The Red Planet goes retrograde
As Mars’s favourable October opposition get closer, watch the planet reverse its apparent motion against the stars this month. Pete Lawrence is your guide
Ahead of opposition next month, explore Mars’s curious orbital loop
Faces of Mars: in the weeks leading up to opposition you can observe different Martian features. A small scope will show the larger markings, while anything over 200mm starts to reveal more detail. In these south-up views Mars rotates right to left with the orientation shown
Mars presents a superb UK observing opportunity in 2020 as the planet reaches opposition at a decent altitude in mid-October. As well as being bright, Mars’s apparent diameter will reach a respectable 22.6 arcseconds, a size that will not be approached again until July 2033.
But there’s another interesting aspect to opposition, because Mars will be in the middle of a ‘retrograde loop’ in the sky, and its apparent motion against the background stars will appear to reverse. So far during 2020, its apparent motion against the stars has been eastward, but this will slow to a stop on 10 September, when Mars reaches a ‘stationary point’. After this, it tracks west and continues in this direction until another stationary point is reached on 16 November, when Mars resumes moving east. The illusion is caused by the relative position and speed of Mars in its orbit compared to Earth: our faster orbit allows us to overtake Mars, and this creates the looped path in the sky.
All the planets appear to perform retrograde loops, decreasing in apparent size with distance from Earth. And this is the key point: it is Mars’s close proximity that makes its opposition and retrograde loop really impressive.
You can reveal the loop by photographing Mars against background stars with a mid- to wide-angle lens on a camera. Take a photograph on every clear night through to the end of the year, load each into a layer-based editor like GIMP and align using the stars. Set the blend mode of all upper layers to lighten and Mars will show through, revealing the loop.
Mars in focus
Through an eyepiece, Mars appears impressive during September as its apparent size increases from
18.9 to 22.4 arcseconds. Its darker albedo markings and bright southern polar cap (SPC) should be obvious with magnification. Bathed in sunlight over past months, the SPC will have reduced in size and during September appears small. As it shrinks, the ‘Mountains of Mitchel’ become visible; a bright region
It is Mars’s close proximity that makes its opposition and retrograde loop really impressive
that in reality is a plain, it appears detached from the SPC, south of Hellas.
At the opposite pole, the northern polar cap (NPC) is cooling and as it does, a shroud known as the north polar hood (NPH) forms above it. The southern edge of the NPH may well be visible during September.
Amateur telescopes show albedo features – dark areas of exposed rock and light areas of desert sands. Imaging under excellent seeing with a telescope over 300mm in diameter may hint at large relief features such as giant craters Schiaparelli and Herschel or the impressive Valles Marineris canyon, but in general what you’re recording are albedo features. Martian weather clouds may also appear and are often better seen close to the limb and terminator of the planet.
Take a tour
The features you’ll be able to see on the disc will be dependent on the date and time you’re observing. V-shaped Syrtis Major is prominent and well presented during this apparition (the current observing window of an object). Clouds tend to form in an equatorial band, possibly reducing the intensity of Syrtis Major. Known as the ‘Syrtis blue cloud’, this is best seen as Syrtis Major approaches sunset. South of Syrtis Major is the giant Hellas basin, its western edge defined by Hellespontus, the eastern edge by Mare Hadriacum.
Extending west at a tangent to Hellas’s northern boundary is Mare Serpentis, eventually merging with Pandorae Fretum, bordered by lighter Deucalionis Regio to the north. Further north lies dark Sinus Sabeaus. Aeria, Arabia, Moab and Eden are light desert regions to the north. At the western end of Sinus Sabaeus sits Sinus Meridiani, a dark rectangular feature with two north-pointing ‘prongs’; it marks the zero point of Martian longitude.
After a lighter patch west of Sinus Meridiani comes dark Margaritifer Sinus with Oxia Palus to its north. West of Margaritifer Sinus is dark Aurorae Sinus, bordered to the north by numerous finger-like extensions. Desert regions Chryse and Xanthe sit north, bordered to their north by dark Niliacus Lacus and eventually Mare Acidalium.
South of Margaritifer Sinus and Aurorae Sinus, extending west of Pandorae Fretum, is Mare Erythraeum with the large Argyre basin to its north. West of Aurorae Sinus is the ‘Eye of Mars’, the central feature being dark Solis Lacus. The immense Vallis Marineris canyon lies northeast of Solis Lacus.
The desert expanses of Tharsis and Amazonis lie northwest of the ‘Eye of Mars’. Here three volcanic peaks named (north-to-south) Ascraeus Mons, Pavonis Mons and Arsia Mons define the southwest edge of the ‘Tharsis quadrangle’. Arranged in a line, they look northwest over the vast volcano Olympus Mons. Bright orographic clouds may form when the Martian atmosphere is forced upwards over these volcanos. These sometimes coalesce on the evening side, forming a south-up ‘W’. A blue or violet filter (W38 up to 250mm, W47 for larger apertures) helps reveal this.
West of Solis Lacus is Mare Sirenum, eventually leading into Mare Cimmerium with its northpointing dark protrusions of Sinus Gomer. West of Mare Cimmerium is lighter Hesperia, dividing Mare Cimmerium from dark Mare Tyrrhenum. The latter eventually joins to Syrtis Major, where we started.
Dust storms may be seen as lighter patches, often yellowish in colour, and they occur seasonally. The way seasons are marked on Mars is to use solar longitude (Ls), the Mars–Sun angle. Storms occurring between Ls = 270˚ to 359˚ (northern winter) can be large and Mars will be in that season: in September Ls = 269˚ to 288˚, while in October Ls = 288˚ to 307˚. Large planet-engulfing storms are fortunately rare; whether we will get one we’ll just have to wait and see.