My son, I've perceived your great ability to learn science, so consider yourself gifted now for I will teach you of that what is within the numbers and proportions. Consider I pray for you, alone then and afterwards in your special way, as having learnt these certain special evidences of the treaties of the astrolabe.
As a philosopher once said "he wraps him up in his friendship, with that but which condescends to the rightful praise of his friend", therefore in as much as I know the latitude of oxford, upon which, by meditation on a technique told now, I propose to teach you a certain number of conclusions (appertaining to the same instrument).
I myself see for certain in this, these causes for a conclusions foremost. The first cause is to trust well that all the conclusions that are discovered, or else possibly might be, in so noble an instrument as is this astrolabe, are unknown in part to any mortal man in this region, as how so I do suppose of the same such conclusions. Another cause is that even a soothing grace said by anyone will be for some a conclusion that by all things performed at her behest, is something about them that's too hard to conceive.
By this treaties, divided in 5 parts, I will show what is known in the full light and rules of naked English words, which in Latin you cannot know fully, my son. Never the less suffice to say that these true conclusions in english as well sufficed for reason with the noble clerks of the Greeks. These very same conclusions were said in greek mind you, and to Arabians in arabic, and to Jews in hebrew, and to the latin folk of Italy. Which Latin folk had first said thereof, out of all the other diverse languages, and written of them in their own tongue, that is also said alike in Latin, I don't know. God would that in all these languages and in many more ways than these, know that we do conclude to sufficiently learn and teach by diverse rules, and as rightly diverse as paths that lead diverse folk the right way to Rom.
Now will I pray concretely that every person that reads or hears this will discretely excuse my rude editing, and superfluous use of words. The reason being for any heavy sentences (and lack of structural form), is only for a standard of a curious child in learning, and lest he forget it all at once.
Lo behold to show more than this in the light of day, that is which my english says true in conclusion on these mater, and not only but to serve as truth in future with as many a subtile conclusions as forthrightly possible. So will been shown either in latin or in any further communication of this treaties of the astrolabe, I can say thankfully. For I pray so that God saves the King, who is lord of this language, and that in all that his faith bears in obedience, in every degree more and less. So consider well that I am no compiler of the labour of old astrologies, mere set to usurp the labours, as have worked hard to translate a lot into english for this doctrine, and like a sword only more shall I shear ones envies.
The first part of this treaties shall state the figures and the members of the astrolabe. With this alone you shall have greater knowing of the instrument.
The second part shall teach the practical workings of the aforesaid conclusions about this small and portable instrument called the astrolabe, as far fetched and narrow as they may be. For well does every astrologer know that the smallest fractions can be shown in this instrument, as are needed in the most subtle tables calculated for any cause.
The third part shall contend diverse tables of longitudes and latitudes of stars fixed for the astrolabe, tables of the declination's of the sun, tables of longitudes of cites and towns, and tables as well for the governance of a clock. All as so to find the altitude of a meridian and many other notable conclusion known by the calendar after the revered efforts of clerks, Friar J. Somes and Friar N. Lenne.
The fourth part shall describe in theory a declaration of the causal motion of the celestial bodies. The fourth part will show a table of the motion of the moon from hour to hour every day and in every sign too, called the almanac. This table follows with a law sufficient to teach you as well the manor of the operation of this all, so in conclusion as to know yourself in detail how the astrolabe shows the moon arising on the horizon by the set latitudes, and by its degree of the zodiac, and more so the arising of any of the planets on the elliptical line.
The fifth part shall be an introduction, after the statutes of medicine, for which thou must learn a great part of the general rules of theory in astrology. In which fifth part shalt thou find tables of equations of houses after the latitude of Oxford, and tables of dignities of planets, and other interesting things, that by God we would vouch safely by and to say for your maiden mother too, I behest.
To master the instrument, one must first recognize the physical layout and members that form the body schema of the astrolabe. These components serve as structural calibrations, translating the infinite alignments of planetary aspecting into localized coordinates.
The Ring (Ansa) is designed to be placed upon the thumb of the observer's right hand. By hanging free, it uses gravitational pull to establish a vertical coordinate, calibrating the instrument's baseline with the local horizon limits of the Earth.
The Turet is a swivel bracket that attaches the Ring to the body of the instrument. It moves freely in a loose socket so that no friction prevents the astrolabe from hanging true, aligning with the gravitational center of the observer's station.
The Mother (Mater) is the thickest brass plate, hollowed like a womb. It contains the coordinate plates (Tympana) and the star-web (Rete), representing the somatic frame that bounds all observation within a geocentric matrix.
