Why is Mars sometimes bright and sometimes faint?
2018 was a good year to view Mars. So was 2020, especially around September, October and November. In both 2018 and 2020, Mars appeared as a blazing red dot of flame in our night sky. On the other hand, in 2019, Mars was mostly faint and barely noticeable in our sky. And – although Mars started this year shining as brightly as the sky’s brightest stars – Mars is now the dimmest of the three planets in the evening sky and will appear quite dim throughout 2021. In May 2021, Mars has faded so much that it has begun to appear inconspicuous among the brighter stars. Mars’ dramatic swings in brightness are part of the reason the early stargazers named Mars for their god of war; sometimes, the war god rests, and sometimes he grows fierce!
Why? Why is Mars bright in some years, but faint in others? And what can you expect from the red planet in 2021? Keep reading to learn why the appearance of Mars varies so widely in our sky, making it one of the most interesting planets to watch.
In this coming northern summer (southern winter), a much-faded Mars will be hovering near the sunset glare. It’ll appear to linger for many months in that position – close to the sun along our line of sight – and finally disappear into the sunset in August 2021.
Then Mars will be gone from our sky for some months. It’ll return by 2021’s end, though, to appear in the east before sunrise.
For specifics on Mars in the coming months, bookmark EarthSky’s monthly planet guide.
To understand why Mars varies so much in brightness in Earth’s sky, first realize that Mars isn’t a very big world. It’s only 4,219 miles (6,790 km) in diameter, making it only slightly more than half Earth’s size (7,922 miles or 12,750 km in diameter).
Consider Mars in contrast to Jupiter, the biggest planet in our solar system. Jupiter is 86,881 miles (140,000 km) in diameter. More than 20 planets the size of Mars could be lined up side by side in front of Jupiter. Jupiter always looks bright, because it’s so big.
Not so for little Mars. Its extremes in brightness have to do with its nearness (or lack of nearness) to Earth.
Mars orbits the sun one step outward from Earth. The distances between Earth and Mars change as both worlds orbit around the sun. Sometimes Earth and Mars are on the same side of the solar system and near one another. Sometimes, as was the case for much of 2019 and will be the case for much of 2021, Mars and Earth are on nearly opposite sides of the sun from each other, and so Mars appears faint.
Look at next few diagrams, below. The first shows Earth and Mars in their respective positions in their orbits around the sun in October 2020, when Earth and Mars were closest for this two-year period. The second shows Earth and Mars this month. The third shows Earth and Mars in August 2021, when Mars will be fading into the sunset glare.
Earth takes a year to orbit the sun once. Mars takes about two years to orbit once. Opposition for Mars – when Earth passes between Mars and the sun – happens every two years and 50 days.
So Mars’ brightness waxes and wanes in our sky about every two years. But that’s not the only cycle of Mars that affects its brightness. There’s also a 15-year cycle of bright and faint oppositions.
Due to that 15-year cycle, 2018 was a very, very special year for Mars, when the planet was brighter than it had been since 2003. Astronomers called it a perihelic opposition (or perihelic apparition) of Mars. In other words, in 2018, we went between Mars and the sun – bringing Mars to opposition in our sky – around the same time Mars came closest to the sun. The word perihelion refers to Mars’ closest point to the sun in orbit.
Maybe you can see that – in years when we pass between Mars and the sun, when Mars is also closest to the sun – Earth and Mars are closest. That’s what happened in 2018.
2003 was the previous perihelic opposition for Mars. The red planet came within 34.6 million miles (55.7 million km) of Earth, closer than at any time in over nearly 60,000 years! That was really something.
In 2020, Mars was still very bright at opposition. But it wasn’t as bright as it had been in 2018, or in 2003.
When is the next opposition of Mars? The next time Mars will appear at its brightest for that two-year period in our sky? You guessed it. 2022!
Bottom line: Mars alternates years in appearing bright and faint in our night sky. 2020 brought another bright year for Mars. But in May 2021, Mars appears faint and inconspicuous again and will remain so for the rest of this year.