Al‐Malik Al‐Ashraf (Mumahhid Al‐Din) Umar Ibn Yūsuf Ibn Umar Ibn Alī Ibn Rasul (), also as Umar Ibn Yusuf (or also Al-Asharaf Umar II) was the third Rasulid sultan and also an mathematician, astronomer and physician.

Biography

Umar Ibn Yusuf was born in 1242 in Yemen and he died in 1296. He is known for writing the first description of the use of a magnetic compass for determining the qibla. Also, his works on astronomy contain important information on earlier sources. In a treatise about astrolabes and sundials, al-Ashraf includes several paragraphs on the construction of a compass bowl (ṭāsa). He then uses the compass to determine the north point, the

meridian Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to Science * Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon * ...

(khaṭṭ niṣf al-nahār), and the Qibla towards Mecca. This is the first mention of a compass in a medieval Islamic scientific text and its earliest known use as a qibla indicator, although al-Ashraf did not claim to be the first to use it for this purpose. http://www.uib.no/jais/v001ht/01-081-132schmidl1.htm#_ftn4 We owe him a famous astrolabe made in 1291, actually in the New York Metropolitan Museum of Art.MET picture
— Sharon Kinoshita ''The Painter, the Warrior, and the Sultan: The World of Marco Polo in hree Portraits'', The Medieval Globe, vol. 2, n° 1, 2016, p. 120 sq.

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1242 births 1296 deaths 13th-century Arabs 13th-century astronomers Astronomers of the medieval Islamic world Yemeni astronomers Rulers of Yemen 13th-century rulers in Asia Rasulid dynasty {{Astronomer-stub