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.

The simple idea

There is one idea in science, that is unquestionably the simplest but at the same time, explains so many things that we see around daily, that it forms the backbone of an entire field of science: Biology. That idea is evolution.

Everybody has heard of evolution, and would readily say that it means we came from monkeys. But I have a suspicion, that not everybody understands it well. That suspicion, comes from the fact that, I did not understand it, till a few years back. You might say, “neither do I understand E=mc², even though I have heard about it. So why does it matter knowing evolution?”. But what is different about evolution by natural selection is that, this is an idea, which, once you learn it, will make you feel, “Oh! I could have thought of that myself”. The basic premise is so extraordinarily simple and straightforward. I bet you cant say that about E=mc².

Evolution by Natural Selection can be explained in 3 points.

1. Features are inherited from parents to offspring while reproduction.

2. Some of these features might vary a little, due to random changes, called mutations, while copying DNA from parent to child. The DNA is the information passed on from parent to child which contains the genetic instructions for building an individual from a single cell.

3. The changes in features (as a result of change in DNA due to mutations) might be beneficial or detrimental to the ability of the individual to survive and reproduce. Individuals whose DNA has changed “positively”, will lead to that individual surviving better in its environment than the others whose DNA has not changed or whose DNA has changed for the worse. Since it is these changed individuals who will survive better, and will have more children, their DNA (inherited from the parents along with the beneficial change) will start spreading through the population.

A hypothetical example: If the previous points sounded a bit vague, imagine a species of deer in a forest. One of the most important things it has to do is to run away from predators, like Cheetah, as quickly as possible, lest it becomes their food. Now the speed and the stamina that the deer possesses while running, is crucial to ensure that it outruns that its predator and survive. Imagine 10 sibling deers out of which there are 2 deers A and B who have undergone changes in DNA while it was being copied from its parents. The rest have exactly the same running genes as its parents, and they run more or less at the same speed of their parents. But A’s DNA underwent a random change (or mutation) (possibly due to errors while copying the DNA from parent to child similar to the way we make mistakes while copying a page of text to another page). Also assume that this change in DNA plays a crucial role in running and helps A to run faster than its parents. Similarly, assume B’s DNA too underwent a random change, but its change was for the worse. That is, its change has led to its running slower than its parents.

In such a scenario it is obvious that when a Cheetah notices these siblings playing together and starts chasing them, there is a very high chance that it is B, that will die first. A and the rest being faster than B (since A has had a positive change, and the rest did NOT have the negative change that B had in its DNA) will survive. In a similar way, after B’s death, A will have a higher chance of survival among the remaining 9. Hence, A has the maximum chance to live to become an adult and reproduce. And while A reproduces, it will pass on its modified faster running gene to all its children. If this change in DNA is crucial and significant enough, then after many generations, all the members of this species in this forest will have the faster running gene, simply because of the fact that anybody who does not have it, will be the first one to fall prey to the Cheetah.

This example captures the essence of Evolution by Natural Selection. Evolution tells us that all the species that exist today have branched off from one single instance of life that was created about 3,500,000,000 (3.5 billion) years ago (as per today’s evidence). Natural selection is the process where nature selects which genes survive. In our example the individual with the faster running gene will survive, since nature, in the form of a Cheetah, decides which of the different varieties of genes live. It is the survival of the genes that is crucial since even though the individual will eventually die, its genes will continue to exist since its children will now have the same genes.

In the above example, the Cheetahs too will possibly undergo evolution since they too need to run faster to capture food. Otherwise they will die of starvation. But let the example not make you think that the expression “Survival of the fittest” is only about running, fighting and other such macho features. Fitness includes everything that enables the individual to survive and pass on its genes. Let us look at a real life example where evolution by natural selection has been observed directly.

A real life example of selection: The peppered moth, is a very common example given to demonstrate how selection works. It is a light coloured moth with black spots. In England, there used to be many such white moths which survived well, since they lived on trees covered with Lichens (a composite of fungus and algae), and hence were very well camouflaged. This prevented the predators from finding the moths. Look at the pictures of a white peppered moth and the Lichens below, to see how easy it would have been for the moths to hide from the predators on Lichen covered trees. But when Industrial revolution happened, the Lichens were killed due to the soot that came from the industries and the trees too were covered by the black soot. This meant that the white moths could not camouflage themselves and were easily found and eaten by their predators. Now because of the soot, black moths began to survive well, since they were camouflaged well against the black trees, and soon the area was full of black moths. The black moths were previously at a disadvantage since predators can find them easily. At that time nature (the predators) chose black peppered moths for consumption, but now it was the turn of the white peppered moths. This is a clear example of how selection works. A curious thing about this episode is that the story did not end there. After environmental regulations came in, white moths have again flourished.

