Planetary Orbits

- Overview

The path of an object under the influence of gravity through space is called its orbit. Planetary orbits are the regular, repeating, curved paths that planets follow as they revolve around a star under the influence of gravity. 

In our solar system, all the planets orbit the Sun in paths that are ellipses (oval-shaped), with the Sun located at one of the two foci of the ellipse. 

Two points in any orbit in our solar system have been given special names:

• The point where the planet is closest to the Sun (helios in Greek) and moves the fastest is called the perihelion of its orbit.
• The place where it is farthest away and moves the most slowly is the aphelion. 
• For the Moon or a satellite orbiting Earth (gee in Greek), the corresponding terms are perigee and apogee.

Key Characteristics of Planetary Orbits:

• Shape (Kepler's First Law): Planets do not move in perfect circles, but in ellipses that are nearly circular. The amount an orbit deviates from a perfect circle is called its eccentricity; most planets in our solar system have low eccentricity.
• Speed (Kepler's Second Law): A planet's orbital speed changes depending on its distance from the Sun. It moves fastest when it is closest to the Sun (at its perihelion) and slowest when it is farthest away (at its aphelion). This is a consequence of the conservation of angular momentum.
• Orbital Period (Kepler's Third Law): There is a precise mathematical relationship between a planet's average distance from the Sun (the semi-major axis of its orbit) and the time it takes to complete one orbit (its orbital period). The farther a planet is from the Sun, the longer its orbital period. For example, Mercury takes only 88 days to orbit the Sun, while Earth takes 365 days, and Neptune requires 165 years.
• Gravitational Influence: The force of gravity from the central star is what keeps the planets in orbit, preventing them from flying off in a straight line into space.
• Orbital Plane: All major planets in the solar system orbit in roughly the same flat, disk-shaped space, known as the ecliptic plane. This is a result of the solar system forming from a rotating disk of gas and dust.

These principles of planetary motion were first described by Johannes Kepler in his three laws and later explained by Sir Isaac Newton's laws of motion and universal gravitation. 

These laws are still used today to predict planetary positions and calculate the trajectories of spacecraft, such as the Apollo missions to the Moon. 

- Orbits of the Planets

The eight planets orbit the Sun in a nearly common plane called the ecliptic, with orbits of low eccentricity, though Mercury's is the most eccentric. 

According to Kepler's laws, Mercury has the shortest orbital period (88 Earth days) and highest speed (48 km/s) due to its proximity to the Sun, while Neptune has the longest period (165 years) and slowest speed (5 km/s) at the furthest extreme. Newton's work allows for precise calculation of these orbits, which are governed by gravitational force and a planet's kinetic energy.

Planetary orbit characteristics:

1. Orbital plane: All major planets orbit close to a common plane, called the ecliptic, which is near Earth's orbital plane. 

2. Eccentricity: Planetary orbits have low eccentricity, meaning they are nearly circular. Mercury has the highest eccentricity (0.21), while others are below 0.1. Mars's relatively higher eccentricity was crucial for Kepler to deduce the elliptical shape of orbits. 

3. Speed and period: A planet's speed is highest at its closest point to the Sun and lowest at its furthest point. 

• Mercury: Has the shortest orbital period (88 Earth days) and highest average speed (48 km/s).
• Neptune: Has the longest orbital period (165 years) and slowest average speed (5 km/s). 

4. Governing forces: The combination of a planet's gravitational potential energy and kinetic energy determines its orbit. A planet moves faster when it is closer to the Sun because the gravitational pull is stronger.

[More to come ...]