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The retrograde motion of Mars

Did the title spook you off?? Come on, don’t lie. Anyway, here it goes.

The Earth revolves around the Sun, and so do other planets. This is common knowledge today, and every child in school would tell you this. But there was a time when almost nobody knew it. Everyone just assumed that the Earth was at the centre of the universe, and that the whole universe revolved around it.

I would not blame them. Though there is not much of the night sky to see now in Chennai today, as is the case with any other urban area, looking at the few visible stars in the sky, and monitoring them for some time, would only make you think that the stars are moving and that you yourself are stationary. The pole star alone seems to be fixed, and the rest seem to move around the earth. (Those with a DSLR can try taking a picture using a tripod, leaving the shutter open for half a minute and you can trace the path of the stars as I did on a camera borrowed from my younger brother who attached on it a telephoto lens bought with money borrowed from my elder brother, which he later repaid by borrowing money from dad. We indeed are a close-knit family) The Earth alone, it feels, is majestically stationary, while the rest of the universe humbly goes on around it. So it is natural to think and seemingly obvious that the Earth was indeed at the centre of the Universe.

But there was a problem. There were some objects in the sky which did not follow this pattern. They followed haphazard paths when seen from the Earth. These were given the name of planets, from the Greek word for wanderer. One of them was Mars. It’s path appeared from the Earth to be as follows.

The orbit of Mars as it appears from the Earth

Sorry about the terrible line drawing, there was really nothing so wavy about the orbit of Mars. But the key aspect of the figure is the loop. Mars seemed to move in one direction till it turned back, made a loop, and continued on in the same direction as it was initially moving in. This was called the retrograde motion of Mars. Nobody knew why the planet was following this path.

This problem was solved when Copernicus arrived. He put forth the argument that it is the Sun that is at the centre, and the Earth and the other planets that revolved around it. This was somewhat unsettling, since if that were the case, we humans would lose the position of being the protagonists of the universal drama. Copernicus reduced us to mere side actors, with the Sun taking on the role of the central character. (What they would not have known then, is that our position was going to be made more and more inconsequential as we continued to learn more and more about the universe.) Thus there was some resistance. To be fair, this idea was not completely new. There were some people previously too who suggested that the Earth was not at the centre, but it was Copernicus who brought it to the fore.

But coming to our own problem of the motion of the Mars, we will see how this view of the Solar system explains the Mars. All the explanation needs is a small animation.

Animation to show how the loop in Mars' orbit is explained (Click on it for a bigger picture)

The yellow circle in the centre is the Sun, the blue dashes are the positions of the Earth at differnt times, the red dashes are the positions of Mars, and the line on top can be seen as our Horizon. The black line shows our line of sight.

Again please overlook the quality of the animation (it is, after all, my first animated gif). But as can be seen, both Earth and Mars are going on doing their jobs of going around the Sun. But from Earth it would appear that Mars goes in a loop, because Earth occasionally “overtakes” Mars, in which case, Mars which was going ahead of us would slow down, fall back behind but would get back ahead again after some time. And this was confusing till we thought that Earth does not move. But once we realised that we too are moving around the Sun, the problem disappeared.

This is a very simple problem. I don’t remember where I first read about the retrograde motion of Mars, but where I read it, I was not given a pictorial description of why the Mars would appear that way from Earth, if we assumed that the Earth was at the centre. But I took out a pencil and a paper, and tried drawing it, and I quickly saw, that it was very simple. One of the things about planets, moons etc is that if you learn some strange fact, there is a good chance that you can visualise it well if you spare some thought to it. Another example is the orbit of the Moon and the orbit of the Earth that causes Solar Eclipses. There are numerous such facts about the Universe that would give us that happiness of understanding. We only need to keep our minds open.

  1. suresh
    November 29, 2010 at 18:56

    Good example for the qoute
    “A picture is equivalent to thoushand words”

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