From the July 2018 issue

Mars closes in

The Red Planet’s opposition this summer comes with a few caveats.
By | Published: July 20, 2018

We’re all excited about this extraordinary Mars opposition. That’s July 27, with Mars shining next to the Full Moon. And Mars comes closest to Earth on the 31st — the nearest martian visit since 2003. It won’t be surpassed until 2035. Very cool stuff.

It’s a “perihelic opposition,” with Mars close to its nearest point to the Sun and Earth near its farthest, placing our two orbits at very nearly their narrowest gap. Mars at magnitude –2.8 is now brighter than Jupiter, which doesn’t happen very often. After Venus sets around nightfall, for the rest of the night, even a total newbie could find Mars just by picking out the sky’s brightest “star.” For confirmation, it’s as orange as a pumpkin — astronomy made easy.

The opposition will be a media event, and one of those cases where popular hyped astronomy is fully in sync with what we actual astronomers enjoy. Not that Mars is immune to hype. In August 2003, we had the closest martian visit in more than 50,000 years, and in the Old Farmer’s Almanac, where I’ve been the astronomy editor since forever, I wrote that through any telescope at a mere 100x, Mars would look bigger than the naked-eye Moon.

Mars will be easy to pick out in the night sky as it shines bright orange, like a cosmic pumpkin.
NASA, ESA, and A. Dyer
Well, guess what? Almost every summer after that, there have been splashy “news” headlines on the web urging people, “Look up — Mars looks bigger than the Moon!” Mars wasn’t even visible most of those summers; it’s simply that websites erroneously assumed this was an annual event. The fake “Mars is bigger than the Moon” business every single summer stems from that Old Farmer’s Almanac article, with the part about “through a telescope” omitted.

So don’t be surprised if you see that same ridiculous headline appear this year — except now, unlike all those previous summers, Mars actually has come close once again. And yes, 100x power will make it appear bigger than the Moon.

It’s fun to compare martian oppositions, which happen every 26 months, to those of, say, Saturn, which occur once a year. In both cases, speedier Earth passes the slower superior planet, making it appear big and bright for a month or two. But while Saturn always looks telescopically amazing, Mars usually has such a tiny disk that it’s hard to see detail on its 4,000-mile-wide (6,000 kilometers) body unless the opposition occurs near the narrowest gap between our orbits. And that occurs only during martian oppositions between July and October. Like right now.

The Hubble Space Telescope imaged Mars when it was 50 million miles (80 million kilometers) from Earth, during the Red Planet’s 2016 opposition. At that distance, the telescope revealed features just 20 to 30 miles (30 to 50 km) across. This year, Mars comes even closer, sitting just 35,785,000 miles (57,590,000 km) away.
NASA, ESA, the Hubble Heritage Team (STScI/AURA), J. Bell (ASU), and M. Wolff (Space Science Institute)
This Mars opposition’s width of 24″ is superb. It’ll stay bigger than 20″ through August, the same apparent size as a dime 2½ city blocks away. That’s big enough to let telescopes discern surface features. But there’s a problem. Although it’s mid-spring in the martian southern hemisphere, so the carbon dioxide-based southern polar cap is still large and striking, the word “south” has a darker side for most of us.

Fantastic up-close martian perihelic oppositions that happen in July and August give Mars a very high southern declination. It’s extremely low in the sky for U.S., Canadian, Japanese, Chinese, and European observers. The present martian declination of –25° makes it even lower than the winter solstice Sun. Its disk shines through three times more air than if it were high overhead, which almost always blurs the image.

This is no problem if you’re merely admiring Mars’ rare extreme brilliance. But if you’re hunting telescopically for surface detail, you need a steady night when the stars are not twinkling. Still, give it a try, because Mars appears this large only a few times in one’s life.

Just for fun, consider Saturn’s oppositions for a comparison. When the ringed planet comes particularly close to us, it always happens during our Northern Hemisphere winter when the planet is at its highest, in Taurus or Gemini. At the same time, its rings are always then tilted most favorably, in a maximally “open” orientation, at their best. So Saturn’s big perihelic oppositions automatically happen with optimal ring angle and maximum sky elevation. Everything comes together like gears meshing. We get three such winners, each a year apart, and then wait 27 years for the next trifecta.

By contrast, a series of two very close martian approaches, a bit more than two years apart, happens every 15 years, but with the planet mostly a low-down smudge — unless you can hang out with Aussie, Kiwi, or South African astronomers who have hit the jackpot.

Still, around here, you play the hand you’re dealt. This summer it’s the god of war at his closest and brightest.