Orbit

Things move in curved paths in space.

This path is called an orbit. A star pulls on a planet. This pull makes the planet move in a curve.

A moon also moves in an orbit. It moves around a planet. Some paths are shaped like a circle. Other paths look like a long oval.

Objects move faster when they are close to a star. They move slower when they are far away. This keeps the paths steady and strong.

An orbit is a curved path in space.

An object follows this path because of a pulling force. This force is called gravity. Gravity pulls a planet toward a star.

This pull makes the planet move in a curve instead of a straight line. If the object moves fast enough, it stays in orbit. It will not fall into the star.

Most orbits are shaped like an ellipse. An ellipse is a long oval shape.

Johannes Kepler found that planets do not move at the same speed. They move faster when they are close to the star. This point is called the periapsis. They move slower when they are far away. This far point is called the apoapsis.

Objects orbit around a shared center. We call this center the barycenter. A large star and a small planet orbit this point. Even two big planets will orbit a barycenter. Most orbits repeat the same path over and over. Some paths might not repeat. Scientists use math to study these paths. Isaac Newton and Albert Einstein helped us understand them.

An orbit is a curved path that an object follows in space.

This happens because of an attracting force, like gravity. A planet might orbit a star, or a moon might orbit a planet. Even humans make artificial satellites to orbit the Earth. Most orbits are paths that repeat the same way over and over. However, some paths do not repeat. An orbit is a dynamic thing that can change over time.

Many different objects, like asteroids and comets, follow these paths in our solar system.

To understand how an orbit works, we can look at how forces act. Without gravity, an object would move in a straight line. This is called inertia. But gravity pulls the moving object toward a larger mass. This pull causes the object to follow a curved path instead of a straight one. If the object has enough speed, it will not fall into the center. It stays in a curved path indefinitely.

As an object gets closer to the center, it moves faster. As it moves farther away, it slows down. This happens because of the way energy changes during the trip.

People have studied these paths for a very long time. Early thinkers believed in celestial spheres, which were perfect rings for stars. They thought the Earth was at the center of everything. Later, Johannes Kepler discovered that orbits are actually shaped like ellipses. An ellipse is an oval shape, not a perfect circle. He found that the Sun is at one focus of this oval. Isaac Newton later showed how his laws of gravity explained Kepler's work. Finally, Albert Einstein explained that gravity is actually the curvature of space-time.

There are many specific facts about how these paths look. An orbit has a closest point called the periapsis.

It also has a farthest point called the apoapsis. For the Earth and Sun, these points have special names. The closest point to the Sun is called perihelion. The farthest point is called aphelion. In our solar system, Mercury has the most eccentric, or stretched out, orbit. Venus and Neptune have the smallest orbital eccentricities.

These numbers help scientists predict exactly where a planet will be.

Orbits connect to many things we see in science today. When we launch a rocket, it must reach a certain speed to stay in orbit.

If the orbit dips too low into the air, the rocket will slow down and fall. Scientists also use gravity to help spacecraft travel. This is called a gravity assist. A spacecraft can use the curve of a planet's orbit to change its speed.

This helps us explore the far reaches of our space.

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.