Orbit

An orbit is the curved trajectory of an object under the influence of an attracting force. This motion is often called an orbital revolution because the object rotates around an axis external to itself.

Common examples include a planet revolving around a star or a natural satellite circling a planet. Humans also create artificial satellites to orbit planets, moons, or specific positions in space like Lagrange points. While most orbits are regularly repeating trajectories, some may be non-repeating. Orbits are dynamic and can be changed by the gravitational pull of other masses, a process known as perturbation.

To understand the mechanism of an orbit, we can look at Newton's laws of motion. First, an object stays in uniform rest or motion unless an external force acts upon it. This property is known as inertia. If there were no gravity, an object would simply travel in a straight line. However, gravity acts as a force that pulls the moving object toward a larger mass.

This pull causes the object to follow a curved path. If the object possesses enough tangential velocity, it will not fall into the gravitating body. Instead, it continues along the curved trajectory indefinitely. This balance of motion and pull creates a stable orbit.

Most planetary orbits are shaped like ellipses, which are oval-like curves. In an elliptical orbit, the bodies revolve around a common center of mass called a barycenter. This barycenter is located at one of the two focal points of the ellipse.

As an object moves along this path, its energy constantly shifts between two types. It has kinetic energy, which is the energy of motion, and potential energy, which relates to its position. As a planet approaches its closest point, called the periapsis, its potential energy decreases and its speed increases. Conversely, as it moves toward the apoapsis, the farthest point, its velocity decreases as potential energy increases.

History shows how our understanding of these paths has evolved. Early Hellenistic astronomers like Eudoxus and Aristotle proposed a model of celestial spheres. They believed stars and planets were attached to perfect, moving rings.

Later, Ptolemy added complex mechanisms called deferents and epicycles to predict planetary positions. This geocentric model placed Earth at the center. Copernicus later modified this by placing the Sun at the center to simplify the math. Johannes Kepler eventually provided the modern foundation by discovering that orbits are elliptical rather than circular. He found the Sun sits at one focus rather than the exact center.

Isaac Newton later demonstrated that Kepler's laws could be derived from his theory of gravitation. Newton showed that gravity follows an inverse-square law, meaning the force depends on the masses and the distance between them.

He also determined that orbits are conic sections. For a pair of bodies, the orbit size, orbital period, and combined masses are mathematically related. This helped explain why planets have different speeds based on their distance from the Sun. In the 19th century, Urbain Le Verrier used these principles to predict the position of Neptune by observing perturbations in Uranus's orbit. This was a major success for classical mechanics.

In 1916, Albert Einstein introduced the general theory of relativity, which changed our understanding again. Einstein explained that gravity is actually the curvature of spacetime. In this view, orbits follow paths called geodesics.

While Newtonian mechanics is still used for most short-term tasks because it is easier, relativity is more accurate. It specifically explains the precession of Mercury's perihelion, which Newton's laws could not fully account for. Today, we know that while Newtonian predictions are very close, the differences in high-gravity environments are measurable.

Specific terms are used to describe the points in an orbit depending on the bodies involved. For objects orbiting the Sun, the closest point is perihelion and the farthest is aphelion. For Earth, these are called perigee and apogee. Objects orbiting the Moon use the terms perilune and apolune. In our solar system, Mercury has the most eccentric orbit, meaning it is the most stretched out.

In contrast, Venus and Neptune have the smallest orbital eccentricities. These measurements allow scientists to map the complex dance of our solar system with great precision.