At long last Mars’s great opposition is upon us, and it’s time to make the most of its closeness to Earth, as well as its brightness and height in the sky. See if you can spot it in the night sky tonight.
Autumn 2020 we’ll see the best opposition of Mars for many years to come. Mars oppositions occur every 2 years and 2 months (or to be precise, every 779.94 days) and in 2020 Mars opposition falls on 13 October.
In the run up, Mars will show a gradual but dramatic brightening, surpassing Jupiter’s brilliance on 24 September to become the second brightest planet after Venus.
Mars will remain bright for weeks afterwards, well into November. Indeed, mid-November may be the time to observe Mars if you have a young family or a fledgling interest (for more on this, read our guide to stargazing for kids).
Even though the Red Planet won’t be as bright then, it will reach its highest position in the sky in the early evenings, which is a convenient time for many observers.
To the naked eye the salmon pink colour is incredible when it becomes bright. It’s an unmistakable sign that you are looking at the Red Planet.
Two dates to mark on the calendar are when Mars comes closest to Earth, a distance of 62 million km, at 15:19 BST (14:19 UT) on 6 October, and a week later, on 13 October, when it reaches opposition.
This year the Red Planet is also notable for reaching an altitude of around 42˚ from the UK and presenting a maximum disc size of 22.6 arcseconds.
After 2020 we won’t see Mars’s apparent disc size exceeding 20 arcseconds again until 2033.
What is opposition in astronomy?
A planet is said to be in opposition when Earth lies directly between it and the Sun. As the planet sits in the opposite part of the sky to the Sun, it’s positioned closest to Earth for its current ‘apparition’, or period of visibility.
It’s a good time to observe a planet because it will appear bigger than usual.
Opposition is a big deal for Mars because it’s a long way from Earth for most of the time, looking dim to the naked eye and small when viewed through a telescope.
But as the distance between our worlds shrinks, Mars brightens considerably, while expanding in size through the eyepiece of a scope to be large enough for surface detail to be seen easily.
The last time Mars was at opposition in 2018, we got a view of it with a large apparent size, the best for some time and larger in fact than this year.
But the planet was low to the horizon as seen from the UK and this spoilt the view for many. This year the Red Planet will get to a much higher altitude in the night sky, reaching at least 40˚ in altitude, in the constellation of Pisces.
With a maximum apparent size only fractionally smaller than that seen in 2018, this makes 2020 the year to get acquainted with this fascinating world.
For more on this, read our guide What is opposition in astronomy?
What will Mars look like during opposition?
It takes 687 days for Mars to orbit the Sun and go through its seasonal changes, experiencing two equinoxes and two solstices.
A Martian equinox occurs when the Sun appears to cross Mars’s celestial equator, while solstices represent the instant when the Sun is at its most northerly or southerly position in the Martian sky.
We divide Earth’s year into 12 months, but on Mars it’s usual to refer to periods in the Martian year (and its seasons) in terms of solar longitude (Ls). This value is measured in degrees from the planet’s northern hemisphere spring equinox
The first Martian ‘month’ is considered to be the period between Ls=0–30˚, the second between Ls=30˚–60˚, and so on.
The northern spring equinox at Ls=0˚ is followed by the northern summer solstice at Ls=90˚. The northern autumn equinox occurs at Ls=180˚ and the northern winter solstice at Ls=270˚.
Between 1 and 31 October, Ls increases from 288˚ to 310˚, Mars being in the grip of a northern winter, the southern hemisphere basking in summer sunshine.
At this October’s opposition, Mars’s southern hemisphere will be tilted towards Earth. When it comes to observing around opposition, telescopes will show us dark and light regions on Mars’s disc.
These ‘albedo’ features are visible because of variations in reflectivity. The lighter regions generally represent desert areas, while the darker regions are exposed rock.
In addition, the planet’s polar caps shine bright. The south polar cap (SPC) should dominate our view, but as it’s well into the southern summer, this will have shrunk to a fraction of its full size, the residual ice cap slightly offset from the planet’s axis of rotation.
The north polar cap (NPC) is currently beyond the northern limb of the planet, but the shroud of cloud currently covering it should be partially visible.
This north polar hood (NPH) will dissipate as Mars enters northern spring in late January and February 2021.
What equipment do you need to observe Mars?
Although useful for conjunction viewing, binoculars won’t reveal much more Martian detail than you see with your eyes, the planet appearing as a small, bright, salmon pink disc against a background star field.
To see features on the Red Planet, you’ll need a scope with a front lens that’s at least 75mm in diameter. With a telescope, the changes to Mars’s appearance in the run up to opposition become very evident.
