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Watch the Red Planet go behind the Moon on Thursday 8th December

Many thanks to our colleague John Flannery for providing the below information on what to watch out for in the skies this month! John is a long-time amateur astronomer with an interest in the history and lore of the sky. He is a member of the Irish Astronomical Society.

Mars plays peekaboo with the Moon in a rare encounter during the early hours of this Thursday (8th December) morning. 

Diana may have clashed with Ares in the 2017 movie Wonder Woman but their celestial counterparts have an equally dramatic encounter during the early hours of this Thursday morning when Mars is occulted, or hidden, by the full moon. If clear skies are forecast then it is a sight you do not want to miss.

Although the Moon regularly hides stars on its travels round the sky, occultations of a planet or the brightest stars are not very common from any one location.

The last time Mars was covered by the Moon for observers in Ireland was in May 2013 – but that was a daylight event requiring a telescope to see. You need to go back to March 2004 for the last evening event, and it will be 2039 before Mars is next occulted when the sky is reasonably dark.  

Mars also reaches opposition on Thursday – and the Moon is full the same day too – when the ochre-coloured planet is at its brightest for the year.

The Red Planet – though it almost never appears that colour – can be found in the constellation Taurus this month and rises in the eastern sky after sunset. Its glowing ember is highest in the south just after midnight about two thirds of the way up the sky. 

Around the time of opposition is a good time to view Mars as it then looks biggest in a telescope and some surface detail can be made out. It is best wait until the planet is reasonably high in the sky though before looking through the eyepiece as atmospheric turbulence can blur the view somewhat when Mars is lower down. 

What will we see during the occultation on December 8th?       

This Thursday’s occultation can be seen with the unaided eye despite the full moon because Mars is presently so bright, but any optical aid will enhance the view.

The bad news is that the occultation occurs just before 5am but the early rise will be worthwhile.

At first, Mars’ brilliant orange spark can be found just above the eastern skyline when it gets dark on Wednesday evening and it climbs higher as the night progresses, while all the time the Moon creeps closer.

The two worlds then really start to cosy up in the early hours of Thursday morning, making for a striking sight.

Plan to get ready for the occultation from 4:30am onwards as Mars disappears at 4:55am for Dublin-based observers but that time will vary by a few minutes depending on where you are in Ireland. You don’t want to miss this!

One thing to watch for is whether the planet slowly fades to the unaided eye or in binoculars or will it suddenly wink out as it slips behind the Moon? The Martian globe is evident in a telescope though whereas stars are pinpoints so it will take 36 seconds for the planet to become fully immersed by the Moon’s leading edge if viewing at high magnification.

After about an hour, Mars pops back into view at the Moon’s opposite edge at 05:56am when both lie in the southwestern sky pre-dawn. The sight in a telescope should be worth seeing as the planet slowly emerges from behind the lunar limb but any instrument will show the two worlds part ways. 

Events like these are a dramatic way of seeing the motion of celestial bodies in real time. Although constellation patterns appear static over many lifetimes, solar system dynamics are more easily appreciated. A total solar eclipse is the ultimate demonstration of this, and some accessible opportunities fall in the next few years, with the path of totality sweeping across the US in 2024 and Iberia in 2026 – get planning now!