Astronomy Glossary

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This Glossary deals mostly with Arabic Terms that have been adopted by Medieval European scholars and given European names as below. The discussion of the history of changes in Arabic terms is included whenever needed.

علم الهيئة/علم هيئة الفلك/علم الفلك
Science of Astronomy

"3elm Al-Hay2a" is the Arabic name used by Avicenna (Ibn Sina) and the other leading Muslim scholars of the medievals. It means "Science of the Appearance of the Celestial Sphere". The modern form of this Arabic name is (Al-Falak الفلك) or the "Science of Celestial Sphere".

Al-Aflaak الأفلاك

Although --in form-- this word is plural of "AlFalak" in Arabic, it is not related to it. The Arabic word "Al-Aflaak" seems to have changed in meaning over the ages from the --perhaps-- original Concentric Celestial Domes. Perhaps that's why the latest of its meaning (Orbit) had to take on a new word in Arabic (Al-Madaar)(المدار). Since "Al-Falak" (singular of Al-Aflaak) means originally Celestial Sphere/Dome, we can assume that Al-Aflaak are actually concentric spheres/domes. This meaning may have been the original one thinking that the 'Sky Dome' is made up of several translucent Domes each has some stars printed on them layer after layer. This works well anyways in the case of Horizontal Coordinates.
Example from Holy Quran:
(وَهُوَ الَّذِي خَلَقَ اللَّيْلَ وَالنَّهَارَ وَالشَّمْسَ وَالْقَمَرَ كُلٌّ فِي فَلَكٍ يَسْبَحُونَ) (Al-Anbiaa 33)

الفلك/الفلك الأعلى
Celestial Sphere

The surrounding Celestial Sphere --as seen from its centre at Earth-- appears as a dome which rotates around the Observer. When Earth became perceived as a Globe, the 'dome' concept evolved to a complete Sphere. Al-Falak Al-A3la (الفلك الأعلى) is the name of this conceptual huge dome/sphere in Arabic and from which the science of Astronomy in Arabic has got its name evolving from the medieval "3elm Hay2ah Al-Falak" which meant literally "Science of the Appearance of the Celestial Dome/Sphere"


They are groups of stars in the sky that have been given a single name such as Orion (Al-Jabbaar الجبار) and which usually could be noticed to represent graphically the form of the name. For example, Orion is in the form of a warrior with a belt and a sword holding a club and a shield ahead. There are 88 contellations covering the entire inner-surface of what is called the Celestial Sphere.

دائرة البروج

Those are certain fixed constellations on the inner-surface of the Celestial Sphere which the Sun passes on during the year--each year it completes a cycle. Otherwise, the Zodiac constellations bear no special characteristics among the other 88 constellation of the sky.

فلك البروج

It's the fixed Circle on the Celestial Sphere which the Sun passes on every year. The Circle is not matching with the Celestial Equator (see Equatorial Coordinates) and is inclined to it by approx 23.5 Degrees. The Ecliptic is intersecting with the Celestial Equator at two points: Vernal Equinox and Autumnal Equinox (الاعتدال الربيعى and الاعتدال الخريفى).


The exact single point right above the head of the Observer at the Celestial Dome. If the star is not exactly at this highest point of the dome, then it's not at the Zenith.
لم يتسامت برج الجبار الليلة

دائرة عظيمة
Great Circle

In Spheres, it's the Circle on the surface that passes by the centre of the sphere. The diameter of a Great Circle is the same as the sphere it's drawn on its surface. Meridians of Longitudes are Great Circles.

Celestial Meridian

A Great Circle passing by the Zenith and the Celestial North Pole (in Northern hemisphere). Think of it as Celestial Longitude.


It's the apparent meeting or passing of two or more celestial bodies in the same degree of the zodiac.

زاوية ارتفاع
Altitude Angle

The vertical angle of a celestial body as measured from Horizon to the body.

البزوغ (أو الإرتفاع) الشمسى
Heliacal Rising

The first rising of a star after and last setting before its invisibility owing to its conjunction with the Sun.


When one planet is on a straight line with another planet with the Sun on the middle of this line.
اليوم يقع كوكب الزهرة فى مداره بحيث يستقبل كوكب الأرض

Excerpts from Prolegomena by Ibn Khaldoun

You may like now to excercise your understanding on the following parts taken directly from the introductory part of Ibn Khaldoun's section on Geography of the world.
نقول إن قطبي الفلك الجنوبي و الشمالي إذا كانا على الأفق فهنالك دائرة عظيمة تقسم الفلك بنصفين هي أعظم الدوائر المارة من المشرق إلى المغرب و تسمى دائرة معدل النهار و قد تبين في موضعه من الهيئة أن الفلك الأعلى متحرك من المشرق إلى المغرب حركة يومية يحرك بها سائر الأفلاك في جوفه قهراً و هذه الحركة محسوسة و كذلك تبين أن للكواكب في أفلاكها حركة مخالفة لهذه الحركة و هي من المغرب إلى المشرق و تختلف آمادها باختلاف حركة الكواكب في السرعة و البطء و ممرات هذه الكواكب في أفلاكها توازيها كلها دائرة عظيمة من الفلك الأعلى تقسمه بنصفين و هي دائرة فلك البروج منقسمة باثني عشر برجاً و هي على ما تبين في موضعه مقاطعة لدائرة معدل النهار على نقطتين متقابلتين من البروج هما أول الحمل و أول الميزان فتقسمهما دائرة معدل النهار بنصفين نصف مائل عن معدل النهار إلى الشمال و هو من أول الحمل إلى آخر السنبلة و نصف مائل عنه إلى الجنوب و هو من أول الميزان إلى آخر الحوت و إذا وقع القطبان على الأفق في جميع نواحي الأرض كان على سطح الأرض خط واحد يسامت دائرة معدل النهار يمر من المغرب إلى المشرق و يسمى خط الاستواء
Astronomy Glossary Cover