Planetary orbit

A planetary orbit is the path a planet follows around a star under gravity. Its shape and motion are governed chiefly by the star’s gravity and the planet’s velocity.

Kepler's laws of planetary motion: Three empirical laws describe planetary orbits as ellipses, their changing speeds, and their orbital periods. These laws summarize the geometry and timing of planetary motion.
Ellipse: A closed plane curve whose points have a constant sum of distances from two foci. An ideal two-body planetary orbit traces this conic with the star at one focus.
Circular orbit: An orbit whose path is a circle, with constant distance from its central body. It is the limiting case of an ellipse with zero eccentricity.
Transit method: A technique for detecting an orbiting body by measuring the dip in a star’s light as the body passes in front. A planet’s orbital alignment and period determine whether and when transits occur.
Newton's law of universal gravitation: A law stating that every mass attracts every other mass with force proportional to their masses and inversely proportional to distance squared. It supplies the force that continually bends a planet’s path around its star.
Semimajor axis: Half the longest diameter of an ellipse, equal to the average of its closest and farthest distances from a focus. It measures an orbit’s overall scale and helps determine its period.
Parabolic trajectory: An unbound path with eccentricity exactly one, representing the threshold between bound and escaping motion. Unlike a planet’s bound orbit, it does not return to the star.
Radial velocity method: A planet-detection technique that measures periodic shifts in a star’s spectral lines caused by its motion around a shared center of mass. The orbit sets the timing and amplitude of the star’s measured wobble.
Orbital eccentricity: A dimensionless measure of how much an orbit’s shape deviates from a circle. It distinguishes nearly circular planetary paths from elongated ellipses.
Periapsis: The point in an orbit at which an orbiting body is closest to its primary. For a planet orbiting a star, this is the orbit’s closest approach.

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