Why is Mars sometimes bright, and sometimes faint, in Earth’s sky?
More than any other bright planet, the appearance of Mars in our night sky changes from year to year. Its wild swings in brightness are part of what make Mars a fascinating planet to watch with the eye alone. What causes these brightness changes? Mars isn’t a very big world. And it orbits the sun one step outward from Earth’s orbit. Earth takes a year to orbit the sun once. Mars takes about two years to orbit once.
The brightness of Mars changes by a lot, depending on whether Mars is close to us in its slightly larger orbit, or whether it’s far across the solar system from us.
Mars is faint and relatively inconspicuous now and will remain so throughout 2017.
As 2017 opens, Mars is near bright Venus in the west. Venus will pass between the Earth and sun in late March, and afterwards Mars will be even less conspicuous in our western twilight sky, sitting there after sunset, far behind Earth in orbit, getting fainter and fainter and fainter.
Its superior conjunction – when it’ll be most directly behind the sun as viewed from Earth – will come on July 27, 2017.
But, by mid-2018, Mars will appear brighter than it has in many years.
We’re relatively close – and Mars appears at its brightest in our sky for that two-year period – every time Earth passes between the sun and Mars. Astronomers call this an opposition of Mars, and it happens every two years and 50 days.
May 22, 2016 is the date Earth last went between the sun and Mars. May 30, 2016 is the date Earth and Mars last were closest. Around that time, Mars was bright in Earth’s sky. See photos of Mars in May 2016.
Throughout 2017 – and in all years lacking a Mars opposition – Mars appears faint in Earth’s sky. In non-opposition years like the current year, Mars lies far across the solar system from Earth, at times hidden from our view by the sun itself.
But Earth and Mars will keep moving in orbit. Earth will eventually catch up to Mars and pass between it and the sun again. The image below shows Mars in mid-July, 2018, when Earth will next pass between the sun and Mars.
This 2018 opposition of Mars isn’t an ordinary opposition. Astronomers will call it a perihelic opposition (or perihelic apparition) of Mars. The word perihelion refers the point in Mars’ orbit when it is closest to the sun. 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 will be happening in 2018, and it’s why the Association of Lunar and Planetary Observers (ALPO) wrote:
The 2018 perihelic apparition of Mars will prove to be one of the most favorable since the 2003 apparition when the Red Planet came closest to Earth in 59,635 years (the year 57,617 BC).
According to ALPO, in 2003, Mars came within 34.6 million miles (55.7 million km) to Earth, closer than at any time in over nearly 60 thousand years! It’ll be only 1.2 million miles (just under 2 million km) farther away in 2018. Closest approach for Mars in 2018 will take place about two weeks after the opposition date, on on July 31.
So 2017 is, indeed, a lousy year for Mars. But just wait! Mars will be grand in 2018.
Bottom line: Mars is bright when it and Earth are on the same side of the sun. It’s faint when it and Earth are on opposite sides of the sun. 2017 is one of the off-years, and Mars will be faint in our sky throughout this year. But, in 2018, we’ll have a grand view of Mars … best since 2003!
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.