Engraved on the back of the Mother is the vertical line descending from the Ring to the lowest border. The upper half is the South Line (Meridian), and the lower half is the North Line (Midnight), dividing the instrument's coordinate field.
The outer edge of the Mater, the Limbus, is divided into 360 degrees. Each quadrant contains 90 degrees, allowing precise measurements of celestial altitude and horizontal orientation.
The border is also divided into 24 equal hours, corresponding to the daily rotation of the Earth. These hours represent the standardized artificial time metric, useful for tracking durations and syncing calculations.
The zodiac ring on the Rete is divided into twelve equal parts of 30 degrees each. This partitions the ecliptic line, establishing a coordinate system for tracking the transit of celestial bodies.
The zodiac signs are named in order: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces. These represent the twelve sectors of planetary aspecting, scaling from self to universal concern.
Engraved on the back of the Mater is a scale showing the days of the year, allowing the operator to directly align any day with the Sun's specific degree on the zodiac ring.
Along the calendar scale, the twelve months are marked, reflecting the solar year and translating calendar dates into solar zodiac coordinates.
Key holy days and saints' days are marked on the calendar scale, serving as temporal reference coordinates for medieval clerks and navigators.
On the back quadrant of the Mother is the shadow scale, divided into Umbra Recta (horizontal shadows) and Umbra Versa (vertical shadows). This scale is used to calculate terrestrial heights and distances by simple geometric ratios.
The Alidade is the sighting rule on the back of the instrument. It has a sights-plate (pinnule) on each end, pierced with small holes. Directing light or sight through these holes aligns the observer with target stars or the Sun.
The Label is the rotating rule on the front of the astrolabe, used to read degrees on the outer border of the Mater when tracing coordinates from the Rete.
The central Pin (Clavis) passes through the center of the plates, locking the entire assembly together. It serves as the physical axis of rotation, representing the geocentric pivot of the cosmos.
The Horse (Cavilla) is a wedge shaped like a horse, inserted into the slot of the central Pin. It locks all the plates tight, representing the tensive focus of attention (tonos) that prevents coordinate slippage.
The thin brass plates or Tympana are interchangeable, each engraved for a specific latitude (such as Oxford). They represent the local spatial horizon and local coordinates.
Engraved on the plates are the Almucantars—circles of altitude parallel to the horizon, rising up to the zenith. They measure the height of celestial bodies above the earth.
The main line dividing the climate plate represents the local Horizon, marking the boundary of visibility where stars rise in the east and set in the west.
The Zenith is the central point on the climate plate directly above the observer, serving as the overhead coordinate for spatial orientation.
The Rete (Net) is the skeletal star-web with pointers for fixed stars (such as Alhabor/Sirius and Aldebaran). It acts as a stellar hologram, rotating over the latitude plates to track celestial transitions.
Having understood the figures of the instrument, we must now apply the mathematical rules of planetology to execute operations and trace coordinates across local and universal horizons.
Direct the alidade toward the Sun until its light shines through both pinnules. Read the altitude on the back scale. Look at the calendar scale on the back to find the day of the year, which directly aligns with the Sun's degree. This calibrates the solar coordinate for the day's homeostatic cycle.
Sight the Sun or a fixed star through the alidade's pinnules. The degree pointed to on the outer Limbus reveals the altitude above the horizon, measuring the coordinate height in the local sky.
Measure the Sun's altitude and align the Sun's zodiac degree on the Rete to the corresponding almucantar. Read the time on the outer border using the Label, and find the Ascendant sign where the zodiac ring crosses the horizon.
Rotate the Rete until the Sun's degree meets the eastern horizon line. The zodiac sign intersecting the horizon is the Ascendant, marking the rising degree and the initiation of house equations.
When measurements vary due to refractive air or manual movement, take the exact mathematical mean between two calculations. This filters out error and establishes a reliable baseline coordinate.
Align the Sun's degree on the Rete to the twilight curve (18 degrees below the horizon). The time elapsed between this point and sunrise or sunset measures the twilight duration, marking the boundaries of the spectronomic matrix.
Rotate the Rete from the point where the Sun rises in the east to the point where it sets in the west. Measure the degrees crossed on the outer border; dividing by 15 gives the day's length in equal hours.
Divide the length of the artificial day by twelve. This yields the length of one unequal hour for that day, showing how hours expand or contract with the seasonal movement of the Sun.
Measure the length of the artificial day and add the duration of both morning and evening twilight. This defines the vulgar day, measuring the total duration of light available to the local community.
Observe which hour curve on the latitude plate intersects the Sun's degree (or the nadir degree by night) on the Rete. The curve's number reveals the current unequal hour of planetary rule.
