Why Mars, anyway?

Some people love Mars. I am not one of them.

If you ask me what my favourite planet is, it’s definitely Saturn. It’s massive and floaty and has a huge set of gorgeous rings. Also, it swallowed an entire satellite, which is metal as fuck. Has Mars ever done that? Thought not. (Although landing on Mars is still pretty hard. Plenty of spacecraft have smashed onto its surface.)

A picture of a potato with a sad face
Not Mars. Credit: banger1977 on Flickr.

On the other hand, Mars is a rusty potato. It looks like an orange dot in the sky, and when you look at it through binoculars, it looks like a slightly bigger orange dot. At some point some Italian guy thought he saw lines on Mars, the English-speaking world decided that these were definitely canals built by aliens, and the whole thing turned out to be an optical illusion caused by looking through shitty tiny telescopes. I’m still convinced we spend so much time looking for life on Mars because the sci-fi books and the fucking canali convinced several generations of scientists that the best place to look for life was the cold, dusty potato and not, I don’t know, not one but two icy moons that might have gigantic undersea oceans.

Half the sphere of Europa, an icy white moon with red streaks over its surface
Europa is one of the four biggest moons of Jupiter and might harbour life in an undersea ocean. Credit: NASA/JPL-Caltech/SETI Institute.
An icy white moon
Enceladus, a moon of Saturn, has a thick icy shell that might harbour an undersea ocean. The moon is warm on the inside and plumes from the ocean occasionally splurt out through the surface. Credit: NASA/JPL-Caltech.

 

 

 

 

 

 

Oh, well. Dusty potato it is.

While I was trying to figure out why the dusty potato is so fascinating to people, I learned some really interesting things, and decided to plop them in this post.

I should stress that for my day job I am a historian of science. I talk to people to figure out how they felt and what they knew about science when they were doing their research (spoiler alert: I have talked to literal professors who emphasise that compared to astronomy in 2020, back in the 1970s we pretty much knew shit all about things in space compared to what we know today). I don’t work backwards from the present day, which is what I’m going to do in this post. Working backwards from the present day and assuming that people in the past had the same knowledge and beliefs we do today is called “presentism”, and it’s widely regarded by historians as a Very Bad Thing.

However, this is not my day job. In my day job I have to write history accurately, say nice things about Mars because everyone loves it, and avoid swearing. This is We Lack Discipline, where it’s okay to say fuck, slag off the Red Planet and make weird historical arguments for the sake of explaining why Mars is more interesting than it might look.

So, why is Mars interesting anyway?

People have been looking at Mars for at least 4,000 years – we have records from Ancient Egypt! So we know that people have been interested in it for thousands of years, because the sky is actually pretty cool. (Also, people had to work a lot harder to amuse themselves before radio, cinemas and TV came along.)

Traditionally, ancient civilisations identified the Red Planet with fire or war because it’s…well, red. In ancient Mesopotamia, the Sumerians and later the Babylonians identified Mars with their god of war, Nergal. The Greeks cribbed their astronomy from the Babylonians and decided that Mars was actually their war god, Ares. The Romans cribbed even more from the Greeks and decided that the red dot in the sky was stella Maris, the star of Mars (Ares’ Roman counterpart). That’s how the Red Planet got its name.

Ancient astronomers noticed that Mars has what’s now called “apparent retrograde motion” – in simpler words, it goes backwards. Most Ancient Greek and Roman astronomers thought that everything revolved around the Earth – after all, the Sun and the Moon look like they go around the Earth. In their minds, so should everything else. So Mars going backward was a real problem for them.

A yellow circle representing Mars moves against a black sky
The retrograde motion of Mars in 2020. Credit: Tomruen.

In the 3rd century BCE, they thought they’d solved it. A mathematician called Apollonius of Perga worked out that you could describe the backwards motion in terms of deferents and epicycles – planets doing complicated, loop-the-loop orbits around Earth. 500 years later, the astronomer Ptolemy refined Apollonius’s ideas and presented them in a book called the Almagest, which basically translates as “the greatest thing”. To be fair, the Almagest was pretty great – it influenced Western astronomy for 14 centuries! It turned out that epicycles worked really well, too…at least for a while.

Some small black circles looping around a big black circle
A basic diagram of the system of deferents and epicycles. The deferent is the big circle in the middle. The epicycles are the little circles on the edge of the big circle. When you trace out the path of the epicycle, you end up with a loop-the-loop motion.

By the 16th century, astronomers could measure the position of Mars more accurately, but matching up Ptolemy’s theories with their observations caused problems. To explain why planets seemed to move faster or slower at certain times of the year, they had to add in an “equant” – an imaginary point where planets looked like they had a uniform speed. Nicolaus Copernicus was so irritated by the equant that it was probably one of the reasons he developed a model where all planets revolved around the Sun! He still kept the epicycles, though.

You’d think the problems stopped there, right? Wrong. Planets going loop-the-loop around the Sun in circles…did not actually solve that many of the problems. At all. To get rid of the epicycles would take an even bigger change.

About half a century after Copernicus, a German astronomer called Johannes Kepler was poring over the observations of Mars made by Tycho Brahe. At the time, these were the most accurate observations in the Western world – like getting stuff from the best telescopes in the world today.

The observations of Mars didn’t match up with Copernicus’s epicycles, and Kepler had to add the equant back in. Even then, Copernicus’s system didn’t work.

Was Kepler going to have to go back to geocentrism – to the Sun orbiting the Earth? For fancy maths reasons, you can just keep adding epicycles and equants to an orbit and you’ll eventually be able to approximate it. You just had to add on more and more. And to people who liked geocentrism, that might have made sense. But Kepler was also a passionate heliocentrist – he was really, really invested in the idea of the Earth orbiting the Sun. As a young student, he’d defended it on both theoretical and religious grounds. His pre-existing preferences shaped what he did and the science that came after him.

So he kept the Earth orbiting the Sun, and got rid of equants, epicycles and circular orbits.

What was his big idea?

Ellipses.

It turns out that when you argue that planets move around in ellipses rather than looping circles, it works much better. (To be fair, they’re not very stretched-out ellipses. They are nearly circular. Easy mistake to make when telescopes have basically only just been invented.) It took a while for Kepler’s ideas to catch on, but they got a huge boost from astronomical observations and from Kepler’s ideas being incorporated into one of the most popular textbooks of the time.

Yeah, maybe Mars is just a tiny dusty potato, but it’s a tiny dusty potato that changed the world and led to us re-evaluating our place in the Universe – literally!

Published by Osnat Katz

PhD student with UCL and the Science Museum; space history, public engagement and all kinds of other fun stuff

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: