Guest post by Alex Diaz
Like every other planet in our solar system, Mars orbits the Sun. But Earth is the 3rd planet from the Sun while as Mars is the 4th planet from the Sun. This means that Earth is closer to the Sun than Mars. In the time it takes Earth to complete 2 revolutions around the Sun, Mars has only gone around once. So sometimes the two planets are on opposite sides of the Sun, very far apart, and other times, Earth catches up with its neighbour (known as retrograde motion where it seems like Mars is moving backwards in the night sky) and passes relatively close to us.
So what is Opposition? During opposition, Mars and the Sun are directly opposite to each other relative to the Earth. From our perspective, Mars will rise in the East just as the Sun sets in the West following the Ecliptic (Path of the Sun). When Mars and the Sun appear on opposite sides of the sky, we say Mars is in “Opposition”. So think about this, if Earth and Mars followed perfectly circular orbits, opposition would be as close as the two planets would ever get.
But of course, nothing about motion in space is ever that simple! Our orbits are actually elliptical (oval-shaped)! This means we travel a little closer to the sun at one end of our orbits than at the other (known as perihelion and aphelion respectively).
Mars oppositions can happen every 26 months. But every 15 or 17 years, opposition occurs within a few weeks of Mars’ perihelion (which is when it is the closest to the Sun in its orbit). This year Mars’ perihelion was May 22, 2016
An opposition can occur anywhere along Mars’ orbit. What makes this year’s opposition special, is that it happens while the red planet is closest to the Sun and Mars will be particularly close to Earth. If Earth and Mars both had perfectly stable orbits, then each perihelic opposition (when both planets are closest to the Sun) would bring the two planets as close as they could be. That’s almost the way it is!
But once again, nature throws in a few wrenches to make it a little complicated. Gravitational tugging by the other planets constantly changes the shape of our orbits just a little bit every time. Massive Jupiter especially influences the orbit of Mars being that the two are next to each other in terms of orbits. Also, the orbits of Earth and Mars don’t lie in quite the same plane, nor do any other planets for that matter. The paths the planets take around the sun are slightly tilted with respect to each other hence we are above and below each other but close enough to say we are following the Ecliptic.
So, with all these complicated factors added in, some perihelic oppositions actually bring us closer together than others. Take the 2003 opposition for example; it was the closest approach in almost 60,000 years!
Mars’ orbit is more elliptical than Earth’s, so the difference between perihelion and aphelion is greater. Over the past centuries, Mars’ orbit has been getting more and more elliptical or elongated, bringing the planet even nearer to the Sun at perihelion and even further away at aphelion. So in the future, perihelic oppositions will bring Earth and Mars much closer together. But we’ll still have bragging rights for awhile because our 2003 record will stand until August 28, 2287!
Happy observing folks!