As Mars reaches opposition, watch as the full Moon occults the Red Planet!
Like that back-ordered telescope we've been waiting months to receive, the truck has finally arrived and delivered Mars to our doorstep. Impressively bright at magnitude –1.8, it rises in Taurus in the scintillating company of the Winter Hexagon gang. Despite this not being a particularly close opposition, the Red Planet sails high in the sky, where improved better atmospheric seeing helps to compensate for its relatively small apparent diameter.
Although Mars reaches opposition on December 8th (December 7th in the Americas), it's closest to Earth a week earlier on December 1st at 2:18 UT when the two planets will sit just 81.45 million kilometers (50.61 million miles) apart. The two dates don't coincide because Mars's elliptical orbit further removes the planet from Earth even as we catch up to join it at opposition. Three remote — or aphelic — oppositions follow this one (in 2025, 2027, and 2029) before the Earth and Mars line up nearly as close again (82.8 million km) in May 2031.
One Step Backward . . .
In early December, we find the Red Planet midway along its retrograde loop, moving west across Taurus as the faster Earth prepares to lap it at opposition. Mars rises near sunset and climbs high enough by 9 p.m. local time for a look through the telescope. Of all the planets it's the only one with a plainly visible surface. But unlike soft-touch Jupiter with its gaudy cloud bands or the gimme of Saturn's rings, Mars keeps its secrets close. Teasing out its low-contrast albedo features requires an equal mix of determination and good atmospheric seeing.
Choosing the Right Filter
I have a 10-inch Dobsonian that I can set up at a moment's notice to check on the Red Planet every clear night. If the disk displays a sharp edge (a rarity!) I immediately employ 286× to look for dark features, clouds, and potential dust storms. When the seeing gods allow I occasionally observe at 357× and even 400×. My favorite tool besides high magnification is a red 23A filter. Red penetrates the planet's hazy atmosphere and better shows the albedo features. It also tames the Martian glare and settles the image. On a good night I'll spend about an hour studying the planet. A blue filter is also handy for discerning water-ice clouds and limb hazes. Although there are no current regional or larger dust storms that would block or blur the surface they can blow up almost anytime. Orange and red filters help to sharpen their outlines and increase contrast.
Start with Syrtis
Fortunately, observers across the Americas can begin their Martian explorations at Syrtis Major (Gulf of Sidra), a low-lying shield volcano shaped like the Indian subcontinent. Few albedo markings are as large, dark and well-positioned as this one. And it will be squarely in view during the first few nights of December, near the central meridian (CM) — an imaginary north-to-south line passing through the planet's center — around 11 p.m. EST. NASA's Mars Perseverance rover landed in Jezero Crater at the northwestern tip of Syrtis Major in search of clues and possible traces of past life. In recent months, it discovered a trove of organic-rich material in the crater's ancient delta.
From Syrtis Major a pair of dark "arms"— Mare Serpentis (Sea of Serpents) and the stick-like Sinus Sabaeus (Bay of Sheba and pronounced SEE-nuss, sa-BEE-us) — form a broad, tilted "Y" to its southeast and east, respectively. Southwest of Syrtis Major look for the thick and muscular form of Mare Tyrrhenum (Tyrrhenian Sea, Ter-RAIN-num), a heavily cratered highland region.
Moving south, Syrtis Major blends into the dark, cratered highlands of Iapygia (Salento, the heel of Italy's "boot," eye-uh-PIJ-ee-uh), which is topped by the bright oval of Hellas (Greece). At 2,300 kilometers across and more than 7 kilometers deep Hellas is the largest impact basin on Mars. South of Hellas and currently small and out of view lies the remnant South Polar Cap. Right now, it's late summer in the planet's southern hemisphere when the cap reaches its smallest extent. Adding to the difficulty, Mars's south polar axis nods just 3.5° in our direction in early December. These two factors place the remaining cap at or just beyond the southern limb. Later in January, when the tip increases to 9°, astrophotographers may have success in recovering it.
We can't see the North Polar Cap (NPC) either. At least for now. For the past few months, a hood of dense, bright clouds, the North Polar Hood (NPH), has cloaked the region. Water and CO2 snow falling from the clouds are fattening up the NPC, which should return to view in mid-January as spring unfolds in the northern hemisphere, and the hood begins to dissipate. Aside from Syrtis Major, the NPH is probably the single easiest feature to spot on Mars, forming a thick white rim along the planet's northern limb.
