Why are the orbits of the planets elliptical(oval shaped)?
363542
If the Earth orbited at EXACTLY the right speed, its orbit would be a perfect circle. But the Earth isn't lucky enough to have just the right speed. When the Earth is going slower, it isn't orbiting fast enough to maintain its distance from the Sun so it starts to fall toward the Sun. As it falls, it speeds up. Some people (a lot of people actually) think this will cause Earth to spiral into the Sun. But that is not what happens. Instead, the speed of the Earth builds up too quickly and the Earth ends up a little closer to the Sun but going too fast. That causes it to want to fly away from the Sun. As it moves away from the Sun it looses speed. It ends up farther away again but now going too slow again. The process repeats endlessly and the resulting shape of the orbit is an ellipse. The mathematics of it is similar to a weight hanging from a spring. The gravity of the Sun is like the spring, holding the Earth, but not rigidly. The weight COULD hang perfectly still, but any little bump will set it bouncing until friction damps out the vibrations. And in space there is no (or at least VERY VERY little) friction.
43539
Planets seem to be spheres, not cubes or cylinders or oddball rocky shapes.  Some smaller bodies such as asteroids or Mars moons Phobos and Deimos, do have odd shapes, but larger bodies like the nine planets and most of their moons do look like spheres.  That's because of the nature of gravity.  You can think of gravity as a force that points inward toward the center of the planet so that every part of the surface is pulled evenly toward the center, resulting in a spherical shape. \nOf course, planets are not perfect spheres because mountains and valleys and even skyscrapers are all deviations from the spherical shape.  However, as planets get larger, gravity gets stronger, until eventually large objects on the surface are crushed under their own weight.  That's why we don't have mountains that are 50 miles high or skyscrapers that are 2,000 stories tall.  Planets stay basically spherical because any large deviations get crushed. \n\nAlthough gravity keeps planets close to spherical, there are other forces that cause deviations from the basic spherical shape.  For example, the rotation of the earth once every 24 hours, causes an apparent centrifugal force which creates a bulge at the equator.  In fact the earth's diameter at the equator is 7,926 miles while the diameter between the poles is only 7,900.
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.
114224
Only bodies with a large gravitational mass are round. There is enough gravity to pack it more tightly and this combined with roation of the object forms a round shape. Smaller objects like asteroids, comets and small moons often have irregular shape because there was not enough gravitational mass to compress the object into a round shape
881618
None of the above....\nSaturn's rings are (presumably) in orbit around that celestial body.  As such, they (presumably) would maintain an equal distance from the (centre) of the planet.  They would, by definition, be always 'widest open' as you say above.
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.
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.
570352
Plain Newtonian Physics and Kepler's Laws; even a circle is a special case of an ellipse with both its focii coinciding (Coordinate Geometry).You may like to know more about properties of ellipses, circles, parabolas, hyperbolas, etc. 3D Coordinate Geometry and spherical trigonometry are pretty helpful.
224099
It's not that strange, why would you expect the Earth's orbit to be a perfect circle.\n\nEverything revolve in a elliptical orbit, because they can't all start moving in perfect circles, that's impossible, so when Earth got closer to the Sun, it basically falls and accelerates because of gravity, but because it's now faster, it can bypass the Sun so it won't crash into it.