The planet’s largest apparent size occurs when it is closest to Earth and this typically occurs a few days adrift of the opposition date.
For 2020, Mars is closest to Earth on 6 October and appears 22.6 arcseconds across, a week before opposition on the 13th.
What does Mars look like through a telescope?
Through a telescope, Mars can be slow to reveal its detail. Features are there but you may find them hard to see, especially if you’re new to observing the Red Planet.
It’s not uncommon to see a pink blurry blob at first, but give yourself time to settle into the view and you’ll gradually see more.
Start using a low magnification, say 25x the size of your telescope’s aperture in inches. For a 4-inch (25mm) this means starting off around 100x.
Increase the magnification if the view is steady. As a general rule, the maximum magnification for any size of telescope is 50x its aperture in inches, but atmospheric conditions rarely make this possible: going too high under wobbly conditions will simply diminish the quality of the view.
With each magnification, give your eye time to get used to the view. Notice the edge of the planet first. In the run up to and from opposition, Mars appears gibbous in shape – more than half but less than a full circle.
During the 2020 opposition, it’s Mars’s southern hemisphere and polar cap that’s tilted towards us.
Features to look out for when observing Mars
As Mars gets larger, its surface detail should be easier to see. As well as the bright southern polar cap, the planet presents areas of light and dark, representing deserts and exposed rock.
These are known as albedo features: areas that appear bright or dark due to the amount of light they reflect. The larger and darker an albedo feature, the easier it is to see through a small telescope.
The most prominent dark feature is the V-shaped form of Syrtis Major, a low-level shield volcano. When it has rotated into view, the ‘point’ of Syrtis Major points north, appearing to extend quite far towards the northern edge of Mars during the 2020 opposition.
Between the southern boundary of Syrtis Major and the southern polar cap lies the Hellas Basin. This 2,300km diameter basin is one of the largest impact craters in the Solar System. Hellas can sometimes appear bright due to clouds that form in the basin.
Although Mars has a very thin atmosphere, there’s enough to support weather and the formation of hazy clouds.
Bright orographic clouds form when the Martian atmosphere is forced to a higher altitude and can be particularly noticeable over the peaks of the vast volcanoes that sit on Mars’s Tharsis plain.
If you’re observing Mars from one night to the next, be aware that the planet’s rotation period is almost 40 minutes longer than Earth’s, at 24 hours, 39 minutes and 35 seconds.
This means that features are centrally located on Mars’s disc 40 minutes later on each consecutive night.
Imagine looking at Mars one night and seeing a dark feature at the centre of its disc (what’s known as Mars’s central meridian). The following night at exactly the same time, that feature would appear slightly further east and take an extra 40 minutes to reach the central meridian once again.
The night after that, viewing at the same time, the feature would take 80 minutes to reach the central meridian.
It looks as if Mars resets position each consecutive night, giving you just a glimpse of extra new surface along the western limb before it then does a re-run of what you saw on previous nights. Eventually of course, you do get to see the whole globe, but this ‘reset and re-run’ can catch observers out.
How to observe Mars through a telescope
Through an eyepiece, Mars appeared impressive during September 2020 as its apparent size increased 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.
As it shrinks, the ‘Mountains of Mitchel’ become visible; a bright region 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.
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.
Observing Mars opposition 2020, week-by-week
As we begin, let’s break the views up week by week during opposition month. Our descriptions are centred on 01:00 BST (midnight UT), the time when Mars will be at its highest.
If you’re observing away from this time, there’ll be some variation in the visible features.
Week 1, centred on 3 October (01:00 BST)
This week reveals the most recognisable of all the Martian albedo features, the V-shaped Syrtis Major. It appears narrowest around Ls=190˚ (5 October) and in the past has shown seasonal changes.
Following a major dust storm in 2001, the seasonal narrowing became more permanent.
South of Syrtis Major lies the Hellas Basin and the brightness of this feature varies considerably; it’s often filled with changeable ‘weather’.
Indeed, reflective frosts can trick you into thinking you are viewing the SPC rather than the basin. During the southern summer, Hellas is likely to be devoid of bright weather-related phenomena, but instead may provide a backdrop for invading dust storms.
A long, dark feature known as Rima Australis may be seen south of Hellas. This thin feature runs southwest of Hellas to skirt the southern edge of the SPC where it joins Magna Depressio.
The dark line continues as Ulyxis Fretum before merging with Mare Chronium. As the southern summer approaches and the SPC recedes, its melting ice reveals an SPC remnant known as the ‘Mountains of Mitchel’, named after the Ohio astronomer Ormsby M Mitchel who first reported it in the 1840s.
This region, known as Novissima Thyle, appears like a bright extension to the SPC, separated by Rima Australis.