Continuing west from Mare Tyrrhenum we encounter a light-toned, diagonal gap, Hesperia (Land of the Setting Sun) followed by the prominent band of Mare Cimmerium (Cimmerium Sea). Cimmerium, known for its water-carved gullies, merges into Mare Sirenum (Sea of Sirens, sih-REE-num), creating a long, festoon-like sash across the planet's southern hemisphere. The vast, smooth plains of Elysium (Greek resting place of heroes) and Amazonis (Land of the Amazon) fill out much of the northern hemisphere, the western portion of which is dominated by ancient volcanoes including Olympus Mons, the largest in the solar system.
Continuing west the planet offers up a great variety of dark markings including Solis Lacus (Lake of the Sun), once called "Oculus" because it resembles a giant, unblinking eye. This region is famous for dust storms, which can cause the feature's shape and contrast to vary at each opposition. Be sure to spend a few minutes looking for a skinny, twig-like extension north of Solis Lacus called Tithonius Lacus (Tithonian Lake). Together with the trilobed Aurorae Sinus (Bay of the Dawn) these dark smudges define the largest canyon system in the solar system — Valles Marineris. For all its visual fragility I get a thrill every time I see it. Mare Erythraeum (Red Sea, air-eh-THREE-um) is a mottled, expansive region that occupies a significant portion of the planet's southern hemisphere. The 1,800-kilometer-wide Argyre impact basin lies along its southern border.
Blueberry pie, anyone?
We wrap our quick review of Mars with a very busy hemisphere that includes the aptly named Sinus Meridiani (Meridian Bay), marking the location of the Martian prime (0°) meridian. Meridiani is also famous for its "blueberries," hematite-rich spherules about 0.3 cm (1/8-inch) across that likely formed in the presence of water. They litter the surface like confetti after a Times Square New Year's celebration. Together with Sinus Sabaeus, Sinus Meridiani forms a distinctive club-shaped marking that reminds me of a turkey drumstick.
Farther east you'll find Margaritifer Sinus (Pearl-bearing Bay), a wispy, triangular patch we know to be rich in clay with abundant evidence of lakes, deltas, and ancient rivers. Oxia Palus (Oxeia Marsh) joins it to Niliacus Lacus / Mare Acidalium, which merge to form a prominent dark patch shaped like a tree stump extending south from the pearly clouds of the NPH.
You can know what's up on the Red Planet anytime by using Sky & Telescope's Mars Profiler. Just type in a time and find out what's crossing the central meridian at the moment. You can also download the Mars Atlas app for iPhone or Mars Atlas for Android. Don't have a telescope? Astronomer Gianluca Masi will live-stream close-up views of Mars on his Virtual Telescope site starting at 3 p.m. EST (20:00 UT), Nov. 30.
Moon Occults Mars on December 7–8
As if opposition weren't enough, the full Cold Moon will occult Mars for much of the North America (except the Southeast and a section of the East Coast), Greenland, and Europe on December 7–8. Even if you're not in the occultation zone, you'll still see an extremely close conjunction between two of the brightest bodies in the night sky. As the time of occultation approaches, the Moon will edge its way toward Mars and then slowly devour the Red Planet by degrees, taking many seconds to completely cover it. Depending on your location, Mars will go missing for minutes to more than an hour until it returns to view at the opposite limb, making for an equally astounding sight.
I'm really curious if we'll continue to see Mars with the naked eye right up to the Moon's edge. I suspect we will. Also, at what point will the planet disappear from view as it dips behind the Moon? Keep those binoculars handy! Telescopic observers will get fabulous views, especially those living within the southern graze zone, where the Moon will partially occult the planet (see below).
For full details, see pages 48–49 in the December 2022 issue of Sky & Telescope magazine or check out the International Occultation Timing Association's (IOTA) page devoted to the event. All times are UT, so remember to subtract 5 hours for EST; 6 for CST; 7 for MST; and 8 for PST.
I hope you observe Mars often enough to feel comfortable navigating the planet just like a real Martian.