Why is Mars sometimes bright and sometimes faint?
Mars is bright sometimes, but not now
2020 was a good year to view Mars. The red planet appeared as a blazing red dot of flame in our night sky. But, although Mars started 2021 shining brightly, it quickly faded. And it went on to appear quite dim throughout this year. In August, Mars disappeared into the sunset. And our two worlds – Earth and Mars – traveled (and traveled) in their orbits around the sun. And finally, now, Mars is in conjunction with the sun, or behind the sun from Earth. When will you see Mars next? And what are the prospects for 2022?
Mars’ conjunction with the sun comes on October 8, 2021 (at about 04:00 UTC). That’s why – for all of October 2021 – Mars will be entirely gone from our sky. It’s traveling across the sky with the sun during the day. But Mars will return before 2021 ends. It’ll appear in the east before sunrise in late November, to start a new cycle of visibility and a much better year in 2022.
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! It’s part of what makes Mars so interesting to watch in the night sky.
Mars isn’t very big
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 one step outward
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 the illustration below, which views the solar system from above. And then look at the illustration at the top of this post. The illustration at top shows Earth and Mars in their respective positions in their orbits around the sun in October 2021. Remember, Mars comes most nearly to passing behind the sun from Earth on October 7-8. The illustration below shows Earth and Mars as they will be around the time of Mars’ 2022 opposition on December 4. At that time, Earth will be passing between the sun and Mars. And Mars will be closest and brightest for all of 2022.
Why is Mars sometimes bright?
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 2021, Mars has been mostly faint and inconspicuous. Why?