Skip to main content

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.








Labels:

Comments

Post a Comment

Popular posts from this blog

Super Void

As mysterious as it can get, Universe has always been a place for strange and weird things. Now, scientists have a “Super-Void” in space which is missing galaxies! So before we get in to the topic, first let’s discuss what is “Void” According to Wikipedia, Voids or Cosmic Voids , as they are called, are vast spaces between galaxy filaments in the universe, which has few or 0 galaxies. Galaxy filaments are largest know structures in the universe, which have thread-like structures. They form boundaries between voids in space. These voids have very less density of matter that is considered for typical observable universe. Voids are believed to be formed in Big Bang, followed by massive collapse its mass and compressed by Baryonic matter . After Big Bang, there were lots of areas with high and low densities, which were changing rapidly. The regions with high density collapsed rapidly under gravity eventually causing foam-like structure of cosmic web of voids. Voids loc...
1 comment
Read more

Binary Black Hole

In early 2016, a group of scientists at LIGO observatory made an incredible announcement-detection of gravitational waves.   Gravitational waves are ripples in curvature of space-time. Till then, gravitational waves were considered as a theoretical concept.   This discovery is not momentous just because gravitational waves were discovered but also because nobody had ever seen two supermassive black holes orbiting so close to each other. This system is called Binary Black Hole (BBH). BINARY BLACK HOLE:- Binary black hole is a system which consists of two super massive black holes orbiting close to each other. Like ordinary black holes, BBH is also divided into stellar (black holes formed by gravitationalcollapse of a massive star) black holes. For many years, proving the existence of binary black hole was difficult because of nature of black holes itself and limited detection methods. When two black holes orbit close to each other, they give out immense amoun...
1 comment
Read more

Information Paradox- Can black holes delete the universe?

We have all know how black holes absorb everything in it. Even light is not sparred. But apart from this they have one more property- they could delete the universe itself. How? Let’s understand how. INFORMATION: Let’s first understand about the “information” I am talking about. It is not about some Roman history or other facts of encyclopedia. It’s the information about the matter in the universe. Everything in the universe is made of matter; how different atoms come together to form different structures to form different things. This is information. The different things may include various things ranging from one carbon compound (diamond or graphite) to squirrels to planets and stars in the universe. The structure of each of these things is called INFORMATION. The basic building block of everything in the universe is the same. Without this information everything would be the same. But black holes eat up this information when they consume the stars and planets. And...
1 comment
Read more