BC Gnomonics
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Gunter's Quadrant
Latitude specific, Altitude
Categorized as an astrolabe just as readily as a sundial, Gunter's quadrant is a powerful instrument. This quadrant can perform many of the same functions as a planispheric astrolabe, despite constituting just one engraved surface with no moving parts apart from a plumb line and bead.
This material efficiency comes at the cost of conceptual ease: this quadrant does not present an intuitive model of the celestial sphere. Several arcs on this quadrant serve multiple purposes and take different conceptual roles depending on the time of year. For example, a given point on the ecliptic can represent four different dates, and the horizon arc can represent both the segment of the horizon between the equator and tropic of cancer and the equator and the tropic of capricorn. Use of this quadrant requires a higher degree of conceptual understanding than other portable dials.
These quadrants are latitude dependant and are analogous to a single tympan of an astrolabe.
If designed for latitudes below ~47º North, the azimuth lines begin to overlap the hour lines, potentially confusing operation and adding to the already conceptually dense mesh of crisscrossing lines. For this reasons, azimuth lines may be left off, though this naturally reduces the instrument's functionality.
Gunter's Quadrants often include shadow squares, though this space may alternatively be taken up by a calendar scale.
How to use (finding time with the sun)
1. Tilt the quadrant back until the thread falls on the date scale marking the date of operation.
The quadrant features two sets of hour lines: the set that arcs to the left is used during fall and winter. The set that arcs to the right is used during spring and summer.
2. With the thread oriented to the current date, slide its bead onto the 12:00pm hour line.
3. With the bead set, let the string fall loose. The dial is now ready to be used.
4. Tilt the dial up so that the shadow of the forwardmost sighting hole falls on the second sight. With this done, the dial is now pointing directly at the sun.
5. Take note of where the string is positioned on the hour scale and clamp it down. The position of the bead on the season-appropriate hour scale indicates the current time.
The top labels of the hour scale are used when taking a reading before noon, and the lower labels are used for the afternoon.
6. To determine civil time, add the appropriate longitude correction and equation of time correction (more information here). If applicable, add one hour during daylight saving time.