Astronomia nova

Publication of Kepler’s Astronomia nova

Eight arcminutes — less than a quarter of the Moon's apparent width — were enough for Kepler to reject a model that otherwise fitted Mars remarkably well.

Heidelberg, 1609

A first edition of Johannes Kepler's Astronomia nova was published at Heidelberg in 1609, printed by Gotthard Vögelin after printing began in 1608. The best biographical evidence places its appearance in summer 1609 but does not establish an exact day.

The book presented Kepler's new theory of Mars, derived from Tycho Brahe's observations, and put into print the ellipse and area principles later numbered as his first and second laws.

Eight arcminutes

Kepler's circular and equant-based models could not satisfy Tycho's Mars observations within their known precision: the maximum mismatch was 8 arcminutes, against typical observational errors of 2 arcminutes or less.

In chapter 58 he concluded that the planet's path could be no figure other than a perfect ellipse; in chapter 60 he treated swept area as the measure of elapsed time.

Not a list of laws

Kepler did not present a numbered list called "Kepler's laws." That terminology and the familiar ordering are later conventions — in his investigation the area principle preceded the ellipse result.

The volume was expressly a physical or causal astronomy as well as a mathematical account, with the Sun assigned a causal role in planetary motion.

No well-supported single-day public reaction is known; the two principles were adopted gradually rather than replacing established astronomy at once.

Sources

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What the sources leave uncertain

The title page securely establishes 1609. A standard Dictionary of Scientific Biography account places publication in summer 1609, but the consulted evidence does not support a particular month or day.
Kepler did not present a numbered list called “Kepler’s laws.” That terminology and the familiar ordering are later conventions; in his investigation the area principle preceded the ellipse result.