Last Sunday on May 22nd, Mars reached its opposition, an astronomical phenomenon which happens every 2 years. Since the orbit of Mars is located outside the orbit of Earth, the Earth, Mars, and the Sun periodically occur in a straight line leading to opposition of Mars, wherewith it lies on the opposite side of the Earth from the Sun. The planet rises at sunset and sets at sunrise and is high overhead at midnight.
Mars is well-known for possessing a distinctive retro-gate motion as it orbits. In essence, the planet appears to move eastwards only to start moving westwards after a while. But, that’s only because Earth has a faster orbital velocity than Mars and thus, at some point, Earth catches up with and overtakes Mars. Indeed, Earth travels around its orbit half the time it takes Mars to complete a circuit and thus, it overtakes it once every 780 days, in which case Mars appears to us to be moving backwards. When opposition occurs, Mars appears quite bright and its apparent diameter exceeds 5 arcseconds, making distinctive features discernible through a 150 mm telescope.
Because the orbits of the Earth and Mars are elliptical in nature, the planets happen to be closer to each other at some positions rather than at others. Indeed, the distance between the two planets is not the same at each opposition. There is thus a synodic period between successively occurring close and distant oppositions every 17 years. In fact, some oppositions can place Mars as close as 55 million kilometres to Earth when it is near perihelion (closest to the sun) while some others can place it as far as 100 million kilometres when it is at aphelion (farthest from the sun). Oppositions thus happen to coincide with Mars when it is at its closest approach to the sun or when it’s farthest away from it. The perihelion and aphelion distances of Mars from the sun are 200 million kilometres and 250 million kilometres respectively.
The best oppositions occur during the month of August, at which time the position of Earth happens to be at closest approach with Mars’ orbit. At this point, Mars appears spectacularly bright with a diameter 25 arcseconds across, large enough such that its icy caps are conspicuous for a small telescope. In fact, on August 27th 2003, Mars made its closest approach to Earth in 60,000 years at its closest point to the Sun. It was within 55, 758,006 kilometres away from Earth and looked 6 times larger than its normal size.The last time Mars came this close (as 2003) was estimated to have taken place on September 12th, 57617 BC and the next closest approach is estimated to take place in the year 2287.
Even though Earth already overtook Mars on May 22th last week, Mars is still inching closer to the sun in its orbit and indeed, it will get closer to Earth than it ever has since November 2005 on May 30th, at which point it will be 75 million kilometres away. At a diameter of 18.6 arcseconds, it promises to offer a great sight. Look into the southeastern sky starting at 5:36 p.m ET but wait until it gets dark. Mars will be the brightest object, appearing as a red shining beacon of light.
Feature image credit: NASA.