Orbit

An orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around a star or a satellite around a planet. Orbits can be either circular or elliptical in shape, and may be prograde (in the same direction as the rotation of the body being orbited) or retrograde (in the opposite direction to the rotation). Objects in orbits are held in place by their mutual gravitation towards each other; orbiting objects continually fall towards each other but never quite collide because they are moving sideways at high speed as well.

The study of orbits dates back to ancient times; Aristotle proposed that planets moved in perfect circles around Earth, while Ptolemy refined this idea by proposing that planets move in small circles whose centers moved slowly in larger circles. Johannes Kepler finally explained orbital motion using his laws of planetary motion, which were based on careful observations made by Tycho Brahe. Newton’s law of universal gravitation later showed that Kepler’s laws were consequences of a more general principle, and correctly predicted many previously unexplained phenomena such as the precession of Mercury’s orbit.

Orbits are commonly described using two principal elements: the semi-major axis, which is half the length of the longest diameter of the ellipse, and eccentricity, which measures how far from perfectly circular an orbit is. The shape of an orbit can also be characterized using angular momentum, which describes how much an object “wants” to move in a straight line rather than following its curving path; this quantity is conserved over time provided no external torque acts on the system. Another useful parameter is periapsis (closest approach) and apoapsis ( furthest distance), which describe where an orbiting body comes closest to or farthest from its primary body respectively.