Resources / Celestial timing

The Astrolabe Compass

Hold a spherical astrolabe of the sky around you: its horizon and compass rose turn with your phone to true north, while the sky turns on its pole by the clock.

Lay the phone flat for the sky map, or press the sphere into the plate an astrolabe maker would have cut for your latitude. The old orientations, the winds of Athens, the quarters of the Golden Dawn, the camps of the angels and the twenty-four mountains among them, are laid on the horizon one tradition at a time, each with its source.

Raising the sphere for your place and moment

The tradition

The planispheric astrolabe is the sky projected from the south celestial pole onto the plane of the equator, the construction of Ptolemy's Planisphaerium: a mother holding plates cut for a latitude, a rete of star pointers turning over them, and a rule laid across both. Chaucer wrote its English treatise for his son in 1391. Its three-dimensional form, the spherical astrolabe, carries the plate's lines on a globe under a turning cage; al-Nayrīzī described it, and the only complete example known is in Oxford (inv. 49687, dated 1480/81).

An astrolabe was laid true by the Sun. Chaucer sets its meridian line "evene south" (II.29) and finds the meridian with a round plate of metal laid level on the ground and a pin set upright at its centre, marking where its shadow crosses a circle morning and afternoon (II.38), as Vitruvius and the Kaogongji do with a gnomon. The needle came later: Shen Kuo noted in 1088 that it points "slightly east, not fully south", and compass sundials of the sixteenth and seventeenth centuries engraved the local variation beside their meridian. This instrument does the same with the World Magnetic Model.

Traditions oriented themselves in different frames: by the horizon (the winds, the twenty-four mountains), by the body of a speaker who faces a quarter (Sefer Yetzirah, the Golden Dawn, the camps of the angels), by the sky (the signs, the mansions, the stars) and by the Sun's risings and settings. Here each is a plate laid in the mater, one at a time, with its frame and its source.

Reading the instrument

The Sphere is the spherical astrolabe seen from just behind and above you. At its centre stands your horizon, a gilt disc with a compass rose and a fleur-de-lis on true north, and, while it follows the phone, an M where a magnetic needle would point; the bright circle rising from it to the zenith is the line you face. The horizon ring, the meridian and the plate's lines stay on the world and turn only as you turn. The equator, the tropics, the ecliptic and Chaucer's star pointers turn with the sky about the pole, by the clock. What lies before you is drawn firm and what lies behind you faded; whatever is below the horizon is dotted.
The Map is the same sky projected onto the ground from the point beneath your feet, with the zenith at the centre, the horizon at the rim and the direction you face at the top, so that right on the screen is right in the world. Lay the phone flat while it follows the compass and the Sphere settles into the Map; only the visible sky is drawn, and bodies below the horizon wait just outside the rim.
The Plate is the flat astrolabe a maker would have cut for your latitude. The limb carries Chaucer's twenty-four letters for the hours, or the twelve Chinese double hours; the rule laid over the Sun reads apparent solar time on it. The curved lines below the horizon are the unequal hours, read by the Sun among them at night and by day by its nadir, the opposite point of the ecliptic (II.12). As engraved shows the plate throne up, as a maker drew it; Laid true, offered while the phone lies flat and follows the compass, turns it so that its lines lie as they lie in the world, as Chaucer laid his astrolabe on the ground (II.29).

Where it descends from

Chaucer, A Treatise on the Astrolabe (ed. Skeat): The English treatise of 1391: the parts of the instrument, its limb letters and unequal hours, and the conclusions worked with it; Skeat's note gives the rete's stars and their names, most of them as his manuscript writes them.
Ptolemy, Planisphaerium: The projection of the sphere from its south pole onto the plane of the equator, on which every astrolabe plate is drawn.
Sacrobosco, De sphaera: The medieval schoolbook of the sphere's circles: horizon, meridian, equator, tropics, colures and polar circles.
al-Nayrīzī on the spherical astrolabe (ed. Seemann and Mittelberger): The earliest full treatise on the instrument, edited with the Castilian treatise of Alfonso X's astronomical books.
History of Science Museum, Oxford, spherical astrolabe inv. 49687: Signed by Mūsā and dated 1480/81: a globe engraved with almucantars and azimuths under a cage rete, the only complete spherical astrolabe known.
Meeus, Astronomical Algorithms: The methods for sidereal time, nutation, refraction, the Moon, rising and setting, and the apparent places of the planets.
Bretagnon and Francou, VSOP87: The planetary theory the planets' places are computed from, shortened for the browser and checked against JPL Horizons from 1800 to 2200.
NOAA NCEI and BGS, World Magnetic Model 2025: Magnetic declination: World Magnetic Model 2025 (NOAA NCEI and British Geological Survey), public domain.
Stuart and Revett, The Antiquities of Athens: The survey of 1762 that recorded the Tower of the Winds, whose eight faces carry the winds of the Tower plate.
Agathemerus on Timosthenes: The twelve winds of Timosthenes of Rhodes, four of them defined by where the Sun rises and sets at the solstices.
Huainanzi: Chapter 3: the four directions with their phases, beasts and seasons, the centre, and the solar terms.
Shuogua: The Discussion of the Trigrams, whose fifth chapter gives six trigrams their directions outright, Dui its season (mid-autumn, the west) and Kun none.
Sefer Yetzirah: The six directions sealed with the six orders of Yod, Heh and Vav (1:13), as the Warsaw printing gives them.
Numbers Rabbah: The four angels about the throne, set by the camps of the tribes in Numbers 2 (2:10).
Pirkei de-Rabbi Eliezer: The four camps of angels about the Presence, Uriel before and Raphael behind (chapter 4), as the bedtime Shema also sets them.
Regardie, The Golden Dawn: The Order's papers, with the Lesser Ritual of the Pentagram and its names and archangels at the quarters.
Marinus, Proclus: His pupil's life of Proclus, who greeted the Sun at its rising, at noon and at its setting.
Corpus Hermeticum: Treatise XIII, whose hymn is said facing the south wind at sunset and the east wind at sunrise (Mead's translation).
Shen Kuo, Mengxi bitan: The notebooks of 1088 in which the lodestone needle points "slightly east, not fully south".
North, Horoscopes and History: The house systems of Campanus and Regiomontanus, which divide the sphere through the north and south points of the horizon.

What this does not claim

The astrolabe draws the sky exactly for your latitude and moment: its lines are computed, not traced, and its stars, Sun, Moon and planets agree with reference ephemerides to seconds of arc. What it cannot know exactly is which way your phone points. Phone compasses read magnetic north, which the instrument corrects with the World Magnetic Model; the result is usually good to a few degrees, and worse near metal. The traditions laid on its plates are shown as their sources give them, one at a time and with their disagreements. They describe how people oriented themselves; the instrument measures nothing about what the directions mean.

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