The changing brightness of the planet Mars
The Moon and the red planet Mars on 27 November 2007. Picture taken by Bonny Foley Brennan, an Aboriginal Artist from the Boolarng Nangamai Aboriginal Corporation, of Gerringong NSW
The above picture was sent to me by Steve to identify the smaller object near the Moon. I calculated what the sky looked like on the given date of 27 November using a suitable computer program and established that the planet Mars was near the Moon that night. So what we have is a nice picture of the Moon together with the red planet Mars.
Mars is fairly bright on the picture, much brighter than Mars is in the sky at the moment. Why does the brightness of the planet change so drastically?
The apparent brightness of a planet in the sky depend on its distance from Earth and its distance from the Sun. For a distant planet like Jupiter there is little change in brightness as there is little change in either distance. With Mars there are large changes as its distance from Earth varies. Mars is further from the Sun than Earth so it moves more slowly. Every two years or so the Earth catches up with Mars as they both circle the Sun and the two planets are in line with the Sun. That is called opposition as the Sun and Mars are then on opposite sides of the Earth. Opposition is when Mars is at its closest to Earth and at its brightest.
Of course, it is more complicated. Mars has an elongated path around the Sun. Sometimes when we catch up with it it is further from the Sun than at other times while sometimes it is closer. If we catch up with Mars at a close point astronomers talk about a favourable opposition. Favourable oppositions occur roughly 15 years apart. Distances of Mars at different oppositions are listed below:
2003 56 million km favourable
2005 69 million km
2007 88 million km
2010 99 million km
2012 101 million km
2014 92 million km
2016 75 million km
2018 58 million km favourable