Observers in the British Isles looking due south close to 6pm GMT on Friday, 7 December will find magnitude +0.1 planet Mars about 30 degrees, or a span and a half of an outstretched hand at arm’s length, above the horizon. What you won’t see unless you have large binoculars or a small telescope is outermost planet Neptune one-tenth of a degree to the Red Planet’s lower right. Look out for Fomalhaut, the most southerly first-magnitude star visible from the UK, twinkling near the southern horizon. AN graphic by Ade Ashford.Since the beginning of the year, Mars has had conjunctions with Jupiter (7 January), Saturn (2 April) and Pluto (26 April), but its closest approach to a planetary sibling this year occurs on Friday, 7 December 2018 at 14:07 UT (2:07pm GMT) when the Red Planet passes just 2 arcminutes – one-thirtieth of a degree – north of Neptune in the constellation of Aquarius.
The instant of least separation between Mars and Neptune occurs in daylight for the British Isles, but observers here only have to wait a further four hours to see them in a dark sky. As astronomical twilight fades to dark around 6pm GMT in the heart of the UK, Mars and Neptune are also at their highest in the sky due south, about 30 degrees above the horizon.In this simulated 10×50 binocular view of Mars and Neptune at 6pm GMT on Friday, 7 December 2018, background field stars in Aquarius to magnitude +8 are shown. Labelled stars λ Aquarii, 78 Aquarii, 81 Aquarii, 82 Aquarii and 83 Aquarii are magnitudes +3.7, +6.2, +6.2, +6.2 and +5.4, respectively. AN graphic by Ade Ashford.By 6pm GMT on 7 December the separation of Mars and Neptune has increased to 6½ arcminutes, or about one-tenth of a degree. This means that you could theoretically use telescope magnifications up to about 400× and still see both planets in the same field of view! (In practice, seeing conditions at an altitude of 30 degrees rarely permits such powers; 150× would be more realistic.)
If you get the chance to see both Mars and Neptune in the same field of view on 7 December, do bear in mind that this conjunction is merely a line-of-sight effect. The magnitude +0.1 Red Planet lies almost 159 million kilometres away, hence its 86% illuminated gibbous disc measures just 8.8 arcseconds across. Neptune has a physical diameter 7¼ times that of Mars but lies slightly more than 28 times farther away this night, hence its pale blue magnitude +7.9 disc spans just 2.3 arcseconds.
On 10 March 2006, NASA’s Mars Reconnaissance Orbiter (MRO) entered into orbit around the Red Planet. A decade later, with its six science instruments all still operating, MRO has delivered huge advances in knowledge about Mars, revealing in unprecedented detail a world that held diverse wet environments billions of years ago and remains dynamic today.
Just a day after producing a spectacular solar eclipse over the North Atlantic and Arctic, the Moon is making a more subtle return to our evening sky just after sunset. See if you can see a sliver of the New Moon this evening.
The UK’s Beagle 2 mission to Mars, which was lost in 2003 as it entered the red planet’s atmosphere, has been rediscovered by NASA’s eagle eye in the Martian sky, the Mars Reconnaissance Orbiter.
