Mars finally comes to opposition this week
BY MARK ARMSTRONG
Posted: 7 April 2014
The planet Mars is at its best on 8 April when it lies opposite the Sun in the sky and is observable for most of the night, shining at magnitude -1.47 among the stars of Virgo, just south of the celestial equator.
Mars is not an easy planet to observe at anytime, even when at opposition, despite it being very easy to spot with the unaided eye when it's this close. Mars is a small planet with an equatorial diameter of 6779 kilometres (4212 miles), compared to Earth's 12,742 kilometres (7917 miles). Not every opposition is the same; Mars also has a much less circular orbit that that of the Earth; it can be as far away as 254.3 million kilometres (158 million miles) when furthest from the Sun (aphelion) and as close as 209.4 million kilometres (130 million miles) at perihelion (closest to the Sun). This makes a pronounced difference to the planet's apparent diameter through the eyepiece, at a perihelic opposition Mars can appear 25 arcseconds across but as small as 14 arcseconds at aphelion. This opposition is a near-aphelic one with Mars 239.4 million kilometres distant from the Sun and 92.9 million kilometres (58 million miles) from Earth. Mars creeps closer to Earth to be at its closest on 14 April at a distance of 92.4 million kilometres.
On opposition night Mars rises at around 7.30pm BST from London and by 10pm has risen above the crucial 20-degree altitude landmark and is ready for observing. Conveniently, astronomical darkness starts around this time (the Sun sinking below 18 degrees in altitude). The planet remains above this mark until about 4am, culminating at 1.30am in the southern sky at a respectable 33 degrees altitude, giving a favourable six-hour observing window and all in astronomical darkness too.
Bright Chryse Planitia, where Viking 1 landed in 1976, lies midway up with dark Margaritifer Sinus, Sinus Sabaeus and Sinus Meridiani in the south close to the Martian limb. By midnight the Martian features will have rotated right to left with south up in an inverting telescope without a star diagonal. By 1am the great 'bulge' of Tharis, a volcanic plateau holding the mighty volcanoes of Pavonis Mons, Ascraeus Mons and Arsia Mons has rotated in to view close to Mars' central meridian. The mightiest of them all, Olympus Mons, is close to the western limb but by 4am will be in the centre of the disc with Mare Sirenum just in view on the southern limb. A Martian day is marginally longer than ours, lasting 24 hours and 37 minutes. This can be rather frustrating for observers in that the hemisphere in view will face us again on the next night, 37 minutes later. So observers only able to view Mars say before midnight will see the same features for a week or two. If you can observe Mars throughout the night then Mars will display nearly 90 degrees of longitude (14.6 degrees per hour). Mars' most famous surface marking, the V-shaped Syrtis Major, won't be visible from the UK until around mid-month.
Mars in America
From the United States of America, Virgo lies substantially higher in the southern sky than from the UK, bringing significant observing benefits. On opposition night from latitudes of New York (40 degrees north) there is an eight-hour observing window when the planet is more than 20 degrees above the horizon. Mars can be observed from 9pm to 5am (EDT), with the red planet culminating due south at 1am at 44 degrees altitude. Further south from latitudes of 30 degrees north Mars is 10 degrees higher with a slightly longer observing window.
From New York the Tharsis plateau is on the central meridian at around 10pm EDT with the broad volcanic plain of Elysium Planitia close to Martian equator around 5am. Syrtis Major is just about to appear from the right (western limb) in an inverting telescope and will be well placed at a more sociable, early evening hour at the end of April.
Mars down under
The red planet is extremely favourably placed from Australia and New Zealand. Mars comes to opposition from Sydney at 7am local time on 9 April. On the 8/9 April Mars rises before 6pm and by 7.30pm has risen to 20 degrees above the eastern horizon. It climbs steeply away from the horizon to transit at a whopping 60 degrees above the northern horizon. By 4.30am Mars has dipped below 20 degrees over the western horizon, bringing to an end a nine-hour observing window for those with the stamina! the Syrtis Major is on show at opposition, transiting the central meridian around midnight with Mare Acidalium rotating onto the disc as the observing night end.
Pick up a copy of the April issue of Astronomy Now for an in-depth and authoritative guide to visual observing and imaging Mars. It's also available for the iPhone and iPad.