Although Mars has a very thin atmosphere, there’s enough to support weather and the formation of hazy clouds 40 minutes longer than Earth’s, at 24 hours, 39 minutes and 35 seconds. This means that features are centrally located on Mars’s disc 40 minutes later on each consecutive night. Imagine looking at Mars one night and seeing a dark feature at the centre of its disc (what’s known as Mars’s central meridian). The following night at exactly the same time, that feature would appear slightly further east and take an extra 40 minutes to reach the central meridian once again. The night after that, viewing at the same time, the feature would take 80 minutes to reach the central meridian. It looks as if Mars resets position each consecutive night, giving you just a glimpse of extra new surface along the western limb before it then does a re-run of what you saw on previous nights. Eventually of course, you do get to see the whole globe, but this ‘reset and re-run’ can catch observers out.
True colours
Pete Lawrence is an experienced astronomer and a co-host of The Sky at Night
The light from Mars is predominantly shifted towards the redder end of the spectrum, which works in our favour because longer wavelengths are less susceptible to being blurred by our turbulent atmosphere. You can improve what you see visually with a telescope by using filters. Visual filters are normally identified by their Wratten numbers. These may be written as W followed by a number and possibly a letter. Although the numbers represent specific colours, there is no sequence to them; W16 is yellow-orange, W18B is very deep violet and W21 is orange for example. The
letters sometimes found after the number represent increasing strength of filter. Yellow filters (W12, W15) will tend to make the lighter, desert regions appear brighter and brown/blue regions darker. Orange (W21, W23A) also helps increase the contrast between the light deserts and dark exposed rocks, further cutting through the weak Martian atmospheric haze. Red (W25, W29) enhances the contrast yet again and is excellent for defining the boundaries between regions. Orange and red are also good filters to use for observing Martian dust storms, if they appear. A green (W57) or blue-green (W64) filter is good for showing seasonal features such as frost patches, fog and irregularities along the edge of the polar caps. Blue (W80A, W38, W38A), deep blue (W46, W47) and magenta (W30, W32) are all good colour filters for detecting Martian weather in the form of white clouds or limb hazes; magenta in particular is good for seeing detail in the Martian polar regions. The last time Mars was at opposition in 2018, we got a view of it with a large apparent size, the best for some time and larger in fact than this year. But the planet was low to the horizon as seen from the UK and this spoilt the view for many. This year the Red Planet will get to a much higher altitude in the night sky, reaching at least 40˚ in altitude, in the constellation of Pisces. With a maximum apparent size only fractionally smaller than that seen in 2018, this makes 2020 the year to get acquainted with this fascinating world.
þ A good selection of coloured
filters are useful additions when observing Mars
July 2020 BBC Sky at Night Magazine 37
