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. Mars has been faint throughout 2017. Now it’s still faint, in the eastern sky before dawn, having a close conjunction with dazzling Venus on October 5, 2017.
But – in 2018 – Mars will begin to brighten. At its brightest in mid-year, it’ll briefly appear brighter than the second-brightest planet, Jupiter.
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), making it only slightly more than half as big as Earth at 7,922 miles (12,750 km) in diameter. So, when it’s bright, its brightness isn’t due to its bigness, as is the case with Jupiter.
The main reason for Mars’ extremes in brightness has to do with the proximity (or lack of proximity) of Earth and Mars, during the orbits of both worlds around the sun. It’s about the nearness in space of our two worlds. Sometimes Earth and Mars are on the same side of the solar system, and hence near one another. At other times, as now, Mars is far across the solar system from us.
Mars orbits the sun just one step outward from Earth’s orbit. Earth takes a year to orbit the sun once. Mars takes about two years to orbit once.
In 2017, Mars was in superior conjunction – most directly behind the sun as viewed from Earth – on July 27. As stated above, the planet is now in the east before dawn; that’s where it always is after superior conjunction, as the line between us and Mars is now slightly to one side of a line between us and the sun. That is, when we look toward Mars in October, 2017, the sun is still below our horizon, just about to rise.
But, by mid-2018, Mars will appear brighter than it has in many years.
That’s because, in mid-2018, Mars and Earth will be relatively close, with Earth passing between the sun and Mars. Astronomers will call it an opposition of Mars, because then Mars and the sun will appear on opposite sides of our sky (with Mars rising in the east at sunset). It happens every two years and 50 days.
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 has been 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.