“What are the uniform and ordered movements, by the assumption of which the apparent movements of the planets can be accounted for?” —Plato 4th Century B.C.
As our ancient ancestors gazed up at the night sky, they were vexed for centuries by “stars” that changed in brightness and moved against the background of other stars. The ancient Greeks called these cosmic vagabonds planetes, or “wanderers.” As the ancients tracked the planets, they noticed that the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn all followed a path in the sky through a fixed set of constellations. This path is called the Ecliptic and it traces through the constellations of the zodiac.
About 100 A.D, the Greek Astronomer Ptolemy came up with an idea that did a pretty good job of explaining the phenomena. Ptolemy’s model placed the Earth at the center of the universe and had the Wanderers all orbiting Earth. While incorrect, this geocentric model was used for 1500 years or so to predict the positions of the Sun, Moon, and the planets visible to the naked eye.
Ptolemy’s model, however, didn’t fully explain why the planets would sometimes appear to stop dead in their tracks and actually move backwards! Nor did geocentrism account for why the planets would vary in brightness. At the dawn of the Renaissance, Copernicus, Galileo, and Kepler challenged the geocentric notions that had prevailed during the Dark Ages with the revolutionary idea that the Earth, her Moon and the planets all revolved around our Sun.
It’s hard to overstate how this revelation rocked the very foundation of our understanding of our place in the universe.
My own exploration of our solar system began on a nondescript day at Paul Revere Junior High School in 1968. I was sitting at a grubby lunch table when my friend Allan Gardner sketched a small drawing of the planet Mars in pencil on the table as he had seen it through his small telescope. The modest drawing showed the southern polar cap.
I was thunderstruck that such things were within the view of anything but the largest and most expensive telescopes.This earthshaking notion prompted me to save up and buy another friend’s second-hand telescope and a lifelong love affair with the heavens began.
What I soon discovered is why the planets vary in brightness, Mars especially. As the Earth and Mars orbit the Sun, the Earth does so in a year’s time (the definition of a year). Mars, at roughly twice the distance from the Sun as the Earth, takes 1.88 terrestrial years to orbit the Sun—so we “lap” Mars every two years or so. When this happens, Mars is large and bright and said to be at opposition. On the other hand, when the Earth and Mars are at opposite sides of the solar system, Mars appears very small and much fainter.
Of course this latter condition was how I first found Mars in my “new” telescope. But one of the many things astronomy has taught me is patience. In 1969, we experienced a very favorable opposition with Mars. Because both the Earth and Mars travel in elliptical orbits, the distance varies each time we “lap” Mars. 1969 was an especially close opposition.
Mars is the most prominent planet in the evening sky right now. To find The Red Planet, look to the southern horizon just after dark. Mars is bright rusty orange in color and will shine with a steady light while stars will twinkle.
This summer we had a favorable opposition with Mars at .38 astronomical units (an astronomical unit is the average distance between the Sun and Earth). The next opposition will occur in October of 2020 when Mars will be .41 AU away from us.
Whether it is the polar ice caps of Mars, rings of Saturn, the stormy clouds and dancing moons of Jupiter, or the dramatic, Moon-like phases of Venus, the Wanderers will entice the backyard astronomer to explore the night sky.