Why is Mars sometimes bright and sometimes faint?

Why was Mars brighter in our sky in 2018 than since 2003? And why is it so much fainter now? What about the rest of this year? Will Mars brighten again in 2019?

glowing red dot

Mars was very bright for several months around July 2018! And it was very red in color. Dennis Chabot of POSNE NightSky captured this photo of Mars on July 21, 2018.

Do any of you remember Mars in 2003? That was the last time the red planet came exceptionally close to Earth. It was closer in 2003 than it had been in some 60,000 years. Now … do you remember Mars in July 2018? In 2018, Mars wasn’t quite as bright as it was in 2003. But nearly! Beginning in early July 2018, Mars appeared brighter in our sky than Jupiter, which is normally the second-brightest planet and fourth-brightest object in the sky after the sun, moon and planet Venus. At its peak in late July, Mars outshone Jupiter by some 1.8 times. It was brighter than all the stars. It remained brighter than Jupiter until around September 7, a blazing red dot of flame in our night sky.

Moon lovers! Order this year’s EarthSky lunar calendar here

Now – some half a year past Mars at its brightest in 2018 – Mars has dimmed. In January 2019, it remains modestly-bright and beautiful, shining as brightly a 1st-magnitude star, that is, as brightly as the brightest stars. What’s more, Mars is the lone bright planet in the January 2019 evening sky. It stays out until late evening throughout January 2019, in both the Northern and Southern Hemispheres.

Yet Mars will be fainter by January’s end than it is now. It’ll be fainter by February’s end than in January, and so on – continuing to dim as the months of 2019 pass, becoming too faint and too close to the sun’s glare around July 2019 – and finally passing behind the sun as viewed from Earth in early September.

Mars can be faint, or it can be a bright planet. Keep reading to learn why the appearance of Mars varies so widely in our sky, making Mars one of the most interesting planets to watch!

Want to see Mars now? Check out EarthSky’s planet guide

sky chart moon and Mars

A fun opportunity to view Mars comes around the evening of January 12, when the moon will sweep past it on our sky’s dome. Read more.

More than any other bright planet, the appearance of Mars in our night sky changes from year to year. Its 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! Mars was faint throughout 2017, then bright in 2018. Now, in 2019, Mars is growing faint again.

Why? Why does Mars sometimes appear very bright, and sometimes very faint?

The first thing to realize is that Mars isn’t a very big world. It is 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).

The small size of Mars is your first clue to its varying brightness. The small size means that, when Mars is bright, its brightness isn’t due to bigness, as is the case with the largest planet in our solar system, Jupiter.

Photos of bright Mars in 2018, from the EarthSky community

montage with large Earth on one side and smaller Mars to scale

Mars isn’t very big, so its brightness – when it is bright – isn’t due to its bigness, as is true of Jupiter. Mars’ brightness, or lack of brightness, is all about how close it is to us. Image via Lunar and Planetary Institute.

long exposure with big Mars with reflection in lake, and Milky Way

Matt Pollack captured Mars from Little Tupper Lake in the Adirondacks of upstate New York in July 2018. Read more about this photo.

Instead, the main reason for Mars’ extremes in brightness has 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 hence near one another. At other times, as it was for much of 2017 and will be again for much of 2019, Mars is moving on the opposite side of the solar system from Earth.

Look at the diagrams below, which show Earth and Mars in their respective orbits around the sun in mid-2018 and in September, 2019 …

diagram of orbits

Earth (blue) passed between between the sun and Mars (red) on July 27, 2018. This was Mars’ opposition. It comes to opposition about every 2 years, and, at such times, Mars is always at its best for that 2-year period. There’s also a 15-year cycle of Mars, whereby the red planet is brighter and fainter at opposition. In July 2018, we were at the peak of the 2-year cycle – and the peak of the 15-year cycle – and Mars was very, very bright! Image via Fourmilab.

diagram of orbits

In September, 2019, Mars (red) will be behind the sun from Earth (blue). As that time approaches, as Earth flies ahead of Mars in its smaller, faster orbit, Mars will appear increasingly faint in our sky. Image via Fourmilab.

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. 2018 was a very, very special year for Mars, when the planet was brighter than 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 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 another 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! It was only 1.2 million miles (just under 2 million km) farther away in 2018.

Closest approach for Mars in 2018 took place on July 31, some four days after its July 27 opposition.

Diagram of Mars oppositions

Diagram by Roy L. Bishop. Copyright Royal Astronomical Society of Canada. Used with permission. Visit the RASC estore to purchase the Observer’s Handbook, a necessary tool for all skywatchers. Read more about this image.

What will happen after September 2019 for Mars? After passing behind the sun from us in September, Mars will take a couple of months to become visible again, this time in the east before sunup. Late 2019 will find Mars hovering low in the east at dawn, shining inconspicuously. In the first part of 2020,though, it’ll become obvious that Mars is brightening again! Earth will then be catching up to Mars again, with the distance between our two worlds decreasing.

Thus – after a poor year for viewing Mars in 2019 – 2020 will be another fine year for viewing Mars!

Sun, Earth, Mars with orbits shown

Artist’s concept of Earth (3rd planet from the sun) passing between the sun and Mars (4th planet from the sun). Not to scale. This is Mars’ opposition, when it appears opposite the sun in our sky. The exact date of opposition is July 27, 2018. Image via NASA.

Bottom line: Mars alternates years in appearing bright and faint in our night sky. In 2018, we had a grand view of Mars … best since 2003! July and August 2018 were the best months to see it. In 2019, we’re in one of Mars’ faint years. The planet begins the year as bright as the sky’s brightest stars, but fades rapidly, becoming very inconspicuous by northern spring 2019. In late northern summer, it finally slips into the sun’s glare, passing behind the sun from Earth in early September 2019.

Deborah Byrd

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