Ancient Persian astronomy, spanning from the Achaemenid Empire (550–330 BCE) to the fall of the Sassanid Empire (224–651 CE), represents a crucial link in the transmission of astronomical knowledge from antiquity to the medieval world. Located at the crossroads of great civilizations, Persia absorbed, preserved, and enriched Babylonian, Greek, and Indian knowledge, playing a key role in transmitting it to the Islamic world, which dominated astronomy for nearly eight centuries.
Islamic astronomy did not develop ex nihilo after the Arab conquests of the 7th century. It was built on the solid foundations established by Sassanid Persian astronomers, who themselves inherited and transformed millennia-old Mesopotamian traditions.
N.B.:
The Achaemenid Persians (6th–4th century BCE) and the Sassanids (3rd–7th century CE) are the two great imperial dynasties of ancient Persia. The Achaemenids built a vast multiethnic empire with a structured administration, while the Sassanids centralized power around the teachings of the prophet Zoroaster and consolidated the administrative apparatus.
When Cyrus II the Great (c. 600–530 BCE) conquered Babylon in 539 BCE, he appropriated a millennia-old scientific heritage. The Achaemenid Persians quickly adopted Babylonian astronomical methods, particularly ephemerides and eclipse prediction techniques developed since the 8th century BCE.
Cuneiform tablets from the Achaemenid period, discovered in Babylon and Uruk, attest to the continuity of Babylonian astronomical observations under Persian rule.
Persian astronomers commented on, criticized, and improved ancient texts. This tradition of scientific synthesis characterized nascent Islamic astronomy, where scholars of diverse origins (Arab, Persian, Turkic, Andalusian) collaborated in a common scientific language: Arabic.
N.B.:
The Academy of Gundishapur, founded in the 6th century under the Sassanids, was the leading intellectual center of its time and served as a model for the House of Wisdom in Baghdad. It combined medical, astronomical, mathematical, and philosophical teaching, attracting scholars from across Western Asia.
| Period / Date | Event or Contribution | Significance | Legacy |
|---|---|---|---|
| 539 BCE | Conquest of Babylon by Cyrus II | Adoption of Babylonian observations (planets, eclipses) | Continuity of observations in Babylon and Uruk |
| Achaemenid Period (550-330 BCE) | Development of the Zoroastrian calendar | Year of 365 days, seasonal stabilization | Used until the Islamic period (8th–15th century CE) |
| Achaemenid Period | Adoption of the Babylonian sexagesimal system | Circle in 360°, hour in 60 min, trigonometric base | System used worldwide today |
| Seleucid Period (312-63 BCE) | Introduction of Greek astronomy | Fusion of Greek models and Babylonian data | Basis of medieval mathematical astronomy |
| Seleucid Period | Adoption of the Greek zodiac and epicycles | Geometric models for planetary motions | Foundation of the Ptolemaic system in Persia |
| 224-242 CE | Reign of Ardashir I | Calendar reform, seasonal correction | Improvement of calendar accuracy |
| 3rd-6th century CE | Persian horoscopic astrology | Babylonian, Greek, and Indian fusion | Influence on Islamic and European astrology |
| 531-579 CE | Reign of Khosrow I Anushirvan | Creation of the Academy of Gundishapur | Major intellectual center before Baghdad |
| Around 550 CE | Introduction of Indian concepts: sine, zero | Improved trigonometry and calculations | Adoption by Islamic astronomers |
| 6th century | Translations of Greek and Indian works | Direct access to the Almagest and numerical methods | Preservation of ancient texts |
| 6th century | Introduction of Indian astronomy (Siddhanta) | Sine function for angular calculations | Enrichment of calculation methods |
| 6th-7th century | Perfection of the planispheric astrolabe | Universal instrument for calculations and navigation | Widely used in the Islamic and European world |
| 6th-7th century | Systematic observations of eclipses | Refinement of orbital parameters | Revision of Ptolemaic parameters |
| Late Sassanid Period | Compilation of the Zīk-i Shahriyārān | Hybrid tables: Babylonian, Greek, and Indian | Model for the first Islamic zijes |
| Late Sassanid Period | Calculation of the precession of the equinoxes | Quantification of the slow movement of Earth's axis | Refinement by Al-Biruni and Islamic astronomers |
| 632-651 CE | Reign of Yazdegerd III | Last Sassanid calendar, astronomical reference | Used by Islamic astronomers |
| 633-654 CE | Arab conquest of Persia | Transmission of Persian methods and tables | Continuity of the Persian astronomical tradition |
| 762 CE | Foundation of Baghdad | Astrological calculation for the city's location | Beginning of the golden age of Islamic astronomy |
| Around 770 CE | Al-Fazari compiles the first Arabic zij | Arabic numerical corpus based on Sassanid tables | First zij of the Islamic world |
| Around 820 CE | Al-Hajjaj translates the Almagest | Dissemination of the Ptolemaic model in Arabic | Basis of classical Islamic astronomy |
| 830 CE | Al-Khwarizmi publishes his zij | Persian, Indian, and Greek synthesis | Model for subsequent zijes for 3 centuries |
Source: Encyclopaedia Iranica and Institute for the History of Arab and Islamic Science.
Alexander the Great's conquest and the Seleucid period introduced Greek astronomy to Persia. The geometric models of Hipparchus and Ptolemy complemented Babylonian arithmetic methods. The Babylonian-Greek synthesis, centered on epicycles and deferents, paved the way for medieval mathematical astronomy, adopting zodiacal divisions and ecliptic longitude.
Under the Sassanids (224–651 CE), especially Khosrow I, Persia became a major intellectual center through the Academy of Gundishapur. A great synthesis occurred between Babylonian, Greek, and Indian traditions: translations of the Almagest and the Siddhanta, introduction of Indian trigonometry, and creation of Persian astronomical tables (zīk) combining observations and geometric models.
The Persians used and perfected instruments inherited from ancient civilizations: gnomon, sundials, armillary sphere, and astrolabe. Observations were methodically recorded in astronomical journals, allowing for the refinement of models and the detection of the limits of the Ptolemaic system.
The calendar, linked to Zoroastrianism, initially 365 days without correction, was progressively reformed. Under Ardashir I and Yazdegerd III, it became more accurate, reflecting Zoroastrian cosmology with 12 months dedicated to the Amesha Spentas and the yazatas, thus integrating astronomical observation and religious piety.
Astrology was inseparable from astronomy and influenced royal decisions. Cosmology conceived the universe as a creation of Ahura Mazda, with a correspondence between the planets and the Amesha Spentas. The concept of Zervanism led to a cyclical vision of time, prefiguring the study of the precession of the equinoxes by Islamic astronomers.
Sassanid astronomers compiled sophisticated tables, called zīk, containing planetary positions, eclipses, and trigonometric data. The Zīk-i Shahriyārān, based on observations accumulated over several centuries, directly influenced the first Islamic zījes. These tables used the Babylonian sexagesimal system and combined Ptolemaic models, Persian observations, and Indian calculation methods.
The Arab conquest preserved and adopted the Persian tradition. The foundation of Baghdad (762 CE) and the Bayt al-Hikma allowed for the translation of Greek and Persian scientific texts into Arabic. Persian astronomers, such as Al-Hajjaj ibn Yusuf and the Banu Musa, ensured the continuity and evolution of astronomy into the classical Islamic period.
Ancient Persian astronomy perfectly illustrates how scientific knowledge is transmitted and enriched across centuries and civilizations. Far from being a mere intermediate phase, it represents a moment of creative synthesis.
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