Why is Mars so bright? And other things to see in the October sky

You look out your window in the evening to the south and spot a brighter-than-usual red ‘star’. It moves across the night sky and is setting/has set by morning. In fact, it is not a star, but Mars!

The red planet, the most visited place in the solar system other than the Earth/Moon system, home to rovers like Opportunity and Curiosity that captured the hearts of the world and inspired people into science.

If you cast your minds back to summer, mars was in the news a lot, with 3 space agencies launching missions. They all launched over the space of a few weeks, as there is only a small window to launch, in order to follow a Hohmann Transfer trajectory, which is the most efficient way to reach mars. I did a youtube video on the mechanics of it, which you can watch here! For that to work, you have to launch not when Mars is closest to Earth, but when it is at 44 degrees relative to earth.

That means a few months later (ie. now, October), Mars has caught up, and is now in line with the Earth. This is called opposition- when the sun, earth and another planet/body all line up! For the moon, an approximate opposition happens once a month… full moon! And when the moon is in perfect opposition (which doesn’t happen every month as it orbits at an angle), we get a lunar eclipse, and the moon turns red. As the planets are much further apart, we don’t get eclipses, but they still line up! This happens roughly once every 2 years.

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Planets, like the moon, appear to go through phases like the moon, although the planets that can be in opposition (any further away from the sun than earth, i.e. not Venus/mercury) don’t show a full phase cycle but appear to oscillate between gibbous (mostly illuminated) when the planet is at 90 degrees to earth and full when it is at opposition. So right now is the best time to image the whole of mars. This also partially explains why it is brighter, as more of the illuminated side faces earth, so more of the light that is reflected can be seen from earth!

But the phase doesn’t play that much of a role: as Mars is only a small planet, even a bit of a decrease in distance can noticeably increase the brightness.

Being at opposition will mean it is roughly at its closest approach to earth, although the dates don’t quite match up: opposition was on October 13th, but it actually came to its closest approach on October 1st! This discrepancy is because orbits aren’t perfectly circular, but elliptical. The closest Mars can get is when Mars is at perihelion (closest to the sun) and Earth is aphelion, although earth’s position causes less of a difference as our orbit is a lot more circular than mars’ (eccentricity 0.017 vs Mars 0.093, where 0 is perfectly circular). This occurs roughly every 15 years, and took place last time mars was in opposition back in 2018. This year its slightly further away, and will get further and further away for the next few oppositions until 2027 when Mars is near aphelion, after which oppositions will start to get closer again! However, even though Mars was closer earlier this month, yesterday still was the brightest (though it was cloudy here, so technically it seemed brighter last week from my location, but today would have been brighter, had the clouds disappeared!) simply because of the angle (or lack thereof) that the light from Mars was reflected towards us. Like when the sun is directly overhead at midday on the equator, all shadows on Mars’ surface disappear, making it look brighter! This effect is called opposition surge.

Don’t worry if you missed the night of opposition (i mean it was a bit cloudy anyway, though you could still see it shining through the haze!), it will stay noticeably bright for a few more weeks. So next time it’s clear, head outside, face South East (just to the right of where the sun rises), look for a bright orange dot… and maybe give Curiosity a wave!

Thanks for reading! Before you go- Jupiter and Saturn have been visible in the sky over the summer months, and I loved observing them… but they are setting earlier and earlier in the evening now, so make sure you take some time over the next few weeks to get one last look. Even though they are setting earlier, its also getting darker earlier, so they are still visible for about an hour or two after dark.

To spot Jupiter and Saturn, head out at about 7:30pm and look South West. You should see 2 bright dots in a line, close ish to the horizon. The lower one is Jupiter, and will appear brighter (you can see its moons with some good binoculars or a telescope) and the slightly brighter, higher one is Saturn.

And in the morning, Venus is very bright around sunrise. If you head out just before 7am on the 15th (tomorrow) you’ll be able to see it just above a very thin crescent moon! (or if you were wondering what the bright ‘star’ by the moon over the past few days was, that would be venus!

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Hope you enjoyed this, let me know in the comments if you’ve spotted any of these planets!

As always, check stellarium for what you can see for your location at a specific date/time!

The diagrams of Mars’ angle come from The Sky Live which is a really cool solar system animation/model!

Further Reading:

https://earthsky.org/tonight/mars-closest-to-earth-october-6

https://earthsky.org/astronomy-essentials/why-is-mars-sometimes-bright-and-sometimes-faint

https://earthsky.org/sky-archive/smallest-martian-phase-on-september-13#:~:text=Because%20Mars%20orbits%20the%20sun,the%20complete%20range%20of%20phases.

https://www.space.com/mars-closest-to-earth-until-2035#:~:text=Today%20at%2010%3A18%20a.m.,high%20in%20the%20eastern%20sky.