A similarly thin, long dark tract passes from the southernmost section of Rima Australis, where it merges with Magna Depressio, and runs west at a tangent to the SPC towards Mare Australe. This feature is Rima Augusta.
The region between Rima Australis and Rima Augusta contains the bright Argenteus Mons, which can also look like a projection of the SPC.
Week 2, starting on 10 October (at 01:00 BST)
This period will present a view of Mars that has rotated by 62˚ compared to the view from 3 October. This means that, as time progresses, Syrtis Major returns to centre stage once more.
At midnight on 10 October, the distinctive dark twin prongs of Sinus Gomer appear near the centre of the Martian disc with Mare Cimmerium preceding (ahead of it as Mars turns), and Hesperia and Mare Tyrrhenum following it in the rotational order across the planet.
To the north is Elysium, which shows seasonal and long-term appearance variations.
An interesting example of this is the Trivium Charontis region. This large, dark feature was prominent in the 1950s, but subsequently weakened in intensity as it became covered by dust and sand.
It may appear as several darker spots in the desert: bright streaks, one running close to the northern limb, with another extended at right angles to it into Elysium, may also be seen in this view. The nature of these isn’t fully understood.
Week 3, starting on 17 October (at 01:00 BST)
This brings us to what is sometimes unkindly described as the ‘boring side’ of Mars. At first glance it lacks complexity: dark albedo features are in short supply, apart from the rabbit-shaped profile of Mare Sirenum in the southern hemisphere.
Of course, this side of Mars is anything but boring, giving us a look at the Amazonis and Tharsis regions, which contain the huge shield volcanos Ascraeus Mons, Pavonis Mons, Arsia Mons and Olympus Mons.
The subtle desert tones are fascinating, as is the way Martian weather interacts with the volcanic peaks. Bright orographic clouds, such as the Arsia ‘long cloud’, sometimes betray their positions.
Most notable during the northern summer (Ls=120˚–160˚), the clouds appear to form a recurring W-shaped pattern as sunset approaches the volcanos.
It will be interesting to see whether this pattern repeats during the southern summer, given the equatorial positioning of the volcanos.
If conditions are very good, careful observation may reveal the central caldera of giant Olympus Mons as a defined spot surrounded by a wider ring – the volcano’s slopes
Week 4, starting on 23 October (at 01:00 BST)
This week brings us face to face with the enigmatic ‘Eye of Mars’, another area which has shown significant variation over the years.
The region is centred on dark Solis Lacus surrounded by lighter Thaumasia, the overall effect resembling an eye. The eye’s intensity varies over time and is affected by dust storms.
Its northwest edge contains Valles Marineris, a 4,000km-long canyon system, 200km across at its widest point and 10km deep. By comparison, Earth’s Grand Canyon is 446km long, 30km wide and 1.6km deep.
Through amateur scopes the relief detail of the canyon is too small to be seen, but a hint of the canyon’s shape can be deduced.
The complex dark region north of Valles Marineris includes Agathadaemon, Melas Lacus, Lus Lacus and Tithonius Lacus bordering the Ophir region to the north.
Week 5, starting on 30 october (at 00:00 UT)
This week gives us a view of Margaritifer Sinus, with the complex ‘fingers’ of the Aurorae Sinus region following.
The fingers appear to have darker spots at the end, which include Aurorae Fretum, Aromatum Promontorium and Juventae Fons.
Using filters to observe Mars
The light from Mars is predominantly shifted towards the redder end of the spectrum, which works in our favour because longer wavelengths are less susceptible to being blurred by our turbulent atmosphere.
You can improve what you see visually with a telescope by using filters. Visual filters are normally identified by their Wratten numbers. These may be written as W followed by a number and possibly a letter.
Although the numbers represent specific colours, there is no sequence to them; W16 is yellow-orange, W18B is very deep violet and W21 is orange, for example. The letters sometimes found after the number represent increasing strength of filter.
- Yellow filters (W12, W15) will tend to make the lighter, desert regions appear brighter and brown/blue regions darker.
- Orange (W21, W23A) also helps increase the contrast between the light deserts and dark exposed rocks, further cutting through the weak Martian atmospheric haze.
- Red (W25, W29) enhances the contrast yet again and is excellent for defining the boundaries between regions. Orange and red are also good filters to use for observing Martian dust storms, if they appear.
- A green (W57) or blue-green (W64) filter is good for showing seasonal features such as frost patches, fog and irregularities along the edge of the polar caps.
- Blue (W80A, W38, W38A), deep blue (W46, W47) and magenta (W30, W32) are all good colour filters for detecting Martian weather in the form of white clouds or limb hazes; magenta in particular is good for seeing detail in the Martian polar regions.
