In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System. But unlike the ellipse followed by a pendulum or an object attached to a spring, the central sun is at a focal point of the ellipse and not at its centre.

- Current understanding of the mechanics of orbital motion is based on Albert Einstein’s general theory of relativity, which accounts for gravity as due to curvature of space-time, with orbits following geodesics.
- For ease of calculation, relativity is commonly approximated by the force-based theory of universal gravitation based on Kepler’s laws of planetary motion.
- Historically, the apparent motions of the planets were first understood geometrically in terms of epicycles, which are the sums of numerous circular motions.
- Second, he found that the orbital speed of each planet is not constant, as had previously been thought, but rather that the speed depends on the planet’s distance from the Sun.
- For the planets, the cubes of their distances from the Sun are proportional to the squares of their orbital periods.
- Isaac Newton demonstrated that Kepler’s laws were derivable from his theory of gravitation and that, in general, the orbits of bodies subject to gravity were conic sections, if the force of gravity propagated instantaneously.
- Where one body is much more massive than the other, it is a convenient approximation to take the center of mass as coinciding with the center of the more massive body.
- Essentially all experimental evidence that can distinguish between the theories agrees with relativity theory to within experimental measurement accuracy, but the differences from Newtonian mechanics are usually very small.
- In the case of an open orbit, the speed at any position of the orbit is at least the escape velocity for that position, in the case of a closed orbit, always less.