The planets may not always stay exactly on the ecliptic, but they always stay near to it. The retrograde motion of the inner planets, Mercury and Venus, is the simplest to comprehend. These planets are closer to the sun than we are, and they orbit it quicker. So when they move from west to east they get farther away from the sun. During these periods they can appear to be going against the flow of the stars.
The ancients believed that these planets influenced individual lives as well as world events. They believed that each planet had an effect on humans similar to its role in astronomy: Mars made warriors; Jupiter, kings; Saturn, slaves. Modern scientists have found evidence of environmental impact from many of them over the ages. This means that people have often interpreted planetary events as representing real changes in their lives or the world around them.
Venus was originally named Phoebe (the goddess of love) before being changed to avoid confusion with Athena (the Greek goddess of wisdom and war). As you can see, both planets share the same name - a common occurrence among planets that orbit the same star. Although astronomers now know that any resemblance between planets is purely coincidental.
In addition to this influence on individuals, there has been speculation about a connection between global events and the movements of the planets.
Venus and Neptune have almost circular orbits with eccentricities of 0.007 and 0.009, respectively, in our solar system, whereas Mercury has the most elliptical orbit with an eccentricity of 0.01. Thus, all three planets have very close-to-circular orbits but Mercury is the only planet that never gets closer than about 47 million km from the Sun or farther away than 48 million km.
This means that over half of its orbit it is always hidden behind the Sun. Only when Venus approaches within 498 million km does it come out from behind the Sun to reach its greatest distance from it. At this time, it is traveling around the Sun at approximately 126 million km per hour in a direction opposite to the Earth's. It then begins to move back toward the Sun until it is again hidden behind it.
It is not possible to see Venus from the Earth during part of its orbit because it goes beyond the Moon. But because it takes about 12 years for Venus to complete one orbit around the Sun, we can see it each time it comes around.
The reason why its orbit is not exactly circular is because it is affected by the gravitational pull of Mars, Jupiter and Saturn. If Venus were a perfect sphere, these forces would make its orbit more elongated or less elongated depending on which body was acting on it at any given time.
Venus is the least elliptical of all the planets, according to one definition. This is due to the fact that its revolution is a near-perfect circle. The Earth is perfectly positioned in relation to the Sun, but Venus is too near. As a result, Venus experiences an average temperature of 450 degrees Celsius (890 degrees Fahrenheit), which means that most of it is covered in water.
Earth's orbit is inclined by about 5 degrees with respect to the plane of the Solar System. This means that we experience both summer and winter here on Earth. But if Earth were in the same position as Venus, then there would be no way for us to survive because we would always be exposed to direct sunlight. Even if there was enough atmospheric pressure for humans to live at the surface, there wouldn't be any way for life to survive such constant exposure.
So why does Venus remain so hot even though it isn't directly facing the Sun? It's because it orbits closer to the Sun than Earth does. Every time Venus goes around the Sun, it passes through all the phases of the Moon from new moon to full moon, but it also passes through every stage of development between night and day. As a result, there are times when the Sun is shining down on some parts of Venus but not on others. These shadows cause temperatures to vary across the planet, giving it a more elliptical shape.
We watch these two planetary disks change phases because the position angle between the Earth, Sun, and Mercury/Venus is continually changing owing to our orbital locations relative to one another. When the angle between Earth's orbit plane and Mercury's orbit plane increases, we see more of the inner planet; when it decreases, we see less of it.
This is exactly what you would expect from simple geometry: As the Earth moves closer to the Sun, it sees more of its neighbor planets than when it is farther away; similarly, as it moves further from the Sun, it sees less of them. Over time, this leads to a periodic variation in the number of days that Mercury or Venus is visible from the Earth. The period of this variation is about 75 to 80 years for both planets.
The reason this happens is because of gravity. Owing to gravity, each planet will always try to keep itself in a spherical shape, so each one has a center of mass that falls where the weight is greatest. For Mars, this means the middle of its mass; for Jupiter, it's somewhere near its center; for Saturn, it's toward its north pole. Because of this gravitational effect, each planet will always try to face towards its central star, but due to random angles involved in planetary formation, some of them will be tipped away from their stars at launch.