The Universal Astrolabe is at a glance quite different from its Planispheric cousins. You can see there is a circular grid, ruler (the Regula) and a bendy, moving arm (Brachiolus). This is all surrounded by concentric rings of calendars; which we’ll get to shortly.
The Universal astrolabe differs from the Planispheric astrolabe in its projection. The planispheric device projects the celestial sphere on the plane of the equator. (Looking up from underneath). The Saphea Arzachelis is projected on a Great Circle that intersects the North and South Poles, with either the Spring or Autumn equinox as the point of the projection (Looking sideways).
One of the instrument’s main functions is to translate coordinates between three grid systems. The lines on the plate are largely unmarked with scales; this is because it would get too busy if we tried to mark all three coordinate systems on there.
An explanation of the different coordinate systems can be found here: https://en.wikipedia.org/wiki/Astronomical_coordinate_systems
We are dealing with the Equatorial (RA/Dec), Horizontal (Alt/Az) and Celestial/Ecliptic (lat/long) systems.
Each grid system makes use of the Polar Arcs (vertical, running north-south) and the Parallel lines (running horizontally). These change function depending on how you look at the device.
The marked numbers on the plate are the Hours, which occur every 15º, meaning each polar arc marks 20 minutes. The upper line is used in the morning and the lower line in the evening.
System 1: Equatorial Projection
In this system, the Equinoctial line is the Equator, and the grid shows Right Ascension (polar arcs) and Declination (parallels).
Right Ascension is measured from 0 at the Meridian, to 6h on the right, then 6h to 18h all the way over to the left, then 18h back to 24/0h at the Meridian again. These are in 15º intervals, like the Hours.
The Polar Arcs can also represent hour angles in this projection.
Declination is positive above the equator and negative below.
System 2: Horizontal Projection
In this projection, the polar arcs are Azimuths. They go from 90º to 270º when facing North, and 270º to 90º when facing South.
The parallels are Altitudes. Generally, only those above the horizon are used, although there are calculations that can be made about objects below the horizon.
System 3: Ecliptic Projection
This is where the Saphea projection really shines – if you tilt the device so that the Ecliptic line is now on the Horizon, then the old horizon becomes the Ecliptic, the polar arcs then show the Celestial Longitude, and the parallels show the Celestial Latitude.
The polar arc scale is marked on the Regula (ruler).
The whole Saphea plate
Here is the whole Saphea plate and accessories.
You can see that the Ecliptic is marked with the Zodiac. Each sign is divided into 30 days, starting with Capricorn 0 on the left and running to Gemini 30 on the right (above the line), then Cancer 0 on the right back to Sagittarius 30 on the left, underneath. Above the line, the projection origin is the Autumnal Equinox. Below the line, the origin is the Vernal (Spring) Equinox. The Sun moves back and forth along the line during the year.
These are also marked on the calendar around the edge of the device (outside the Saphea plate) and can be used to convert back and forth from normal calendar dates.
The Regula (ruler) sits across the front of the plate. It is marked in degrees for use with the Ecliptic projection. (They’re also useful in Horizontal mode, at least if you’re facing north). It also represents the local horizon when rotated to the polar distance (top left quadrant) for a given latitude.
Attached to the Regula is the bendy arm of the Brachiolus. This is a pointer that can be moved to any point on the plate, and is used for marking coordinates that you want to convert. It should move in concert with the regula; i.e. when you move the regula, the brachiolus should move with it. (If it doesn’t, try tightening the nut and bolt).
Around the edge of the plate are marked four 90º quadrants. The top right quadrant is marked from 0º at the equator to 90º at the North Pole, for use when the polar arcs are declinations or latitudes, in Equatorial or Ecliptic modes. The top left quadrant is reversed, from 90º at the equator to 0º at the pole, which shows the polar distance and is used to orient the local horizon, i.e. if your location on Earth is at 30º latitude, you would measure that figure on the left side.
The whole instrument
Here is the whole side of the instrument showing how the Saphea plate sits within the calendar rings.
There are named and numeric rings for the normal calendar in the middle,
Named and numeric rings for the Zodiac,
And degrees of Ecliptic Longitude around the edge.
In this way, you can translate between each of the calendars.
A note about the Zodiac
The Zodiac, when used with Astrolabes, is purely a notation that describes a direction in the sky. It has nothing to do with astrology.
The year is divided into twelve sets of 30, named after the constellations that were in them at the time they were denoted – stellar precession means that the constellations have shifted after hundreds of years, so the names no longer match the stars contained within them.
It is a useful shorthand to describe the progression of the Sun; you can use either the Zodiac or degrees of ecliptic longitude, they will tell you the same thing. However, since the Saphea plate traditionally uses the Zodiac along the ecliptic, that is what we will go with here; so you will find the calendar rings useful, and also the extra Zodiac names on the rete on the other side.
What’s on the back?
The other side of the instrument is a Planispheric astrolabe, with a few differences to the standard models. For one thing, it is missing the Equation of Time loop, Unequal Hours, and the Shadow Square; simply because there is nowhere to put them on this device. Also, the ruler is a combined ruler/alidade; it has sighting points on there to allow the user to take elevations of sky objects.
There is only room for one plate disc, although if you own one of my Planispheric astrolabes, they use the same size discs so you can interchange them here. I’m currently only offering a traditional carved rete since it seems to suit the character of the instrument, although really there’s no reason why you could’t have an acrylic one if you’d like – please ask and I’ll add it if there’s interest, or see my Spare Parts listings to add one in addition to the wooden version.
