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Johannes Kepler inferred quantitative laws of planetary motion from astronomical observations. In Astronomia nova by Johannes Kepler, published in 1609, the problem of Mars's path led him to an ellipse with the Sun at one focus and an equal-area timing law.

The equal-area law states that the line from the Sun to the planet sweeps out equal areas in equal times. Together, a geometric orbit and a timing law specify where the planet moves and how its speed changes. Their later connection to a central force and an inverse-square law is explained in inferring an inverse-square force from Kepler laws; those dynamical deductions should be distinguished from Kepler's original empirical construction.^[ref]