Mars at Opposition January 29 2010 Mars is by now a very conspicuous object in the eastern sky shortly after sunset. The planet comes to opposition on January 29th at 19h 42m U.T. Owing to the eccentric nature of Mars's orbit about the Sun (see Dia. 1), oppositions of the planet from Earth vary a good deal one from the other. This year's opposition is one of the more favourable for observers in the northern hemisphere since it takes place with Mars in high declination and in winter, offering long hours of darkness. However, this advantage in altitude has to be set against the small apparent diameter of Mars.
The table below gives the essential data for a number of Mars oppositions, past, present and future. Opposition in longitude for Mars 1993 to 2027
As may be seen from the table, the distance from Earth is least on August 28, 2003 and greatest on February 19, 2027. This corresponds to apparent diameters for Mars of 25.1" (mag. -2.9) and 13.81" (mag. –1.2). At this year's opposition we have: apparent diameter 14.09" (mag. –1.3). This is approximately 1/4 the apparent diameter of Venus close to inferior conjunction or about 1/3 of he apparent equatorial diameter for Jupiter at opposition this year on September 21, one of the six closest approaches to Earth by the giant planet in 100 years from 1950 to 2050. (See Dia. 2. )
Dia. 1 shows the relative position for Mars with respect to the Earth and Sun for January 8th 2010. (Note: for the sake of clarity the orbits of Venus and Mercury have been omitted.) The arrows show the direction of travel for the two planets with respect to the Sun. Owning to the Earth’s greater angular velocity, our planet will catch up with Mars by the 29th January, hence the description “opposition” (opposite the Sun) when all three bodies will be in line if not quite in the same plane. From all of the above we conclude that, where Mars is concerned, observers in the southern hemisphere are at a distinct advantage over their counterparts in northern latitudes. Again, due to Mars’s eccentric path about the Sun, its closest approach to Earth takes place a day earlier than opposition itself.
Mars is not an easy object on the eye when viewed through a telescope. Apart from its small apparent diameter, the planet’s albedo (reflective index) is relatively high and so when close to opposition, in order to have tolerable contrast between the dark sky background, high powers must be used. This demands good atmospheric seeing, a rare event in itself in the skies over the British Isles. In fact, the best views of Mars (as with Venus) are often achieved in strong twilight, or even full daylight, some time before or after actual opposition. Currently both polar caps are visible in good telescopes able to support powers of 150x or more, the south cap being the most extensive. Due to the ruddy aspect of most of the disc the snowy caps often appear almost detached.. So far as we can tell, there is no sign of global warming on Mars as yet! J.V. 07/01/10 |
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