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circumstantial evidence indicates that the marine sandglass was probably invented by Muslims.

Abū Ishāq Ibrāhīm al-Zarqālī (1029–1087) published an improved estimate of the length of the Mediterranean Sea that was only 67% of the estimate made many centuries earlier by Ptolemy. Al-Zarqālī did not say how he made the estimate of longitude.
Al-Zarqālī could have had someone record star positions before departing, record star positions after arriving, and cabin boys could have turned over primitive marine sandglasses to record elapsed time. Thus he would have known the difference in local time between the 2 locations.

Marine sandglasses were hung from a low ceiling to keep them vertical, and were sealed to prevent moisture from entering.

Europeans did not have glass-blowing technology until centuries later, but Muslims could have learned glass blowing from the Jews in Israel. Muslims had more access to dry sand than Europeans did. Muslims wrote about the longitude of a city im China.


[A painting of a Muslim observatory in what is now Istanbul Turkey depicts a sandglass similar to a marine sandglass, but the painting is from the 1500s.]

In or before the year 1300, European Christians, who lagged behind the Muslims in glass-blowing technology, were purchasing marine sandglasses for their ships.

image by illustrator in “The new American practical navigator”, Nathaniel Bowditch and Jonathan Ingersoll Bowditch, 1859, via Wikimedia Commons.
image credit

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