Mars is the fourth planet from the Sun and second only to Mercury as the smallest planet in the Solar System. The Red Planet is of great interest to NASA – who hope to send astronauts there in 2020 – due to its similarities to Earth. However, if you look at Mars from Earth, you will notice something bizarre happen every two weeks.
All eight planets in the Solar System – as well as dwarf planet Pluto – orbit the Sun and can be visible in the night sky.
Early scientists tracked planets and noticed their positions changed over time – they would start moving from west to east, then loop back from east to west.
However, Brian Cox revealed the key moment astronomers changed their view during his BBC “Wonders of the Solar System” series.
He said in 2010: “Up there is Polaris and it is the North Star – it’s almost exactly aligned with the Earth’s spinning axis, which means as the Earth rotates all the stars rotate around that point.
Mars has a trange rotation (Image: GETTY/BBC)
NASA has probed the Red Planet for years (Image: GETTY)
To understand the Earth’s real position we need to look at the one set of bodies that does not behave as predictably as the stars
“So it looks for all the world as if the Earth is at the centre of the universe and the stars rotate around it.
“And that, of course, is what the ancients thought for thousands of years – and why not?
“To understand the Earth’s real position we need to look at the one set of bodies that donot behave as predictably as the stars.”
Dr Cox went on to reveal how, by looking at Mars’ rotation around the Sun, scientists discovered what is known as apparent retrograde motion.
He added: “This is Mars photographed once a week for a period of months.
“Rather than travelling in a straight line across the background of the stars it occasionally changes direction and loops back on itself.
“It’s very hard to explain these loops if the Earth is at the centre of the universe.
“Understanding the retrograde rotation of Mars did not come easily – that’s why it took 2,000 years.”
The apparent retrograde motion simply means that the object appears to be moving in a prograde – or direct motion – before changing to a retrograde – or indirect – motion.
However, it is actually just an optical illusion and Dr Cox displayed this using sticks and stones.
Mars appears to travel in a straight line before turning back (Image: BBC)
Brian Cox showed the optical illusion (Image: BBC)
He continued: “The key thing is the Earth is not at the centre of the Solar System, the Sun is.
“The Earth and Mars go round it in an almost circular orbit.
“So when Mars is viewed from Earth it can be seen in the sky – when Mars moves and the Earth moves.
“But what happens when the Earth overtakes it? Then, to the line of sight, Mars [has] moved back.
“It’s reversed its rotation and will continue to do that until the Earth gets back in the line of sight.
“Understanding the retrograde loops was one of the major achievements of early astronomy, it created the concept of the Solar System.”
Last month, Dr Cox brought out a new BBC documentary called “The Planets”, where he revealed Saturn’s moon Enceladus is “leaking salty water” into space.
In 2005, NASA’s Cassini probe came within 30 miles of the surface of Enceladus, discovering water-rich plumes venting from the south polar region.
Scientists found cryovolcanoes shooting geyser-like jets of water vapour, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space.
He said: “The craft passed within just 48 kilometres of the surface of Enceladus and Cassini was able to touch the plumes.
“What we discovered, thanks to a series of these flybys, was breathtaking.
“The ice in the plumes was actually frozen particles of salty water.
“We had found a subsurface ocean leaking into space.
“Far, far from the Sun, Enceladus was harbouring an ocean of liquid water.
“And it was there because of Saturn.”