SUNDIALS

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This is the oldest post in category SUNDIALS AND CLOCKS

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Synopsis:
In the 14th century the astronomer ibn al-Shatir in Damascus tilted the face of his sundial to be inclined at an angle of 56.5 degrees (90 degrees minus his latitude).
This made the face of the sundial parallel to the plane of the equator, so the sundial told time the same way as a sundial at the North Pole. It divided the day into 24 hours of equal length.
A problem was that prayer time in Islam is instead based on sundial hours that are 1/12 of the time from sunrise to sunset.
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550px-moothallsundial

Photograph of a modern vertical declining sundial similar to ibn al-Shatir’s sundial
(Ibn al-Shatir would not have used Roman numerals.)

VERTICAL DECLINING SUNDIAL
In the 14th century the astronomer ibn al-Shatir in Damascus tilted the face of his sundial to be inclined at an angle of 56.5 degrees (90 degrees minus his latitude).
This made the face of the sundial parallel to the plane of the equator, so the sundial told time the same way as a sundial at the North Pole. It divided the day into 24 hours of equal length.
A problem was that prayer time in Islam is instead based on sundial hours that are 1/12 of the time from sunrise to sunset.
Another problem is that in some locations, no shadow appears.

EARLIER SUNDIALS
Prior to ibn al-Shatir, accurate sundials needed a different set of markings for each astrological month of the year, and the markings had to be custom designed for your latitude. [Because prayer time is based on sundial time,  people prefered that the markings be for hours that were 1/12 of the time from sunrise to sunset, but it was possible to create markings that were for hours that were 1/24 of a day.]
Various Greeks (such as Theodosius of Bithynia in the 2nd century BC) and Muslims (such as al-Khwarizmi in the 9th century and Abu Ali al-Hassan al-Marrakushi in the 13th century) had attempted to devise sundials that solved some of the problems of an ordinary sundial.

Image by Wilson44691, via Wikimedia Commons.
image credit https://commons.wikimedia.org/wiki/File:MootHallSundial.JPG#mw-jump-to-license

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