Why do planets have a fixed orbit for revolution around the sun ?
350331
God created them to move that way
267480
It was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the Sun at all times, in the same way that the same side of the Moon always faces the Earth. However, radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury's orbit makes this resonance stable. The original reason astronomers thought it was synchronously locked was because whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, hence showing the same face. Due to Mercury's 3:2 spin-orbit resonance, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days. A sidereal day (the period of rotation) lasts about 58.7 Earth days.
453860
Because the same face of the planet is not shown to sun all the time. The planet revolves as well as spin about its axis. so face that is exposed to sun changes with time. This is the very reason for day and night.
1056449
Yup, it's orbital period is the same as its rotational period due to tidal forces. \n\nAs it orbits the Earth it spins on it's axis, it takes the same amount of time to make one rotation as it does one orbit so it looks like it's not spinning at all from down here.
12606
Our world, Earth, is a planet, and planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it. With its large body and internal heating from radioactive elements, a planet behaves like a fluid, and over long periods of time succumbs to the gravitational pull from its center of gravity. The only way to get all the mass as close to planet's center of gravity as possible is to form a sphere. The technical name for this process is "isostatic adjustment."\n\nIf the shape of the Earth were due solely to gravity it would be a perfect sphere (round in 3 dimensions) but it's not, the Earth is shaped like what is called an oblate spheroid. As the Earth spins on it's axis, it produces a centrifugal force that pulls away from the Earth (in opposition to gravity)  at 90 degrees from the spin axis, this corresponds to the equatorial regions. So the Earth bulges out a little around it's middle.
433264
Objects orbit one another because they have a curved trajectory that results from the vector addition of their momentum (in the direction of travel, tangential to the orbit) and the gravitational attraction to the other object (normal to the orbit).\n\nThis vector is constantly changing in direction (but not to any significant degree in magnitude) because of the object's motion, but it always keeps the satellite's acceleration pointed towards the object is it orbiting, resulting in smooth, roughly circular motion.
472607
planets orbit stars, satellites orbit planets
44602
Why are any of the planets round?
739091
because the earth rotates on an axis
1094922
The Earth is continually rotating on its axis.\n\nIn short, the Earth rotates because it has to in order to help conserve angular momentum in the universe.\n\nThere is a physical Law which states that angular momentum must always be conserved.\n\nThe big ball of gas and debris which out solar system was formed from had some value for its angular momentum (even if that value was zero).  When the sun and the planets formed from that material, in order to conserve the angular momentum, the planets and sun started rotating.  If you add up all the angular momentum there is now between all the planets/the sun, moons/.... it should come out to be the same value as the angular momentum of the material which formed out solar system.
818196
caus they hv fixed orbits n they kind of follow trafic rules!\nlol!!!!!!!!!!!!!!!!!!!
319780
Yes, the Sun revolves around it's own axis, just like the Earth.  It spins around every 27 days.  It just doesn't ORBIT anything.\n\nedit: actually, it orbits the galaxy.  Takes a few hundred million years.
699589
Kepler figured out the laws that describe planetary orbits.  This enabled much more accurate planetary positions (which Kepler published), and set the stage for Isaac Newton to explain that gravity  was the force that held the planets in their orbits.\n\nUp until Kepler, everyone was sure that planetary orbits must be some form of perfect circle. His discovery that they are actually elliptical was an important turning point in the history of science, away from the Aristotelian method of applying philosophical concepts to physical reality, and towards modern empirical science.
592819
When the Sun has Sunspots you can see that they revolve around the Sun.\n\nIt basically rotates similar to how the planets rotate.\n\nRotate not orbit.
963778
Eliptical orbits. If the orbit were circular, then you'd expect the period to be the same. ...The planet speeds up and slows down and spends more time at the part of the orbit more distant from the sun.
108530
Not all the planets do rotate on their axes with constant angular velocity (see reference) and even two rotate in the opposite direction to earth (counter clockwise when viewed from the celestial north pole); venus and uranus (called retrograde motion). \n\nThe rough alignment of all the angular momentum vectors in the solar system, including orbital and rotational angular momenta of all the planets, moons and the sun is presumably a left over relic from the angular momentum when the system was formed from a rotating gaseous collection of material.
464601
Earth revolves around the sun because the giant nebula out of which the solar system formed was spinning and the angular momentum is conserved, and earth spins because of the initial config. Why it revolves in an ellipse, see this\nexplanation at http://www.mathsnet.net/cabri/feynman.html