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RETROGRADE AND PROGRADE



Since childhood we have learnt that all the planets go around the Sun. Meaning, the planets and other celestial objects rotate around the Sun in specific directions. But did you know that not all follow the same directions? Some follow the Sun’s rotation while some go in the opposite direction. After observing these motions of certain planets and comets, astronomers termed these rotations as “Prograde” and “Retrograde”.

Before we get into these terms, let’s first understand how the direction of the motion is determined. The direction is determined by the primary object in system that forms the system’s hub. In our case, it is the Sun. So rotation of all the planets and celestial bodies is decided by the Sun’s direction of rotation.

Now that we know how the motion is determined, let’s get into the above mentioned terms.

Prograde:  It is the motion in the direction of that of the Sun. Meaning, most of the planets and some comets move in the direction of the Sun, which is counterclockwise.

 Retrograde:The celestial bodies which move in the opposite direction as that of the Sun are said to have retrograde motion. They are few planets, comets and satellites which have this motion. Let’s see which those are.

PLANETS:

Of all the eight planets in our Solar System, six have prograde motion. While the remaining two have retrograde motion. These two planets are Venus and Uranus. The reason why they have retrograde motion is because of their axes.  Venus has axial tilt of 177 degrees whereas Uranus has axial tilt of 97.77 degrees. This means that Venus is spinning almost in the opposite direction to its orbit and Uranus is almost parallel with the plane of the Solar system.

DWARF PLANETS:

All the known dwarf planets in our Solar system have prograde motion. But there’s an exception. Pluto, the most controversial planet, has retrograde motion.  It is because Pluto’s axial tilt is 120 degrees.

NATURAL SATELLITES AND RINGS:

The motion of satellites is determined is by the following factors:
  1. If it is formed in the gravity field of the planet, by collision, then it will have a prograde motion and such a satellite will be called “regular moon”.
  2. But is the object is formed somewhere else and then captured into the planet’s orbit, it can either have retrograde or prograde motion, depending on whether the planet for rotating away or towards the object when it approached it. Such moon is called as "irregular moon".
So considering the above factors, all the asteroid-sized moons have retrograde motions. Whereas the moons larger in size, with an exception Triton, have prograde motion. Earth’s satellite has prograde motion.

Neptune’s moon Triton has retrograde motion. Saturn’s ring has retrograde motion too, as they were formed from the Saturn’s irregular moon Phoebe.

ASTEROIDS AND COMETS:

Most of the asteroids have prograde motions. There are very few asteroids with retrograde motion. These asteroids are mostly burnt-out comets.

Comets from the Oort cloud have retrograde motion. Our favorite, Halley’s Comet too have retrograde motion around the Sun.








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