Andreas Osiander (19 December 1498 - 17 October 1552) was a German Lutheran theologian and mathematician best known in the history of science for writing the anonymous preface to Nicolaus Copernicus's De revolutionibus orbium coelestium (1543), the foundational text of heliocentric astronomy. He was also a significant figure in the Protestant Reformation, active primarily in Nuremberg and later in Königsberg (now Kaliningrad, Russia).

Osiander was born in Gunzenhausen, in what is now Bavaria. He studied at the University of Ingolstadt and was ordained a Catholic priest before converting to Lutheranism. From 1522 onward he served as a preacher in Nuremberg, where he became one of the leading reformers in the city. He was notably learned in Hebrew and engaged seriously with Jewish-Christian textual relations, including a 1529 tract arguing against blood libel accusations - an unusual position for a Christian theologian of the period. He played a role in the Nuremberg religious discussions of the 1520s and maintained correspondence with major figures of the Reformation, including Martin Luther and Philip Melanchthon, though he held several theological positions that put him at odds with Lutheran orthodoxy.

In 1549, following political and religious difficulties in Nuremberg, Osiander accepted a position in Königsberg under Duke Albert of Prussia. There he continued to develop and publish his theological views, particularly on the doctrine of justification, which generated substantial controversy within Lutheranism and led to the so-called Osiandrian controversy after his death.

Osiander's most consequential act in the history of science was adding an unauthorized preface to Copernicus's De revolutionibus (1543) without identifying himself as its author. The preface argued that the heliocentric model should be understood as a mathematical device useful for calculation rather than a literal description of physical reality - a position now associated with instrumentalism in the philosophy of science.

Copernicus himself did not write or approve the preface, and its anonymous insertion obscured his own stated view that heliocentrism described the actual structure of the cosmos. The astronomer and mathematician Georg Joachim Rheticus, who had arranged for the printing of De revolutionibus, was absent from Nuremberg when the final sheets went to press, leaving Osiander in a supervisory role he appears to have used to insert the preface. The preface was not publicly attributed to Osiander until Johannes Kepler identified him as the author in 1609.

The preface has been interpreted variously as an act of theological caution intended to protect the work from ecclesiastical censure, as a sincere expression of Osiander's own instrumentalist philosophy of mathematics, or as an unauthorized misrepresentation of Copernicus's intent. See De revolutionibus for fuller context.

Osiander's theological writings focused heavily on mystical and Neoplatonic themes uncommon in mainstream Lutheranism. His doctrine of justification held that the believer is made righteous through union with the divine nature of Christ - a position his critics argued departed from the Lutheran emphasis on forensic justification through the imputation of Christ's righteousness. This dispute, which intensified after his death, is treated in Osiandrian Controversy.

There is broad historical consensus that Osiander wrote the De revolutionibus preface and that it was inserted without Copernicus's knowledge or approval. This consensus is based on Kepler's attribution, corroborating correspondence, and subsequent scholarly examination. See De revolutionibus - History Consensus.

• Osiander as cautious protector - Some historians argue Osiander's preface was a pragmatic attempt to shield the work from condemnation by framing heliocentrism as hypothesis rather than physical claim. See De revolutionibus - Cautious Protector Viewpoint.
• Osiander as instrumentalist - Others hold that the preface reflects Osiander's genuine philosophical view about the status of mathematical models, independent of any protective motive. See De revolutionibus - Instrumentalist Viewpoint.
• Osiander as usurper of authorial intent - A third reading treats the preface as a distortion of Copernicus's realist intentions that shaped - and arguably distorted - the early reception of heliocentric astronomy. See De revolutionibus - Authorial Intent Viewpoint.

• De revolutionibus orbium coelestium
• De revolutionibus - History
• Osiandrian Controversy
• Nicolaus Copernicus
• Instrumentalism (Philosophy of Science)
• Protestant Reformation

1. Westman, Robert S. The Copernican Question: Prognostication, Skepticism, and Celestial Order. University of California Press, 2011.
2. Rosen, Edward. “Was Copernicus's Revolutions Approved by the Pope?” Journal of the History of Ideas 36, no. 3 (1975): 531-542.
3. Kepler, Johannes. Astronomia Nova. 1609. Kepler identifies Osiander as the preface's author in his introduction.
4. Stupperich, Robert. “Osiander, Andreas.” In Theologische Realenzyklopädie. Berlin: de Gruyter, 1995.
5. Jardine, Nicholas. “The Significance of the Copernican Orbs.” Journal for the History of Astronomy 13 (1982): 168-194.