Mars Opposition 2020

In the skies above us, the planets orbit at different rates. Sometimes those orbits align to give us a close-up view of something special and over the next couple of months it is Mars that will loom larger.

Hundreds of years ago, in a world before telescopes, if you spent your evenings gazing upwards you might well have developed a good understanding of how the night sky changed with the passage of time.

Every so often – in strange but semi-regular intervals – you might have noticed that the fiery orange spot in the sky had silently crept closer once again. Some years it would edge even closer than usual, appearing brighter than ever before.

As it edged closer its movements might have appeared to change too. It may have seemed to inch its way westward, week by week, slowly, not quite keeping up with the westward march of the stars. Its next trick? Speeding up before slowing again. And finally reach a stunning crescendo of fiery orange before seeming to edge further westwards, in pursuit of the Sun, and gradually becoming less visible – and for a shorter time – each night, and then fading entirely.

But you would have come to know it was a cycle, and sure enough you would spot it again a few months later on the other side of the sky, a tiny dot emerging from the glare of the rising Sun. Now you know that in the months and years ahead, it will come closer, it will be brighter again – but you don’t know if it will achieve the same bright orange blaze or edge quite so near.

You yearn for an invention that would bring that fiery orange spot even closer.

Mars at opposition

That fiery orange spot is of course Mars and the pre-telescope imaginings above provide a picture of what’s happening in the sky right now and what will continue to unfold into October and early November. Mars won’t be this close or bright again until July 2033. And then in September 2035 even better views will be possible – trumping the viewing opportunities of Mars afforded back in 2018. Perhaps there’ll be boot prints in Martian dust by the time 2035 rolls around?

The ‘Red Planet’ (curiously more orange than red) will be at opposition on 13 October 2020. It will be at its closest approach on 6 October at just 62.1 million km from Earth. So, in the evenings be sure to look up to the northeast and wave ‘hello’ to Mars from September onwards right up until November. After this, it will fade quickly.

When a planet is at ‘opposition’, Earth is directly between it and the Sun, so together they form a line with Earth in the middle. Earth orbits the Sun about 25% faster than Mars, so every 26 months Earth overtakes Mars on the ‘inside lane’.

At this time, they are of course, closer together, so we see Mars not only much brighter but also larger when viewed through a telescope. Earth passes by Mars quite quickly, so the time for best telescope viewing is just a few weeks before and after the opposition.

Not all Mars oppositions are equal

The orbit of Earth is close to a circle, but Mars’ orbit is much more elliptical, deviating from a circle by a whopping 9.3%!

A planet’s closest approach to the Sun is its perihelion, while its opposite, aphelion, is its furthest distance. The most intimate encounters of the Mars kind occur when Earth’s aphelion and Mars’ perihelion nearly coincide with each other and with Earth’s overtaking manoeuvre.

This happens roughly every 26 months or 780 days. Astronomers need to be more patient than this, however. As alluded to in the story, the more distant oppositions outnumber the closer ones! The 2003 opposition was calculated to have been the best in 60,000 years. The 2018 event was pretty good too, being only 3% more distant.

Martian dust storms and telescope viewing

Mars is well known for its dust storms. The larger ones can last from weeks to months and can even extend across the planet. These big storms typically only happen during Mars’ southern hemisphere summer.

To understand why these Mars storms happen when they do, you need to think about the perihelion and aphelion again. Mars is nearly 10% closer to the Sun at perihelion than at aphelion and while this might not sound like much, it makes a difference. Planets move faster in their orbits during perihelion, and since Mars’ perihelia coincide with its southern hemisphere summers, the southern summers are short and hot (relatively speaking). It’s thought that these short hot summers give rise to seasonal dust storms every 26 months.

Unfortunately, at the time of the 2018 opposition, a months-long planetwide dust storm was underway. Though the naked eye brightness of Mars wasn’t much affected, our telescope viewing definitely was! The surface details we’d hoped to observe, were sadly obscured by the storm. The storm also saw the demise of NASA’s Opportunity Rover. On the bright side, it created an unprecedented opportunity for the study of Martian dust storms!

Caption: Global dust storm as seen by MRO in 2018 (above). Credit: NASA/JPL

The good news for us is that these planetwide storms don’t usually happen every southern summer, so while Mars won’t be quite as close as in 2018, we’re very hopeful for crystal clear views.

In late September and early October, we may need to wait until about 9.30 pm for Mars to clear the hills for our courtyard telescopes, but it will rise earlier as the nights go by. All the while, Jupiter and Saturn will be beautifully placed for telescope viewing, so if the weather is kind, it will be open season on planets.

For more information on viewing Mars during this window, check out our session times here for Te Wiki O Matawhero – The Week of Mars.

Mars moving in reverse?

In the opening observation there was talk about Mars edging westward even though the planet typically moves eastwards against the stellar background. Around the time of opposition, Mars does appear to temporarily accelerate westwards with respect to the stars but it’s just an illusion. The phenomenon is known as ‘apparent retrograde motion’ since Mars is not really changing direction or speed! Read more: Apparent Retrograde Motion and path of Mars

Beware the Mars hoax!

Every time Mars nears the Earth, during the Mars opposition, a crazy rumour starts circulating. The rumour would have you believe that Mars will look as big as the Moon in our night sky. That is never going to happen. Even during its closest approaches, Mars is still some 150 times more distant than the Moon. Mars is about twice the size of the Moon, and some simple arithmetic tells us that at its closest it will appear just 1/75th of the Moon’s apparent size in the sky!

Caption: The Mars Hoax.   Credit: NASA

Even if Mars won’t appear as large as the Moon, it’s definitely a treat to see the dazzling red planet lighting up our spring-time skies.