  1. The Mater is the body of the astrolabe, onto which circles of the tropics and equator are drawn. It may have separate Plates installed on top, which are used for different latitudes, but in this case we are using a single latitude whose arcs are drawn directly onto the device.

2) The Limb exists around the edge of the Mater and carries the degree and hour scales.

3) The Rete shows the Ecliptic Circle and the various stars of the sky. I offer this in two types; you may have a transparent acrylic disc, or the traditionally-shaped wooden version. On the acrylic one, it is engraved with stars up to magnitude 4, and magnitude 1 stars are labelled. The wooden one uses sharp points to denote the locations of the Mag 1 stars (see image at the bottom of the page).

4) The Ruler is used to point to the scale on the Limb. The scale is used to measure Declination of stars.

5) The Alidade is attached on the reverse side of the instrument but is shown for clarity here.

Below are the parts for the Multiple Plate version. Items 1-5 remain the same. Item 6 is the removable Plate. You may have more than one of these, depending on what you ordered. The Plate is normally engraved with one latitude on each side, and has a notch at the bottom so you can only insert it the correct way up.

Here is the reverse side of the Astrolabe:

Back

  1. Scales of degrees, ecliptic longitude (zodiac) and the calendar.
  2. Equation of Time curve for finding the offset of the Sun for a date.
  3. Unequal Hours
  4. Shadow square for finding distances and heights of objects (eg trees).
  5. The Alidade is used for sighting objects, finding dates and measuring the Equation of Time.

Brass hardware is used to bolt it all together. It’s #8-32 size if you ever need to replace anything. (Metric users – 4mm bolts will probably fit; 3mm certainly will)

The Plate

This will look different depending on the latitude you’re using – it becomes more circular the further north you are, and flattens out for latitudes towards the Equator.

Plate 25º Latitude
Plate 55º Latitude

Astrolabes were traditionally shipped with a set of plates, spaced 5° apart, for use around the world.

There are two conventions used for the Azimuth markings; one is traditionally based on degrees from the Meridian, and the other uses degrees from North. Astrolabes purchased before December 2021 have the Meridian markings, but we have now switched to North-based for greater compatibility with normal compasses and ease of use.

Don’t fret if you have a Meridian based astrolabe – it’s easy to convert the readings. If you’re reading off the East side, subtract the given reading from 180. If you’re on the West side, add 180 to the reading. For example, if your azimuth reading is -120º, then 180-120=60º from North. On the West side, a reading of 180+150=330º.

Plate with Meridian-based azimuth markings
Plate with North-based azimuth markings

If you have the Single Plate version of the Astrolabe, then the plate is drawn directly onto the instrument and cannot be changed. This is the version shown in the photos in this guide.

If you have the Multi Plate version, then the plates are stored inside the instrument. Remove the nut and bolt, place the smaller wooden pieces and the plastic Rete to one side, then give the Astrolabe a good tap on the back, and the plate discs will fall out from the front. They are engraved front and back with plates for different latitudes. Find the one showing your current latitude and replace them all into the instrument with your chosen one on top. Then reassemble the device and you are good to go.

  1. North Pole
  2. Zenith (the point in the sky directly above your head)
  3. Tropic of Cancer
  4. Equator
  5. Tropic of Capricorn
  6. Horizon
  7. Civil, Nautical and Astronomical Twilight Arcs
  8. Altitude circles go around the zenith, from 0° at the horizon to 90° at the Zenith
  9. Azimuth circles converge at the zenith, from 180° facing South to 0° facing North

Here’s a basic diagram to show how the lines on the plate relate to the sky above you:

This star has an Azimuth of 45º and an altitude of 60º

The Calendar

The calendar shows the following scales:

  1. Quarter-circle degrees
  2. Sun’s ecliptic longitude in degrees
  3. Signs of the Zodiac
  4. Days
  5. Months 

The Alidade

The ‘minutes’ scale on one side is used with the Equation of Time curve.

The ‘altitude’ scale is used with the Unequal Hours.

The Wooden Rete

Below is an astrolabe with a wooden rete. This is more like the traditional astrolabes you find in museums. You’ll notice this has an extra layer around the limb to protect the rete from damage. The sharp points represent the brightest stars in the sky; these are named on the rete. The celestial ring is the same on both types of rete.

Using an acrylic rete on a wooden-rete instrument

Some people choose to have both retes, in which case I make the astrolabe with a wooden rete and add the acrylic as a spare. The acrylic rete fits in the same hole as the wooden one, but then it doesn’t receive light from the edges – some people find it a bit dark to read in that case. One of my lovely customers has provided a spacer, here: https://www.thingiverse.com/thing:6130822 which can be 3D printed to insert behind and partially lift the rete out of the hole, giving it a little extra illumination. You need to put the spacer behind any/all plates in the instrument. It only lifts the plates a little, so they stay locked in position, otherwise they might spin along with the rete.

Introduction
Parts & Plates
Finding Times
Finding Positions
Unequal Hours & Shadow Square