AL-KASHĪ’S SLIDE RULE

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In the early 1400’s, astronomer al-Kashi invented a table-sized linear interpolation device. Unlike modern slide rules, it could not do multiplication. It had markings at equal intervals, not at logarithmic intervals.

It was used for astrology purposes such as calculating when Jupiter would align with Mars.
To align non-adjacent sliding parts, it did not use a fine central hairline in a sliding cursor, but a pivoting part.
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250px-Slide_rule_cursor

photograph of a 20th century hand-held slide rule

In the early 15th century, al-Kashi (who was the first director at prince Ulugh Beg’s observatory) invented a table-sized linear interpolation device. Muslim websites call it an “analog computing device”.

Unlike modern slide rules, it could not do multiplication. It had markings at equal intervals, not at logarithmic intervals. It was used for astrology purposes to calculate the time of conjunctions of heavenly bodies (such as when Jupiter aligns with Mars).

It had 2 grooves into which were inserted 3 sliding rulers. In a 20th century slide rule, the non-moving portion with markings is shaped like a long rectangle, but al-Kashi’s was triangular in shape. To align non-adjacent sliding parts, 20th century slide rules use a
fine central hair line in the transparent sliding cursor, but he used a pivoting part instead of the hair line.
He called his slide rule device the “Plate of Conjunctions”. [He also built another device called the “Plate of Zones”, that was more complicated, and could do such things as predicting eclipses of the sun.]
To set up the positions of the sliding pieces for a calculation, an almanac of rising and setting times of heavenly bodies needed to entered for a day prior to the event and for a day after the event.
An alhidade (for sighting the position of a heavenly body in the sky) was used with his “Plate of Conjunctions”.
Muslim websites praise the “Plate of Conjunctions”, describing it as the invention of the analog computer.

A scientific article by E. S. Kennedy of the History of Science Society describing the device was printed in 1947 by the University of Chicago Press, and can be read for free by following the instructions athttps://www.jstor.org/stable/225450?seq=1#page_scan_tab_contents for registering as a scholar.

Image by en.wikipedia
image credithttps://en.wikipedia.org/wiki/Slide_rule

 

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