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.
PLANETARY ORBITS AND OPPOSITIONS
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.