To convert unequal hours into standard equal clock hours, multiply the number of unequal hours by their calculated length and divide by 60, reconciling natural and artificial time metrics.
The first unequal hour of sunrise is ruled by the day's planet (e.g. Sunday by the Sun). Succeeding hours follow the Chaldean order (Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon) to determine the planetary hour.
Direct the alidade to the Sun at exact midday when it reaches its highest point. The height shown on the outer border is the meridional altitude, the solar zenith coordinate for that day.
Subtract the Sun's declination from its meridional altitude at noon. Subtract this result from 90 degrees. The remaining degrees reveal the latitude of your station (e.g., 51° 45' for Oxford).
Measure the Sun's midday altitude and compare it to the equinox altitude. The difference is the declination north or south of the celestial equator, mapping the Sun's seasonal variance.
Measure the Sun's altitude at noon. Find the corresponding declination and look it up on the calendar ring on the back of the Mother to find the date, reversing space into calendar time.
Align the Rete with the stars in the night sky. Find the degree of the zodiac that lies on the meridian line at the same moment as the star. This defines the star's longitude in the celestial matrix.
Rotate the Rete until the star's pointer meets the eastern horizon line. The degree of the zodiac sign crossing the horizon at that moment rises with the star, defining its rising coordinate.
Set the Rete for the star's coordinates and observe where the star-pointer meets the horizon. This marks the star's rising and setting boundaries in your local topological sector.
Compare the planet's position with the ecliptic ring on the Rete. The angular distance north or south of the ecliptic line measures the planet's latitude within the solar system.
Measure the altitude of the North Star (Polaris) at its highest and lowest points during the night. The mathematical mean of these two altitudes is the exact latitude of your geographic station.
Measure the Sun's altitude at noon on the day of the equinox (Aries 1st point). Subtract this altitude from 90 degrees to find the latitude of your location, locking geographical coordinates via solar alignment.
Determine your latitude, then locate the corresponding degree circle on the plates. The point directly vertical to this altitude at the meridian intersection is the Zenith, the overhead center.
Using the unequal hour lines and the horizon curves, divide the sky above and below the earth into twelve sectors or houses, establishing the fields of planetary aspecting.
Observe the planet's daily transit. If its zodiac degree increases, its motion is direct; if the degree decreases, it is retrograde, tracking coordinate reversals in real-time.
Set the Rete for the equator (right circle). Rotate the zodiac sign across the meridian line and measure the degrees crossed on the Limbus to find the sign's right ascension.
Rotate the chosen zodiac sign across the oblique horizon line of your local plate (e.g. Oxford) and measure the degrees crossed on the Limbus, determining its oblique ascension.
Measure the right ascension of a star from the 1st point of Aries along the equinox line. This calculates its stellar longitude relative to the equator's coordinate grid.
Observe the intersections of the horizon, meridian, and equinoctial lines on the plates. This defines the cardinal directions—East, West, North, South—in the local space.
Place the astrolabe flat on a level surface. Align the South Line with the shadow cast by a vertical pin at noon, orienting the instrument to the physical compass coordinates of the Earth.
Find the intersection of the Sun's degree with the azimuth lines on your plate. This measures the horizontal direction or compass bearing of the Sun at that moment.
Adjust the Rete until the azimuthal lines align with the local horizon, locking horizontal bearing calculations in sync with the celestial aspect angles.
Measure the Sun's altitude. Find the corresponding almucantar line on your plate and align the Sun's zodiac degree with it. Read the hour directly on the outer border using the Label.
Measure the Moon's altitude. Locate its degree relative to the fixed stars on the Rete. Compare with the almanac to calculate its exact zodiac degree, mapping lunar transitions.
Measure the Moon's meridional altitude. Subtract the equator altitude; the result is the Moon's declination north or south, charting its complex orbit relative to the equator.
Rotate the Rete in 30-degree increments of ascension starting from the Ascendant degree. Note where these points cross the horizon to define the boundaries of the twelve houses.
Align the Sun's nadir degree (180 degrees opposite the Sun) with the nocturnal hour curves beneath the horizon, calculating planetary rulers of the night.
Align the alidade with the sun or target height and read the shadow index on the Umbra Recta scale. This determines heights by shadow lengths and geometric ratios.
Measure the angle of an object's vertical shadow on the Umbra Versa scale. Use the ratio to calculate distances, translating shadow scales into spatial lengths.
Observe the planet's altitude. Align its Rete position with the altitude curve, and read its ascension degree on the Limbus to locate it in the timespace matrix.