A white peppered moth - Source: http://en.wikipedia.org/wiki/File:Biston.betularia.7200.jpg

A tree covered with Lichens - Source: http://en.wikipedia.org/wiki/File:Lichen-covered_tree,_Tresco.jpg

A black peppered moth - Source: http://en.wikipedia.org/wiki/File:Biston.betularia.f.carbonaria.7209.jpg

Speciation: You might still say, “This might explain how white moths evolve into black moths, but they are all still moths, right? How can this explain the enormous variety of life that we see around. It is easy to see how features within a species might change like a cheetah or a deer running faster than its peers. But How do a cheetah and deer come about? How do different species come about?”. Speciation (how species come about) is an important field of Biology. But we can easily see how the previous examples can be extended to see how species evolve. And for that let us consider a few members of a species, say rats, that has been introduced into a new island where no rats existed before. Being a new island, the conditions might be drastically different from where they lived previously. Let us say that one of the changes in conditions is that it is very cold. Thus if one of the children of the pioneering rats, has slightly more fur than the rest, then it might survive better, and have more children thus producing more rats with fur. The ones without fur will have lesser chance of surviving. Thus, over many generations rats might evolve fur. Another change in condition could be that there could be lesser predators here, which could mean that they would rather spend their energy on other things like finding food and reproducing, rather than running around avoiding being food themselves. Thus rats that will have lesser speed will survive better, since for them the effort spent of running could be spent on something else more useful for their new environment. Another change in condition could be that the island could be full of water-bodies, and hence some rats which can survive a bit longer in water, might have a better chance of catching a fish for food. Thus slowly, over generations, the rats that survive longer in water might have better chances of surviving. To move around in water, they might start evolving fin like things. And so on. Thus a rat with fur, of a bigger size, with fins and that can survive in water will become completely different from the rats that were introduced first into the island. Thus a new species is created. This is just an example, that I came up with, for understanding how species could evolve.

Gradual change: Note that in the example above, one single rat does not grow fur or start swimming in its own lifetime. The change happens over generations. Change is for the species not for the individual. Moreover, an ordinary rat does not give birth to a new species of rat. At every stage of reproduction, the children vary only very slightly from the parent. It is only if you look at the starting and ending products, that you will see the drastic change. Evolution of every species is also the same. A monkey like ancestor does not give birth to a human being overnight. No individual of a species gives birth to an individual of another species. Features change very slowly, gradually. So gradually, that at no stage can you say that the offspring is different from the parent.

If this sounds weird, imagine that since your birth, your doting parents had taken photographs of you every day and arranged them chronologically in a flip book (like the flip books I used to see when in school, where you can see a batsman playing cover drive. Here is an example.) you can see yourself growing up in front of your eyes.

If you compare each page of the your life’s flip book with the previous one, you will never be able to say that on this particular day you have changed a lot from the previous day. Every day’s photo seems to the same as the previous day’s.   But over a 60-70 year period, the amount of change that can accrue is unbelievable. There is no clear delineation from a child to an adult to an old person. What we know is that at 5 years one is a child, at 30, definitely an adult, and at 70, an old person. But when exactly does one become an adult from a child, is tough to say.

In the same way, it is tough to say when the monkey like ancestor become a human being, or to answer who was the first human being (can you tell me the first day when you were an adult?) or the first chimpanzee, the first dinosaur or any species for that matter. But there is one example which clearly explains the impossibility of drawing boundaries between species.

Thus we are, through a continuous unbroken series of lineage, connected to the first life form that existed about 3.5 billion years ago. And each individual living today (including the whales, the lizards, the plants, the bacteria, the fungi, the peppered moth, the lichens) you can trace your relationship, in exactly the same way as you can trace it for your close human cousins. We are all cousins, (with varying degrees of removal) literally. Take any two individuals, and they would have had a common ancestor some time back (you and your brother’s common ancestors are your parents, you and your first cousin’s are your grand parents and so on).

Given two species, this website, TimeTree, can tell you when the common ancestor between the two species lived (Mya stands for Million years ago, which is the unit used in the website). You can spend quite a bit of your time, finding common ancestors.

This idea was that of Charles Robert Darwin and Alfred Russell Wallace, who came to these conclusions independently. Isn’t it wonderful to know that the mosquito you just killed is your cousin, and so is the okra that you just cut for your lunch (as I once told my mom)? And all this knowledge from the simple idea that those that are better at surviving, survive. Neat, isn’t it??