RETROGRADE MOVEMENT OF MARS AND OTHER PLANETS
Mars rises late, after 10 pm this December, and can be recognized as a brilliant red star close to the eastern horizon. It is increasing in brightness as our Earth approaches Mars in its motion around the Sun.
Mars is now in the stage of retrograde motion since 20th December. Retrograde means opposite, so this describes the opposite way that Mars is moving currently. Normally ALL planets (and the Moon) move from West to East. Our normal observation of for example the Moon is that it moves from West to East, since we know that during New Moon the Moon is in the West while as the days go by it appears further and further East until after about 15 day it will appear in the East rising at sunset. Hence movement from of planets and the Moon from East to West is rather contrary to our intuitive understanding. The term “retrograde” tries to capture this dilemma.
The only way to notice this retrograde movement of planets is by comparing their positions against the fixed background of the stars and constellations.
The way we can notice this retrograde motion of Mars is by comparing its position against the background of stars in the background. Before December, Mars was constellation of Pices, Fish and had shifted day by day towards the constellation of Leo, the Lion in its normal motion. At the moment, since 20th December, it has started shifting away slowly from Leo back towards Pices, that is in the opposite, or retrograde direction. This will continue until the beginning of March 2010 when it will be closest to Pices constellation and its movement will turn back towards Leo and resumes its normal motion in the sky.
A very nice animation of retrograde motion of Mars can be seen at this website: http://www.shadowandsubstance.com/
Retrograde motion happens because Mars is in an external orbit around the Sun compared to Earth so our Earth regularly, once a year, overtakes Mars. When Earth approaches Mars near its closest approach the parallax effect makes Mars appear to move in the opposite direction to its normal movement. We notice parallax movement when we are moving in a car or a bus and see the trees and houses go by backwards. When we (on Earth) are far enough away from Mars, the normal movement of the planet is the most noticeable one, that is from west toward the East.
(Remember that all planets as well as the Moon travel in their orbits from West towards East, that is, in an anticlockwise direction when observed from a point far above our North pole. Also remember that the Earth’s rotation is also in this same west to east direction, that is western portions of earth are moving towards the eastern direction, and indeed that is why we see the early morning sun rise in the east! Everything in our solar system, except a few things such as planet Neptune have anticlockwise rotation when viewed from a point well above our North pole. This is because of a Physics principle that angular momentum (motion) does not change unless acted upon by an external force – similar to Newton’s first Law of Motion for straight line motion.)
Now coming back to the movement of Mars, as long as the earth is far enough away, the Mars is seen to have a normal west to east motion is seen from earth across the background stars and constellations. However, once the earth comes very close as it overtakes Mars, the parallax movement takes effect and we see Mars passing back as Earth overtakes us (like trees passing back when we are in a vehicle). Which means it will move from east to west and hence it is seen in its retrograde motion.
All planets in external orbit, that is, Jupiter, Saturn etc show retrograde motion but Mars shows the biggest retrograde motion because it is closes to us.
What about the inner planets, Venus and Mercury – do they show retrograde motion? Indeed they do, and over a large part of the sky compared to Mars, especially Venus. For example you will see that when Venus makes its evening appearance in the west as an evening star, it first rises higher and higher in the sky in the west as the days go by until it reaches nearly 45 degrees above the horizon at sunset. It never goes higher than this so this is one way of identifying Venus when it is confused with Jupiter, which can be just as bright as Venus. You can see Jupiter high in the overhead skies but never Venus. Once Venus has reached its highest elevation it over the next few weeks it moves rapidly back towards the western horizon and hence undergoes retrograde motion (that is from East towards the West)