Where is Mars Now?

Where is Mars now?

Due to our ability to harness electricity and create light pollution, where I now live on the east coast of the United States, it's well nigh impossible to pick out where Mars is in the night sky. I know, you can download a free app that shows you the position of the planets and the stars and thus you can see where Mars is right now, but it's not the same.

In the Australian outback, it's easy to see Mars as an angry red dot on the sky. No doubt the various Australian aboriginal tribes who lived beneath those skies for 65,000 years were familiar with the red planet (Hamacher, 2017).

'Planet' is derived from the Greek word planētēs (πλανήτης) which literally means 'wanderer'. To ancient peoples observing the sky and the steady transits of the stars across the skies, Mars was a red anomaly, which not only didn't match the cycle of the stars, but even travelled in a retrograde motion and became brighter at the same time. Many European cultures associated this intruder with the god of war, as did the Greeks.

The Orbit of Mars

When Galileo finally turned his telescope upon the moon in 1609 and discovered the four Galilean moons in 1610, he only had 8x magnification and really terrible color aberration by today's standards. Half a century later, in 1666, another Italian, Giovanni Cassini, was able to use an improved telescope to observe surface features on Mars, and he used these to determine the length of the Martian day as 24 hours and 40 minutes long. Today we know the Martian day to be 24 hours 37 minutes 22.66 seconds long. In 1672, Cassini and a colleague to French Guiana so they could conduct simultaneous measurements and make a parallax measurement of Mars in both locations to measure its differing position on the fixed stars when viewed from Paris and French Guiana. He was thus able to establish the distance from Sun to Mars - Mars is 1.524 times further away from the Sun than the Earth is. Because we live on Earth, we call it's distance from the Sun 1 astronomical unit (or a.u. for short).

While Galileo conducted observations with his telescope, Johannes Kepler was busy improving Copernicus heliocentric model of the Solar system. In 1609, as Galileo reported his findings of mountains on the Moon, Kepler eventually hit upon the idea of using ellipses to fit the locations of Mars using his mentor Tycho Brahe's measurements (Kepler, 1609). We won't discuss too much the fact that Brahe was famously incorrigible and Kepler had to wait until after he died to obtain full access to observational results that he had taken on the Danish island of Uraninborg. Using insights from these observations of Mars, Kepler was later able to come up with his third law which relates the time it takes for a planet to go around a star to the distance of that planet from the star (Kepler, 1619). Kepler's final work of 1619 is called "Harmonices Mundi" or the Harmony of the Worlds. Inspired by the idea of a harmony existing between geometry of polygons and the regularity and harmony of the orbits of the planets, Kepler was able to deduce a relationship between the distance of a planet to its star, and the amount of time it took to orbit the star:

t^2 = d^3 <eqn 1>

d is the distance from Mars to the sun, and if we insert that into the equation, we get:

t^2 = (1.524)^3 = 3.54

t = sqrt(3.54) = 1.881

Therefore the Martian year is 1.88 times longer than Earth. Given that Earth has 365.25 days in a year, the Martian year is 687 Earth days long.

Seasons on Mars

In 1800, one hundred and fifty years after Cassini discovered how far Mars was from the Earth, a German born British astronomer, William Herschel, who can rightly be called the first (Father?) spectroscopist. Herschel recognised the phenomenon of infrared radiation by carrying out a very cool heat experiment. Herschel used a prism to split the light from the sun into different colours "of the rainbow". He then used a thermometer to measure the amount of heat delivered across the spectrum from blue to red. He then moved his detector into the region beyond the red colours, and found that an even greater amount of energy was being deposited in this "infrared" region by invisible light. In carrying out this first infrared (IR) experiment, he had built the first spectroscope. As we shall discuss below, the story of how this instrument has been improved over the past 200 years is key to understanding how our knowledge of the seasons on Mars.

Not only did Father Spectroscopy discover the IR, he was also an avid telescope developer. In fact, his IR discovery was the result of him chasing down sources of glare in his telescope. He also became the first discoverer of a planet since antiquity when he discovered Uranus. But he is part of this Martian story because he improved Cassini's rotation period of Mars to 24 hours, 37 minutes and 8 seconds long, very close to today's accepted value. He also measured the inclination of Mars to the ecliptic (angle of rotation relative to the Sun-Mars line) as 24 degrees, and recognised it as similar to that of the Earth. In the course of these observations, he was able to observe the white polar caps of Mars and see that they varied over time. He speculated that the polar caps of Mars might consist of water ice, by analogy with the Earth. He also speculated that Mars may, as a result, be inhabited. However, he also carried out an observation of stars traveling behind Mars and noted that they did not 'twinkle' for a large period as they were obscured by Mars, which led him to infer (correctly) that today's Martian atmosphere must be relatively thin.