Opposition describes the occasion marked by the sun, Earth and Mars all lining up perfectly. Earth is in the middle, so the sun is on one side while Mars is on the other. That means Mars will be at the opposite point in the sky, above the horizon after the sun has set.
It also means Mars will appear fully illuminated from the vantage point of Earth-dwellers, causing it to appear especially bright.
Where to look
Mars was closest to Earth a week ago on Oct. 6, in fact the closest in 15 years, but appears more brilliant Tuesday night. That’s because it’s in a better position to reflect more sunlight back at us. Last week, it was doing so at a slanted angle, acutely diminishing its apparent magnitude.
If you’re looking to catch Mars at its most effulgent, all you have to do is look east an hour or two after sunset. Mars will be highest toward midnight.
You’ll be able to tell which one is Mars based on its brightness and color. Only Venus and the Moon will be more scintillating. But Venus makes its appearance in the mornings.
You’ll also see a reddish tinge to Mars, resulting from the iron oxide-rich surface that gives it a rusty hue.
What is opposition, and how often does it occur?
Mars is the fourth planet from the sun. Earth is located about 93 million miles from the sun; Mars averages 142 million miles. It’s the last of the solar system’s solid, dense inner planets, which also include Mercury, Venus and Earth.
Earth rotates around the sun once every 365 days; Mars takes 687 days to do the same, so a year on Mars is longer. That also means that Earth and Mars are usually in different places in their orbits about the sun.
Picture two cars driving around a traffic circle, at different speeds. Even if they start out next to each other, the faster car will outrun the slower until it comes back around again. That’s sort of how Earth and Mars behave.
As a result, opposition occurs every 26 months or so, when the more quickly orbiting Earth swings by on the “inside lane” as it passes by Mars.
Why opposition doesn’t always mean the closest point to Earth
Common sense dictates that when we pass Mars in a perfect line with the sun, Mars should be at its closest point to Earth. But that’s not exactly true. Mars was closer a week ago. SpaceWeather.com reports that was its closest until 2035.
The reason? Mars has a slightly eccentric, or elliptical, orbit. In addition, the gravity of Jupiter tugs on Mars and causes its orbit to be a bit off-kilter compared with ours. Those orbital quirks make it so Mars can sometimes be closest before opposition, and therefore before it appears brightest.
According to EarthSky.org, Mars will next reach its closest point to Earth on Dec. 1, 2022, but the next opposition will occur a week later.