Mars is approaching but not too close – the unfavourable Mars opposition of March 2012
The paths of Mars and the Earth around the Sun with the positions of the 2012 and 2003 oppositions indicated. Drawing Nick Lomb
Every two years or so the red planet Mars is at opposition, when it is on the opposite side of the Earth to the Sun. At such times the planet is relatively close, but some oppositions are more favourable than others. The coming opposition in early March 2012 is a particularly unfavourable one, yet it is still worth looking for Mars with the unaided eye or examining it through a telescope.
The changing distance between Mars and Earth at oppositions is mainly due to the oval-shaped path of Mars around the Sun. The spectacular Mars opposition of August 2003 when the red planet was only 56 million km from Earth took place when Mars was near perihelion, its closest distance to the Sun. Conversely, at the current opposition the distance of Mars from Earth will be 101 million km, almost twice the distance in 2003, as Mars is near its aphelion, its furthest distance from the Sun.
Mars is at opposition on Sunday 4 March at 7:10 am. Interestingly, Mars is not at its closest to Earth until two days later on Tuesday 6 March at 4:00 am when it has moved about 60 000 km closer to Earth. The time difference between opposition and when Mars is actually at its closest is again due to the oval shape of the path of Mars around the Sun.
Favourable oppositions of Mars take place in a cycle of around 17 years. The last favourable one was in 2003 and the next one will be in October 2020. The best known favourable opposition of Mars was in September 1877. That opposition allowed American astronomer Asaph Hall to discover the two tiny satellites circling Mars using the large 26-inch (66-cm) lens telescope at the US Naval Observatory in Washington DC. He named the two moons Phobos meaning Fear and Deimos meaning Flight.
Another observer in 1877 was the Italian astronomer Giovanni Virginio Schiaparelli who made careful observations of the planet during the opposition in order to draw a a new map of its surface features. On his map he included some linear features that he called ‘canali’, which in Italian can mean natural features. However, the word was translated as ‘canals’ in English and so began a whole culture of belief in intelligent beings on Mars or Martians.
Oppositions of Mars are useful not just to provide the best times to examine the surface of the planet, but they also provide opportunities to launch spacecraft towards it with the minimum expenditure of energy and so fuel. Spacecraft to Mars are launched into a Hohmann Transfer Orbit before opposition and reach Mars eight and a half months later.
Finding chart for Mars in the eastern sky calculated for 9:30 pm on 5 March 2012 as seen from Sydney. Drawing Nick Lomb
Although the March 2012 opposition of Mars in not a favourable one it is still worth looking for it in the night sky. As indicated in the diagram above in the early evening Mars is low in the eastern sky near the bright sky Regulus. It is the brightest object in that part of the sky and with its red colour is quite unmistakeable. Through a telescope some features including its north polar cap should be visible. Good observing!