astronomy

What is Retrograde and Prograde motion of Planets?

When we study the behaviour of celestial bodies then two major terms come into observation, they are Retrograde Motion and Prograde Motion. In general, the retrograde motion happens in the direction exactly opposite to the movement of another object under consideration. Note that Retrograde and Prograde motions are exactly contrary to each other where Prograde motion is defined as the orbit of one object with respect to another object or in other words it is also defined as the rotation of single body across its axis.
If we talk specifically about the celestial systems then retrograde motion actually defines the motion that is contrary with respect to motion of primary object; here this primary object use to form the hub of the system under consideration.
In the Solar System, all planets and several objects that use to orbit the Sun follow Prograde direction that means they orbit in the same direction as that of rotation followed by the Sun. Studies reveal that most of planets in space use to follow the Prograde motion where Uranus and Venus are common exceptions as they use to follow retrograde motion. There are so many satellites in the space and the fact is that they use to follow prograde motion around their dedicated planets. However, there are few satellites that are observed to follow rotation in retrograde motion but the fact is that they are smaller in size and stay at higher distance from specific planet; the only exception to this case is Triton that is satellite of Neptune and is close as well as large enough to this planet.
Essential Orbital Parameters:
Inclination:
The inclination of a celestial object represents information about whether orbit of that particular object is of retrograde type or prograde type. In scientific terms, the inclination of any celestial body is actually the angle defined between its orbital planes; as well as it can be considered with respect to another reference frame for example the equatorial plane of primary part of any object. In the solar system, the planet inclination is measured from ecliptic plane. Note that this ecliptic plane is plane of the Earth’s Orbit with respect to Sun. Similarly, moon’s inclination is measured from equator of specific planet about which they follow orbital motion. An object is observed to have inclination when it is revolving in the direction as that of primary object with range between 0 degree and 90 degree. Any object that follows inclination value of exactly 90 degree use to have a perpendicular orbit or it is neither following the retrograde motion nor prograde. The retrograde orbit is observed when inclination of object lies somewhere between 90 degrees to 180 degrees.
Axial Tilt:
The axial tilt of any celestial object defines whether rotation of this object is retrograde or prograde. Object that has axial tilt value up to 90 degrees is observed to have orbital plane that passes exactly from centre of object. Axial tilt equal to 90 degree represents perpendicular orbit whereas axial tilt measured somewhere between 90 to 180 degrees represent rotation opposite to the direction of orbit.