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:
- 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”.
- 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".
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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