Kepler's first law

A planet follows an elliptical orbit with the Sun at one focus of the ellipse. It describes orbital shape, not how quickly the planet moves along its path.

Ellipse: A closed curve whose points have a constant sum of distances from two foci. An ellipse is the geometric path specified by the law.
Focus (geometry): Either of two fixed points that define an ellipse through sums of distances to points on its curve. The Sun occupies one of the ellipse’s defining points.
Johannes Kepler: A German astronomer who formulated three laws of planetary motion in the early seventeenth century. Kepler established the elliptical-orbit rule from astronomical data.
Circular orbit: An orbit whose distance from its central body remains constant. A circle is an ellipse with zero eccentricity, but planets generally have nonzero eccentricity.
Orbital elements: Parameters that specify the size, shape, orientation, and position of an orbit. Eccentricity and semimajor axis encode the ellipse in orbit descriptions.
Newton's law of universal gravitation: The law that every pair of masses attracts with a force proportional to their product and inversely proportional to distance squared. Newtonian gravity explains why a planet’s orbit takes a conic-section shape.
Semimajor axis: Half the longest diameter of an ellipse, measured from its center to its boundary. It sets the characteristic size of the orbital ellipse.
Tycho Brahe: A Danish astronomer whose precise pre-telescope observations recorded planetary positions. His measurements of Mars enabled Kepler to reject circular-orbit models.
Kepler's second law: The law that a line from the Sun to a planet sweeps out equal areas in equal times. It describes orbital speed, which the first law leaves unspecified.
Orbit determination: The process of inferring an object's orbit from observations of its positions and motion. Fitted elliptical orbits predict planetary and spacecraft positions.

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Kepler's laws of planetary motionBroader topic: This law fixes the geometric shape shared by idealized planetary orbits.