Mars Opposition 2016

That increasingly bright and obviously red object rising in the southeast late at night (around 10:30 pm EDT from Lynchburg) is Mars. It will rise ever earlier as it moves into position exactly opposite the sun in our sky on May 22, when it will rise around 8:30 EDT. This every-26-month event is a Mars opposition, and this is a reasonably good one.

What makes one opposition “better” than another? Therein lies a tale of orbital peculiarities that allowed the true nature of our solar system to come to light.


The orbits of planets around the sun are not perfect circles, they are in fact ellipses, circles that have been pulled and stretched. The ellipse below varies from circularity far more than any planetary orbit in our solar system, but it illustrates the point.


Earth’s orbit is elliptical, but not very much so. We are roughly 3 million miles (5 million kilometers) closer to the sun in January than we are in July, with an average distance of 93 million miles (150 million kilometers). But Mars! Mars has the second most (after Mercury) elliptical orbit of the eight planets (sorry, Pluto lovers) and that means that not all Mars oppositions are created equal.

The wonderful diagram below shows the positions of both Earth and Mars for all oppositions between 2012 and 2027. The distances between the two planets are given in astronomical units (AU) where one AU is that average distance between Earth and the sun. Mars, further from the sun than the Earth and therefore moving more slowly around it, takes 687 days for one orbit. The oppositions will occur at different places around that orbit, and only when the faster-moving Earth has caught up to the more stately motion of its sister planet.


The opposition of 2027 is an example of a “bad” opposition. Mars is near its aphelion (farthest distance from the sun, marked by the orange A), and so the distance between the two planets is 0.6780 AU, or 63 million miles (101 million kilometers). By contrast, the 2018 opposition is a very “good” one. Mars and Earth line up almost exactly halfway between perihelion (closest distance to the sun) for Mars and aphelion for Earth. The planetary distance is 0.3862 AU: 36 million miles (58 million kilometers). Quite a difference!

The difference between these two oppositions is seen in the greater brightness of Mars in our skies with the nearer opposition, and the greater apparent size of its disc. In the days before we had robots roaming its surface, Mars was eagerly scanned with the most powerful telescopes of the day at each opposition, particularly at very favorable ones.


A careful observer of the motion of Mars against the background stars around the time of opposition would see the planet appear to reverse its daily west to east motion and travel for a few weeks from east to west, then resume its west to east motion until the next opposition. The moon travels in a similar fashion from west to east against the background stars as it moves through its phases.

This motion is hard to explain with an Earth-centered geocentric model of the solar system. Attempts to do so by adding epicycles—circles on circles—somewhat alleviated the difficulties.


But as measurements became more precise, it became more apparent that something was amiss. The timing was off, and the observed motions of Mars required ever more elaborate epicycles with planets speeding up or slowing down for no reason other than a requirement to fit the observations.

Although many original works are lost, there are references among ancient Greeks suggesting that they recognized the true nature of this motion, that it was apparent not real, and that it resulted from the motion of the two planets around a common center—the sun.


Or in a stationary image:


But for all sorts of socio-politico-techno-religious reasons, it was not until centuries later that the heliocentric model began to be widely accepted. And only about a century later that the remaining irregularities were resolved by the realization that planetary orbits were not perfect circles, but ellipses.

A wonderful quote from “The Copernicus Complex” by Caleb Scharf:

“Many of our discoveries about nature have come to light only after examining tiny details—the little nagging inconsistencies that at first seem so technical and esoteric and only later seem so marvelous. Breakthroughs have happened when someone became intrigued with the slightly wayward motions of planets…”





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