Why is Mars sometimes bright and sometimes faint?

In 2018, Mars was brighter in our sky than it had been since 2003. In 2019, Mars was mostly faint and inconspicuous. But 2020, here we come!

Several small white dots on black background with one big glowing red dot.

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

Mars was brighter in 2018 than all the stars. It was even brighter than Jupiter. It was a blazing red dot of flame in our night sky for several months. This year, 2019, Mars was mostly faint. It was barely noticeable in our sky. Why? Why is Mars bright in some years, but faint in others? And what are the prospects for Mars in 2020?

Let’s talk about this week first. The week of December 9, 2019, is a good time to come to know Mars. That’s because – this week – Mars is near the star Zubenelgenubi in the east before sunup. Mars is faint now. But you can use the juxtaposition of Mars and this star to locate the planet on the sky’s dome. Read more.

So 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 tonight or in the coming months? Bookmark EarthSky’s planet guide

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Slanted green line of ecliptic with Mars and three labeled stars.

A fun opportunity to see Mars comes around the morning of December 10, 2019. Look east before sunrise for the star Zubenelgenubi – Libra the Scales’ alpha star – above brighter Mars. 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, bright in 2018, and has been faint again for most of 2019. Mars is relatively faint now, a not-very-noticeable light in our eastern predawn sky.

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.

Montage with photos of large Earth 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 was again for much of 2019, Mars was 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 this month, December 2019 …

Heliocentric chart showing planetary orbits with positions of planets.

Earth (blue) last 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.

Heliocentric chart showing planetary orbits with positions of planets in December 2019.

Mars passed most directly behind the sun from Earth in September 2019. This chart shows December 2019, when Mars (red) is still far across the solar system from Earth (blue). But, in 2020, Earth will catch up to Mars again! See the chart below. Image via Fourmilab.

Large heliocentric chart showing planetary orbits with positions of planets October 13, 2020.

This chart shows the relative positions of Earth (blue) and Mars (red) at the time of Mars’ coming opposition on October 13, 2020. Around that time, Mars will appear bright in our sky again – and in the sky all night long – but it won’t be as bright as it was in 2018. 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. Last year, 2018, was a very, very special year for Mars, when the planet was brighter than it has 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.

Concentric circles with Mars different sizes at different points around its orbit.

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.

And now? Earth will pass between Mars and the sun next on October 13, 2020. The red planet will appear brightest in our sky – very bright indeed and fiery red – around that time.

Mars is now in that part of its cycle with respect to Earth in which it’ll hover – faint and not at all noticeable – in our predawn sky for some months. Mars was in conjunction with the sun, that is, behind the sun as seen from Earth – on September 2, 2019. And now it remains far across the solar system from us, with Earth speeding around in its orbit, trying to catch Mars again. But it’ll be some months before we catch it.

And thus Mars alternates years in being bright in our sky, or faint. 2019 was a dull year, but 2020 will be an exciting one, for Mars! It’ll start slowly, though, with Mars sitting low in the east (much as it is now) as the year begins. January, February, March, April 2020 … you’ll still find Mars doing nothing much, low in the east before sunup. Earth will still be far across the solar system from Mars, but rushing along in our smaller, faster orbit, trying to catch up. As northern summer 2020 approaches, Mars will begin to change. It’ll begin to brighten more dramatically as, finally, Earth begins to catch up to Mars.

Watch for it!

Sun, Earth, Mars lined up 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. 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! In 2019, we were in one of Mars’ faint years. But 2020 will – once again – be a bright year for Mars.

Photos of bright Mars in 2018, from the EarthSky community

Deborah Byrd