What Does It Mean for Mars to Be in 'Full Opposition'?

What does it mean for Mars to be in full opposition? How rarely is the red planet so close to Earth, and how can I get the best view?

Sabrina Stierwalt, PhD
4-minute read
Episode #192

On May 22, 2016, Mars will be in what is known as “full opposition.” At this time, the Sun, the Earth, and Mars will sit in a straight line with the Earth directly between our planetary neighbor and our star. All planets that are farther from the Sun than the Earth pass through similar opposition points. Due to the ellipticity of the Martian and Earthly orbits (i.e., their deviations from purely circular orbits in a flat plain), this alignment does not occur regularly based, for example, on our calendar system, but instead happens once about every 26 months.

Since night occurs for us when our side of the Earth faces away from the Sun, Mars will then be most clearly visible at opposition roughly around midnight for any given local time. This positioning is also coincident (to within about a week) with perigee, the point in the orbit of the red planet where Mars is closest to Earth and thus appears brightest and largest in the sky. At this year’s perigee Mars will be roughly 0.5 astronomical units away from us (that’s about half the distance from Earth to the Sun). This is the closest Mars has been since 2005 (although it will be even closer in 2018).

Thus next week will make for some of the best observations of Mars with amateur telescopes in recent history. Making Mars even easier to find this year, the planet will form a triangle on the sky with two other bright sources: the ringed planet Saturn and Antares, a bright star in the constellation Scorpius. For more help locating Mars on the sky even with your naked eye, check out earthsky.org for some helpful finding charts.

Also worth keeping in mind is that, due to the rotation of the Earth, objects on the sky appear at the same location four minutes earlier each night. So even though May 22 marks the point of full opposition, Mars will still be relatively bright for several weeks.

Mars in Retrograde

This gradual, four-minute change in position each night also helps explain why sometimes Mars appears to move backwards in the sky, a phenomenon known as retrograde motion. Since from a given vantage point on the Earth, the Earth’s rotation brings you to view the same spot on the sky four minutes earlier each night, celestial objects, including Mars, will appear to move eastward from one night to the next. However, approximately every two years there is a period of time when Mars appears to double back and move from west to east for a few months before returning to its normal eastward motion.

You may have heard someone blame a particularly hectic day or a string of bad luck on Mars being in retrograde. In reality there are no mysterious forces at work in the universe that are making you constantly misplace your keys or putting you in a less than stellar mood. It’s just geometry!

Mars appears to move backwards across the sky in the period when Earth overtakes the red planet in its orbit. Since the Earth travels faster than Mars and in a tighter orbit around the Sun, our planet is able to make two trips around our star in roughly the time Mars needs to make only one lap. Thus a year on Mars is closer to two Earth years. As the Earth passes Mars, like a race car on a tighter, inner track, the relative motion between the two planets means that Mars appears to slow down, stop, and then briefly head back in the other direction before once again returning to normal.


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About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.