Mars at close opposition (October-November 2005)

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(October-November 2005)

By Salvador J. Ribas

During the last stretch of October and the first of November, the indisputable main object of night skies was the planet Mars.

Mars as seen by the Hubble space telescope during the approximation of 2003.
(Courtesy of NASA and the STScI)

A superior planet is said to be at opposition when it is directly on the opposite side of the Earth from the Sun. This implies it rises at the sunset, passes the meridian at the middle of the night, and finally sets at the very moment the Sun rises again from the east skyline. It also implies that the planet is brightest than at any other position of its orbit, because we can see its whole disc directly lighted up by the Sun.

Earth and Mars configuration at the moment of the opposition.
(By Salvador J. Ribas)

Mars spends about 687 terrestrial days to completely revolve around the Sun, following a slightly elliptical orbit. That is why two special positions exist: the nearest to the Sun (perihelion), and the furthest (aphelion). If we take into account the time the Earth spends revolving the Sun, it is easy to conclude that every 26 months (2.2 years) an opposition occurs.

Because of the orbits of planets are elliptical, oppositions can be more or less appropiate for observation, depending on the exact position the planet occupies on its orbit. In fact, oppositions are always the moment of gratest brightness, but if they occur when the planet is near its perihelion, obviously a grater amount of light reflects on its disc and so we see it still brighter. In summer 2003 an exceptional close opposition took place, the best for 58 thousand years. The distance between Mars and the Earth was only 56 million kilometers and as a result, Mars's apparent size was 25.1 arcseconds.

The opposition we are talking about was quite favourable and occured on the early morning of 30th October. The distance from Mars to us was 69 million kilometers and the apparent size was significantly over 20 arcseconds.

Agrupació d'Astronomia d'Alella

5th November 2005, Alella (Barcelona)

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