The Red Planet, an ember growing bright
Mars is now past opposition, but as Paul G Abel explains, the planet will remain with us in evening skies for many more months of useful observations
Mars may be past opposition, but there’s still plenty to see
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first Martian month sees the value of Ls go from 0° to 30°, while the second month sees the Ls range go from 30° to 60° and corresponds to later summer in the south, meanwhile the northern hemisphere has passed into spring. We continue in this way until we have broken the Martian year up into 12 months, each spanning 30° of the Red Planet’s orbit.
Of course, Martian months are longer than Earth’s, since a Martian year is about twice that of our own. By looking up the value of Ls, we can tell exactly where Mars is in its orbit and work out what the seasons should be in either the northern or southern hemisphere. We can also mark out other important events: perihelion (when Mars is closest to the Sun) occurs at Ls=251°. So, when opposition occurs close to this value of Ls, we know it will be a ‘perihelic’ opposition – as was the case in October. You can find the value of Ls by using the free software WINJUPOS (which can be downloaded at http://jupos.org/gh/ download.htm); it’s the value called ‘longitude of the Sun’ in the ‘Ephemerides’ tab. The Handbook of the British Astronomical Association also gives the values in its ‘Mars Section’. >
Seasonal changes
At the start of November, Ls has a value of around 310° and so it’s late summer in the southern hemisphere. A number of changes have started to occur – the southern polar cap (SPC) has now shrunk and this has returned volatiles, such as dust and water vapour, back into the Martian atmosphere. Watch out for brilliant white clouds that collect around the Tharsis
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