Use Sunquest to determine latitude and direction

The Sunquest sundial itself can be used to determine latitude and direction from the sun.

Sunquest-001-8Start with an approximate north orientation and latitude setting.  Early in the day, turn the gnomon on its axis until the sun shines broadly through the slot, falling upon the time-scale between two shadows.  Observe which portion of the gnomon slot permits the sunlight to reach the time-scale.  Now mask, with tape or paper on the gnomon slot, all but the small square of light reaching the time scale ridge.  Watch this small light patch as it progresses along the time scale at intervals during the day and toward late afternoon, turning the gnomon on its axis as necessary.  If the light patch wanders off the time scale the settings need correction.

Next sunny day repeat, but turn the entire sundial on its vertical axis about the base plate (loosening the bolt through the pedestal halves) in a direction to cause the light-patch to return to the time scale ridge at equal intervals before and after noon, even though it wanders off in mid day.  Then this occurs, hold the orientation or direction but increase or decrease the angular elevation of the gnomon axis until the light patch has a consistent path along the time-scale ridge throughout any one day.

This procedure may take several days (not necessarily consecutive) with a new position of the mask for each day’s observation.  Then the plane of the latitude crescent (and the axis of the gnomon) serve as a pointer to true north,, and the latitude can be read on the latitude scale.  A latitude-and-direction finder!

Text by Richard L. Schmoyer, 1983.