From the October 2020 issue

Mars opposition 2020: See the Red Planet loom large

Give yourself an early Halloween treat this year by spending some quality time observing Mars at its best.
By | Published: October 3, 2020
Mars’ opposition was the target of the Hubble Space Telescope in April and May 1999. At the time, the Red Planet was 54 million miles (87 million km) from Earth. From left to right, the images show: the region of Acidalia Planitia; Tharsis Montes, the largest volcanic region on the planet; Elysium Planitia, the largest of Mars’ 12 plains; and the great albedo feature Syrtis Major. 
Steve Lee (Univ. of Colorado)/Jim Bell (Cornell Univ.)/Mike Wolff (Space Science Institute)/NASA/ESA
Planetary observers have been waiting two long years for another opportunity to see Mars dominate the night sky. And although it won’t be quite as big or bright as it was in 2018, the Red Planet will stand much higher in the sky this fall. Amateur astronomers who point their telescopes and cameras at it won’t have to deal with as much of Earth’s obscuring atmosphere, so views and images will be superior to those of its previous appearance.
Mars appeared as the destination along a road near Moab, Utah, at 4:36 a.m. MDT on May 16, 2018. The photographer mounted his camera on a tripod and took a 13-second exposure. 
John Chumack

Just the facts

The best time to observe Mars is when it is directly across from the Sun from our perspective. Astronomers refer to this as opposition, and the Red Planet reaches it at 7:20 p.m. EDT on October 13. When a celestial object is at opposition, it rises at sunset, is visible all night long, and sets at sunrise. Also, because sunlight is falling on it from behind us, it appears brightest.

Mars actually comes closest to Earth a week earlier, on October 6, at 10:19 a.m. At that time, it will lie 38,568,243 miles (62,069,570 kilometers), or 0.415 of an astronomical unit (AU; 1 AU is the average distance between Earth and the Sun), away from our planet. On this date, Mars will attain its maximum size of 22.6″. (It will appear 22.4″ across at opposition.) In fact, Mars’ will span 20″ or more for nearly two months, from September 4 through November 2. As a comparison, the Full Moon’s average diameter is 1,865″, so the Red Planet’s diameter is, at its largest, 1.2 percent that of the Moon’s.

Mars spends September, October, and November in the constellation Pisces the Fish. It begins retrograde (westward) motion September 9, and resumes direct (eastward) motion November 14. 
Astronomy: Roen Kelly
Comparing this apparition to the one in 2018, Mars will appear 1.7″ smaller, but it will stand 31° higher in the sky. An object’s altitude is an important consideration for viewing because the less air you look through, the better the image will be. If you observe Mars at opposition from a site at 40° north latitude, its altitude at midnight (when it’s highest in the south) will be a worthy 55°. If your observing location is more northerly, Mars will lie 1° lower for each degree of latitude. But south of 40°, it will appear 1° higher per degree of latitude. I look forward to seeing it standing 63° high in the south from my location in Tucson.

Mars also will appear 0.2 magnitude fainter than it did two years ago. That said, it still will blaze at magnitude –2.6 from October 5-16. In fact, it will be brighter than magnitude –2.0 from September 7 through November 6; and brighter than Sirius (Alpha [a] Canis Majoris), which is the brightest star at magnitude –1.46, from August 17 through November 20.

Throughout this entire period, Mars will appear within the boundaries of the constellation Pisces the Fish. During most of its orbit, Mars moves eastward through the backdrop of stars. But when Earth overtakes and passes it, the Red Planet appears to travel westward — movement that astronomers refer to as retrograde motion. For this apparition, Mars will be in retrograde from September 9 through November 14.

Mars will be brilliant throughout October, and will appear even more so because it lies in one of the darkest parts of the sky. At opposition, the two closest 1st-magnitude stars will be Aldebaran (Alpha Tauri) and Fomalhaut (Alpha Piscis Austrini). Each will lie 49° away.


How big and bright Mars appears depends on when you observe it. This graph shows how the Red Planet’s apparent size grows before opposition and shrinks afterward. Try to view it as close as you can to when it’s at its largest.

Astronomy: Roen Kelly

Start observing

Astronomers call one martian day a sol. One sol is 37.4 minutes longer than an Earth day. So, if you observe Mars at the same time each night, it will appear to rotate slowly (just 9.11° each day) to the west. This means that in 39½ days, Mars will go through a complete rotation, albeit backward. All its major features will, on one night or another, appear on the planet’s meridian, an imaginary line from north to south that divides the planet in half.

One class of targets observers usually seek on Mars are albedo features. Albedo is a measure of how much light something reflects. These features, therefore, can be bright (high albedo), dark (low albedo), or a mix of both. The most prominent albedo feature is Syrtis Major Planum, a dark high plain that spans some 750 miles (1,200 km), making it the eighth-largest such feature on Mars.

Although farther from Earth and smaller than it appeared in 2003, Mars in 2020 will look much larger than during its worst opposition, more than 800 years from now. 
Astronomy: Roen Kelly
Beyond Syrtis Major, the best albedo features to look for include the Hellas basin, Elysium, Chryse, Libya, and Solis Lacus. It’s worth revisiting these every week or so because their appearances change through Mars’ seasonal cycle. Some normally light features may appear darker, and vice versa.

Albedo features are definitely worth searching for and spending serious time on. But if you have your scope pointed at Mars and a non-astronomy family member or friend wants to have a look, the one thing you can point out and be sure they’ll see is the South Polar Cap (SPC). And on October 13, Mars’ south pole has an advantageous 21° tilt toward Earth.

Temperatures fluctuate widely at Mars’ poles, creating two types of caps: residual and seasonal. Residual caps are permanent and are made mostly of water ice. Seasonal ice caps are composed of mostly dry ice (frozen carbon dioxide) and some water ice. As one pole tilts toward the Sun during the martian summer, the seasonal cap sublimes (changes directly from solid to gas), which allows for the formation of ground frost and thin cirrus clouds. The residual cap at Mars’ south pole spans 200 miles (320 km).

Mars rises near Barker Dam in Joshua Tree National Park in California. The photographer’s 30-second exposure captured the Red Planet and the central part of the Milky Way at 10:30 p.m. PDT on July 5, 2018. 
Rami Ammoun
Oh, and a word of caution: Observers sometimes confuse the Hellas basin with the SPC. Hellas is an impact basin with lots of bright dust; when it’s at the planet’s edge, it can look like a polar cap. But you’ll be able to tell the difference in about an hour. The true SPC won’t appear to move, while Hellas will slowly drift due to Mars’ rotation.

Quality time

Once every 26 months, the Red Planet reaches opposition. But not all oppositions are equal. Some occur when Mars is near its farthest point from the Sun, and others, like this one, occur when it’s much closer.

And although Mars will appear at its biggest and brightest on October 6, it will look nearly as good two weeks before to two weeks after that date. That gives you ample time to revisit the planet through your scope and follow its rotation, looking for any changes in its albedo features.

And best of all, you don’t even need a dark sky to enjoy what Mars has to offer. Although, I do wish you a clear and steady one.