Let’s get off this confounded planet for a little while, shall we?
As discussed in a previous post, Earth sits precariously close to the edge of the Goldilocks zone in our solar system – the window of distance from our Sun that permits the formation and sustenance of life as we know it. This, among other things, gives us a sense of being something special in the universe. But our world is not even the only one in our own solar system that resides within the Goldilocks zone. Orbiting 60-ish million miles farther out is an entire other world, albeit somewhat smaller than our own, which may at one time have harbored life, and which may one day harbor us: the planet Mars.
Particularly over the last couple of centuries, Mars has fascinated us. Before that, it wasn’t viewed as any more special than the other planets we knew about. But those planets as a group were known to be something special dating back to the second millennium B.C. This new series of posts will explore Mars – from our first notions about it all the way through when we might go there, and whether we will stay. We’ll start with a broader look at the planets we’ve been seeing for a very long time, only to understand them relatively recently.
It’s quite likely that our species has been drawn to the night sky and observed its patterns for many thousands of years. That includes some attention to the fact that the planets appear to move through the sky quite differently from the stars. But the first of us to write those observations down were the Babylonian astronomers. They likely did so starting in the 17th century B.C., but the oldest surviving copy of that information is in the Venus tablet of Ammisaduqa, from the 7th century B.C. The tablet contains 21 years of observations of Venus, including its rise times and visibility.
The planets move uniquely through our night skies for two driving reasons. First, they are much closer than the stars. The nearest star is around 25 trillion miles away – over a million times farther than Mars is from Earth. So even though the planets and the stars are both moving through space at breakneck speeds, it takes a lot longer for us to notice it with the stars – so long that they appear not to move at all. Second, the planets orbit our Sun, which the stars don’t. Since Earth also orbits our Sun, the relative movements of the planets can look rather complex, almost as if the planets are just wandering through the sky (foreshadowing). The Babylonians didn’t know that the planets are orbiting the Sun, but they did track their movements. And so it was that the worlds we now know as Mercury, Venus, Mars, Jupiter, and Saturn were given distinct identities in our collective consciousness.
The word “planet” comes from the Ancient Greek asters planetai, which means “wandering stars”. The Greeks named the red planet after their god of war, Ares. The Romans, as they did after conquering the Greeks, renamed the god of war to Mars, and the planet followed suit. Mars has had many other names in other cultures, and usually those names are based on its reddish color. We now know that hue comes from the oxidizing (rusting) of the large amount of iron on its surface. So we can add iron swords and shields to the god of war imagery, if we like.
Another way the planets distinguish themselves from the stars is that they don’t twinkle. Stars are so far away that they appear as mere points of light, no matter how strong a telescope you might use. The planets in our solar system also look like points of light to the naked eye, but in a telescope, they show up as disks. The atmosphere between our eyes and a star or planet is rather turbulent, and with a single point of light, that turbulence can cause the star to “wink out” a bit, hence the twinkle. But with more rays of light traveling toward us from a planet’s disk, those twinkles get lost in the shuffle, and we just see a continuous solid light.
It is remarkable how long it took from the earliest recorded observations of Venus to having any real understanding of the planets. The Greek astronomer Aristarchus – in the third century B.C. – boldly proposed that the Sun was the center of the universe, with Earth and all other bodies revolving around it. While not completely correct, this view was much closer to the truth than the view that eventually won out and persisted for nearly two millennia – the notion that everything revolves around Earth. Four giants of science changed all of that over the course of two breathtaking centuries. Books have been written about each of these geniuses, of course, but I’ll try to sum it all up in one paragraph. And,… go.
In 1514, Nicolaus Copernicus began the first outline of the same model of the universe proposed by Aristarchus – with everything orbiting the Sun instead of Earth. This was a notion counter to the mainstream, to say the least, and Copernicus held it close until the year of his death in 1543. That year, the publication of On the Revolutions of the Celestial Spheres began a scientific revolution of its own, describing the reasons we should believe the planets orbit the Sun. Fast forward to Johannes Kepler, who in 1596 published the first defense of the Copernican view, entitled The Cosmographic Mystery. Some time after that, Kepler provided an endorsement of some observations made by this dude named Galileo Galilei, who in 1610 discovered the four largest moons of Jupiter, the observation of which clearly indicated those moons revolve around Jupiter, and not the Earth. Galileo made those observations with one of his newly designed and fabricated telescopes, which subsequently allowed him to see a full set of phases on Venus, similar to what we see with our Moon. The nature of these phases were proof that Venus, at the very least, must be orbiting the Sun – and once you open that door, why would the other planets be any different? Later, based on Galileo’s findings and many other observations, Kepler published the three volumes of Epitome of Copernican Astronomy, which described the three laws of planetary motion. These laws use mathematics to describe how the planets orbit the Sun. Finally, born in 1642 (the same year Galileo died), Isaac Newton would go on to do a few things himself, among them developing his universal theory of gravitation, which describes how the same force and physical laws governing a falling apple on Earth also govern the orbit of a planet around its star – effectively showing us why the planets orbit the Sun. Those laws are described in Newton’s Mathematical Principles of Natural Philosophy, published in 1687.
Galileo’s early advances with the telescope began a thread of technology that culminated a couple of centuries later in massive observatories, which would lead to an entirely new picture of Mars. We’ll talk about that in the next post. In the meantime, a toast to the wanderers in the night sky, and to the wandering minds that helped us understand them.