Wanderers of the Solar Family

In the previous blog, we realized that the sun shows patterns in the sky when observed over the course of a year in a periodic manner. Now, what do the planets or ‘planetes’, as the Greeks like to call them, have to show us? First things first, you gotta know what planet’ means. According to the observations led by ancient Greek astronomers, these objects move in a weird motion and thus named them planetes which literally means ‘wanderers’. Pretty straightforward, huh?

Let us start off with our twin planet — Venus. What would your mom tell you when you start cleaning your messy room all of a sudden?

“Did the Sun rise from the west today?”

But it is actually possible for the sun to rise in the west — not on Earth but on Venus! While the Earth rotates on its axis eastwards, Venus rotates westwards. If Venus had residents, they would apparently view the Sun rising from the west. Can’t be helped since it is this way. After all, considering Venus as our Twin makes sense — the polar opposite twin obviously!

Axial positions of Earth and Venus. Image Source: Youtube

The weird motion of planets can also be observed from anywhere on the Earth! Go on a trip to the countryside or a place with less or no light pollution. Plan a night sky watch session — you don’t need a telescope or a DSLR. All you need is your eyes with a little bit of patience. Watch the heavenly bodies rise from the east. Observe the motion carefully. Now, since our planet rotates eastwards, you should see every object in the sky moving the same way including the Sun. When you observe planets, you may find something absurd. Take Mars for example. Every two years, we get to see Mars looping back and forth its position — usually moves eastwards but takes a turn westwards, goes on a loop, and comes back to its usual direction. The time-lapse image below of the position of Mars observed by NASA will give you a better idea of how it looks to the observer.

Retrograde motion of Mars viewed from Earth. Image Source: NASA

Now, this is not something that is only observed on Mars. Every planet observed from the surface of the earth would show this type of motion. But does this actually mean the planets change their path over the course of time? What you see is not actually how it works. The planets appear to loop around from Earth but in reality, they all move in the same direction as others. In short, it is an illusion that has been tricking us for ages! Both in the case of Venus and Mars, the motion is called retrograde, meaning moving backward, while the Earth possesses prograde motion, meaning moving forwards.

So, what is happening?

At some point, Earth tends to overtake Mars which results in the reversal of direction. But then, Mars gears up and crosses Earth which makes it appear like Mars has travelled a loop before retracing its original path. So, who won the race? None. (Heh. Jokes on y’all! )

Greek astronomers were baffled observing such unrealistic behaviour of the heavenly bodies. It was the time when people believed in geocentricity. According to that idea, Earth was stationary and remained in the centre of the universe, and all other objects revolved around it. Hipparchus devised some geometrical models that could actually satisfy the loophole created by retrogrades. He placed the planets on small rotating circles called epicycles which in turn moved on a large circle called deferent. This idea somehow convinced other astronomers of that time. Claudius Ptolemy refined this model by moving the planet away from the centre of the deferent, which he called the equant. But every model proposed till now had its own shortcomings, yet they were reaching the right destination step by step!

The Ptolemaic model. Image Source: Salem State University

Polish-born astronomer Nicolaus Copernicus suggested heliocentricity, meaning the centre of the system is the Sun which further lifted off the thick fog that has been pestering the astronomers all this time. Ptolemy’s model started making sense after the introduction of this idea that also explained the reason for the change in brightness of the planets observed during the period of retrograde. Kepler’s idea of introducing eccentricity to the perfect circle-like orbits made things clearer than before. Geometry and Mathematics have played a major role in bringing Astronomy to where it is now. All it requires is the subtle art of connecting knowledge and ideas!

Moral: What you see doesn’t actually have to be the reality :)



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Physics Graduate | Passionate about astro stuff| Techie