Not all Mars oppositions are the same
The Red Planet reaches opposition every few years, but there are also cycles that play out over longer periods of time.
Some Mars oppositions are distinctly better than others. The maximum apparent size of Mars varies in a cyclical fashion through subsequent oppositions, because the orbits of Earth and Mars are not exactly circular: they’re elliptical.
When Mars is very far from opposition, its disc can shrink as small as 3.5 arcseconds – similar to the apparent size of Uranus.
In contrast, at a really favourable opposition the largest size the Red Planet can reach is 25.1 arcseconds, and at the 2018 opposition we saw the apparent diameter of Mars get very close to that, at 24.2 arcseconds.
For the UK at least, the 2018 opposition took place when Mars was low in the sky. This was a ‘perihelic opposition’, one taking place when Mars was close to (technically within 90° of) perihelion, the position where the planet has its smallest orbital distance from the Sun.
The maximum size of Mars in 2020, also technically a perihelic opposition, will be 22.6 arcseconds, smaller than the 24.2 arcseconds presented during 2018, but still a reasonable value.
Opposition diameters will continue to shrink for a number of future oppositions. At the 2022 opposition, the biggest the Red Planet’s disc gets is 17.0 arcseconds, while the 2025 opposition presents a 14.5 arcsecond disc.
It shrinks further still for the 2027 opposition, when it will appear 13.8 arcseconds across.
With these forecasts in mind, we can see that the 2020 opposition of Mars will be the most favourable for UK viewing for some time – it won’t be this big again until 2035.
How to record your observations of Mars
Making an observation of Mars isn’t hard. Make it useful for wider reference by recording the date and time (in Universal Time, UT). Find out more about this in our guide on how to keep an astronomical log book.
Include your name, location and conditions, making a seeing estimate through the eyepiece. The five-point Antoniadi scale is useful for this:
I Perfect stability
II Slight quivering
III Moderate steadiness with large air tremors
IV Poor stability
V Very poor stability
Record the observing instrument along with details of magnifications used. Also record whether filters were used.
Images and drawings are normally (but not exclusively) presented south-up with the following (F) and preceding (P) limb directions indicated. Features rotate into view around the F limb, disappearing behind the P limb.
It’s optional but useful to include values for solar longitude (Ls) and the planet’s central meridian longitude (CM). Get these from a program such as the freeware WinJUPOS.
Once completed, submit your efforts to an organisation such as the British Astronomical Association (BAA) Mars section or the Association of Lunar & Planetary Observers (ALPO).
How to photograph Mars
When it’s close to a favourable opposition, Mars is a great imaging target. For best results you need a planetary camera that can record many still frames in rapid succession. These can then be processed using freeware programs such as AutoStakkert! or RegiStax.
Find out how to take your Red Planet imaging to the next level with our guide on how to capture scientific images of Mars.
The larger aperture of telescope you can use, the better. Allow your scope time to cool before use, typically 1-2 hours (large scopes may need longer). When imaging, re-focus after each filter change and ensure focus is accurate.
A colour camera or monochrome camera with filters, eg, RGB (Red, Green, Blue), is ideal. However, colour can suffer from atmospheric dispersion, an effect that chromatically blurs detail.
This worsens the closer you get to the horizon, fine detail becoming less distinct and colour fringes appearing, but these effects can be reduced by using an atmospheric dispersion corrector (ADC).
Mars withstands seeing conditions well due to much of its light being in the redder, longer-wavelength part of the spectrum. If your camera is infrared (IR) sensitive, an IR-pass filter can also deliver sharp, high-contrast results.
Optimum focal lengths are dictated by camera pixel size. For decent seeing, an image scale of 0.25 arcseconds/pixel is recommended. For superb seeing, 0.1 arcseconds/pixel may also work.
The best focal length can be calculated from: FL = (Ps x 825) for 0.25 arcseconds/pixel, or FL = (Ps x 2060) for 0.1 arcseconds per pixel; where FL = telescope focal length (mm) and Ps = pixel size (microns).
Close to opposition, Mars rotates 0.25 arcseconds in about 5 minutes and 0.1 arcseconds every 2 minutes. Consequently, cumulative capture times (eg, total for R+G+B) must be shorter to avoid rotational motion blur.
So make sure you take advantage of this opportunity to view the fascinating world of Mars, when it’s better presented than at any other time for years to come – and it’ll be visible in the coming months after opposition too.
Just think, when you view it through a scope you could be looking at a world which one day may have humans living on, or beneath, its surface.
Pete Lawrence is an experienced astronomer and a co-host of The Sky at Night. This guide originally appeared in the July and August 2020 issues of BBC Sky at Night Magazine.