astrolabe

Subtype evidence

Record the physical evidence supporting a subtype assignment and any conflicting features.

1. Which combination of celestial representation, graduated body, suspension and sighting components supports the proposed subtype? definition
2. What observed feature distinguishes this instrument from a quadrant, sextant, armillary sphere or modern planisphere? boundary

Artefact status

Record whether the instrument is an original, reproduction, reconstruction or decorative interpretation without assuming that age determines functionality.

1. What evidence establishes whether this is a historical instrument, later reproduction, reconstruction or decorative object? provenance
2. Which supported operations have been demonstrated rather than inferred from appearance? action

Assembly coherence

Record how the body, applicable plates and rotating or sighting parts fit together and which are absent or replacements.

1. Which body, plate, rete, rule, alidade, pivot and suspension elements are present and expected for this subtype? definition
2. What evidence shows that interchangeable or replacement components belong together geometrically and mechanically? provenance

Representation applicability

Establish the projection, orientation and latitude dependence where present, explicitly marking their absence in subtypes that do not use them.

1. What projection, celestial orientation and horizon geometry are represented, if any? definition
2. For which latitude and hemisphere is each plate or configuration intended, and how was that applicability established? measurement

Graduation interpretation

Record scale units, origin, direction and any conversion required before reporting a result.

1. Which scales express altitude, zenith distance, time, calendar position or another quantity, and where does each begin? definition
2. What is the smallest readable division of each operational scale, including any damaged or ambiguous regions? measurement

Reference identification

Connect labels and pointers to evidenced celestial identities, epochs and calendar conventions.

1. Which stars or celestial positions do the pointers represent, and what reading or comparison supports each identification? provenance
2. What epoch and calendar conventions are evidenced, and how do unresolved assumptions limit the proposed calculation? boundary

Sighting procedure

Record a procedure appropriate to the instrument and target, including alignment and safe solar observation constraints.

1. How must this instrument be suspended or supported, aligned and read to obtain the intended angle? action
2. If solar observation is proposed, what documented shadow or projection procedure avoids direct viewing of the Sun? action

Operation validity

Associate each proposed calculation with its necessary inputs, applicable geometry and observed uncertainty.

1. Which operation is supported by the actual markings and components, and what date, location or observation inputs does it require? action
2. How closely does its result agree with an independent reference under recorded conditions, and what limits that comparison? measurement

Performance-affecting defects

Connect observed wear, distortion or corrosion to particular reading errors or handling failures.

1. Is pivot play, binding, body distortion or suspension damage present, and how does it affect alignment or movement? measurement
2. Which damaged graduations, inscriptions or star pointers prevent particular readings or identifications? boundary

Operation and conservation limits

Record restrictions on suspension, rotation, plate removal and cleaning based on the instrument's condition and custodial requirements.

1. May the instrument be lifted by its suspension, rotated or opened to exchange plates, and what condition evidence supports each permission? action
2. Which proposed cleaning, lubrication or adjustment requires conservation assessment because it could alter markings, surfaces or original components? action

Check these first

Recalled without web access and unsourced; every item is a lead to verify.

• The definition centers on the planispheric astrolabe; the mariner's astrolabe is a simpler altitude-measuring instrument without its projected sky calculator.
• The altitude range describes the measured quantity, not a universal specification for instrument graduations, dimensions or precision.
• No modern product standard or universal identifier is asserted; historical instruments vary substantially in construction and scales.

1. Which of these check these first hold for the sense of astrolabe this model covers, and on what evidence? provenance

Kinds and varieties

Recalled without web access and unsourced; every item is a lead to verify.

• Planispheric astrolabe
• Universal astrolabe
• Spherical astrolabe
• Mariner's astrolabe

1. Which of these kinds and varieties hold for the sense of astrolabe this model covers, and on what evidence? provenance

Real-world use

Recalled without web access and unsourced; every item is a lead to verify.

• Measuring the altitude of the Sun or a visible star above the horizon.
• Determining local time from celestial observations and known date and latitude.
• Solving astronomical problems involving rising, setting and the positions of celestial objects.
• Supporting historical prayer-time determination.
• Estimating latitude at sea using a mariner's astrolabe and appropriate astronomical information.

1. Which of these real-world use hold for the sense of astrolabe this model covers, and on what evidence? provenance

Typical measurements

Recalled without web access and unsourced; every item is a lead to verify.

• Celestial altitude above the horizon - 0-90 - degree

1. Which of these typical measurements hold for the sense of astrolabe this model covers, and on what evidence? provenance

Failure modes and hazards

Recalled without web access and unsourced; every item is a lead to verify.

• A bent sighting rule, displaced sights or worn pivot introduces angular error.
• Using a latitude-specific plate at an unsuitable latitude produces incorrect results.
• Incorrect date settings or star identification produces incorrect time or position estimates.
• Wind, vessel motion and failure to suspend the instrument vertically impair altitude measurements.
• Looking directly at the Sun through the sights can injure the eyes; historical solar observations could use shadow alignment.

1. Which of these failure modes and hazards hold for the sense of astrolabe this model covers, and on what evidence? provenance

Regional variation

Recalled without web access and unsourced; every item is a lead to verify.

• Islamicate instruments may include Arabic or Persian inscriptions and scales supporting religious timekeeping.
• European instruments often use Latin or vernacular inscriptions and locally appropriate latitude plates.
• Portuguese and Spanish maritime traditions used robust mariner's astrolabes adapted for observations at sea.

1. Which of these regional variation hold for the sense of astrolabe this model covers, and on what evidence? provenance

Neighbouring kinds and how to tell them apart

Recalled without web access and unsourced; every item is a lead to verify.

• Quadrant - A quadrant uses a quarter-circle scale; a conventional planispheric astrolabe uses a circular projected sky with a rotating star framework.
• Sextant - A sextant measures angular separation using double reflection, whereas an astrolabe normally measures altitude with a sighting rule and gravitational vertical.
• Armillary sphere - An armillary sphere represents celestial reference circles with rings; a planispheric astrolabe represents them through a flat projection.
• Planisphere - A modern handheld planisphere primarily shows which stars are visible for a date and time; an astrolabe adds observational and astronomical calculation functions.

1. Which of these neighbouring kinds and how to tell them apart hold for the sense of astrolabe this model covers, and on what evidence? provenance