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Our solar system is in constant motion, with the eight planets – and the dwarf planet Pluto – orbiting the sun. Most of these planets are visible from Earth, even to the naked eye. If you follow them in the sky night after night, you will notice that their position changes slightly; as they orbit around the sun, they seem to move from west to east on a fixed background of distant stars.
However, sometimes, some of them seem to reverse direction and travel back – from east to west – for weeks at a time, before resuming their usual course. This movement is called "retrograde movement". But what does it mean, and what exactly is happening here?
The retrograde movement is actually an illusion. Earth surrounds the sun faster than planets that are farther away from the sun. And when Earth passes one of those distant planets in its journey around the sun, for those of us standing on dry land, it seems that this distant object reverses the direction – but it's just a trick of your brain. The planet is moving in the same direction that it always has, but our perspective is different. [Seeing Things on Mars: A History of Martian Illusions]
Think of it this way – you are in a car on the highway, and you pass another car in the next lane. By the way, it looks like this other car is backing away. Obviously, the driver did not suddenly start driving in reverse. But compared to your car and your momentum, it seems that the other car is actively moving in the opposite direction.
Now, let's apply that to Mars. About every two years, Mars seems to be changing course in the sky and spending a few months traveling backwards. In 2018, the retrograde movement began on June 28, with Mars appearing to move from west to east in our sky until August 28, and then resume its normal path.
But during these two months, it is not Mars that is doing something different – it's Earth.
It takes 365 days to Earth to orbit the sun. Mars needs 687 Earth days to make a complete circuit. We are both in motion, but Mars still has some way to go until the end. Every 26 months, Earth catches Mars and passes. As our orbital trajectory carries us beyond the red planet, we have the illusion that Mars is moving away from us, rather than the reality – that Earth is moving away from Mars.
After a few months of this, our perception of how our planets move hits the reset button, and Mars seems to be moving forward again.
A rapidly tilting planet
And if that is not weird enough, because the Earth and Mars have different inclinations to their orbital trajectories, the shape of the path that follows the movement toward the lorry. back of Mars can change between retrograde events. If you observe and mark the position of Mars night after night during retrograde, you will see a shape emerge – sometimes it's a closed loop and sometimes it's no longer a zigzag – depending on where it's going. the planets are on their inclined axes. [19659002] If the Earth and Mars orbit at the same rate and remain in fixed positions relative to each other throughout their orbits, Mars would still seem to move in the same east-west direction. As they do not, every two years, Mars is temporarily left behind.
The retrograde movement was even visible to early astronomers, who were completely confused when they saw this and had difficulty in explaining it. But it was impossible for them to find a solution that also corresponded to the popular idea that the Earth was the center of the solar system. It was only until the 16th century – when the Polish astronomer Copernicus placed the sun at the center of the solar system – that all this retrograde movement suddenly made sense.
Original article on Live Science.
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