726348
Well, that is one hypothesis, which is "conjecture", which is just to say it's a "great big guess".  No astronomical explanation has thus far been a satisfactory explanation, because the hypotheses fail to explain "why" comets orbit the Sun clockwise, while everything in our Solar System rotates counter-clockwise along with the counter-clockwise rotation of the Sun.\n\nMy analysis has determined that our Solar System and the Sirius Binary System are interlocked in an electromagnetic deadly dance of death in which the two systems cyclically and recurrently interpenetrate through each other every 12,000-year period.  Hence, our Solar System is on a collision course with the Sirian System.  This transversion is an extinction level event that I have identified as the Sirian-Solar Reconvergence.  The Sirian Binary rotates clockwise, thus comets are dislodged debris from the Sirian System captured within our Solar System from each Reconvergent event in which the comets retain their original clockwise rotation due to their immense velocity that cannot be overcome by the rotational influence of our counter-clockwise system. \n\nThe Sirian-Solar Reconvergence also explains all the crater impacting that has scared all the solid-body planets.  As the cosmocataclysm ensues all the debris from the various debris belts in both systems dislodge and rains down mass celestial bombardment on every object in both systems, particularly when the Reconvergence reaches its critical vertex of transversion.\n\nAdditionally, the Sirian Solar Reconvergence would completely explain why the planet Venus spins clockwise and backwards (opposite to the spin of the other planets) while revolving around the Sun counter-clockwise, thus creating the unusual phenomenon of one day being one year, because it completes one revolution in one year.  Clearly this is an indication that Venus was originally a planet torn out of its clockwise orbit in the clockwise Sirian System during one of the Reconvergence, and was captured by our Sun.\n\nThe reason why the two systems are locked in this process is because they were formerly unified as one solar system; our system was originally the Osirian Trinary Stellar System.  Sirius B is our original paternal star, Sirius A was the outer star that orbited the body of the system, our Sun (Sol) was technically Sirius C, originally a binary companion with Sirius B at the core of our former Trinary System, and there were originally a total of eleven planets.  The split occurred because Sirius B after depleting its helium as a Red Giant exploded into a Nebula Discontinuity Event, creating the Local Compression Bubble and precipitating the celestial dynamics that tore the system in half.  The two halves journey off to their greatest distance, and then invert and return to the path around the galactic center that they were originally blown apart from, being the recyclic event of the Sirian-Solar Reconvergence.  This cosmocataclysm wipes all human civilization off the face of the Earth and catapults Mankind back to a State of Neobarbarism to begin the slow and incremental process of re-civilization all over again.  It is also responsible for suddenly laying down the Geological Column and entombing the charred bones of beasts and men into fossilization within the stratification, because the event is so violent, catapulting the Earth out of its orbit and thrusting the oceanic waters out of their basins, overrunning the continents.\n\nAs our Solar System and the Sirian System near ever closer, signs of the next Sirian-Solar Reconvergence are mounting, being the environmental deterioration of the terrestrial environment, increased volcanism on the oceanic floors that is heating up the seas and affecting the climatic temperatures, the melting of Earth’s and Mars’s polar ice caps, new red spot storms on Jupiter, the addition of a third belt to the Van Allen Radiation Belts that surround the Earth, the vio
288737
The ellipse is a result of the mutual gravitation of two massive bodies. The center of mass of each sun-planet system is somewhere between the center of mass of the two bodies (it's still inside the sun, though), and each body revolves around the center of mass. Since both masses are moving, they orbit around each other, resulting in elliptical orbits for both (however, since the sun is always orbiting a point within the sun, it's not as noticeable. Unless you're an astronomer.
1123910
Only "large" celestial objects are round. Small ones, like asteroids, can be any shape.\n\nIt is the high gravity of large objects that makes them round. As they accumulated from smaller pieces, any piece that was out of round was pulled by gravity "downhill" so the overall shape of the body is round.
530735
two reasons, gravity and angular momentum.  Gravity tends to pull matter into one centralized spot, but angular momentum must be conserved.  That mass that escaped being part of the sun attracted mass to centralized locations, what became the planets, flattening things out to a disk shape
967418
yes there are straight lines. what you describe is according to the earth and the solar systems that appear round due to the force that acts from all directions creating a universal container (the sphere) which is round. But look at the BIGGER picture a straight line has to exist even if it is as small as an atom (universe - meter scale). also there are other geometrical forms. infact the only reason why round is soo common in all universal shapes (objects). well think of all forces acting on a body from all sides then reason the kind of shape it will produce when all forces level out the object.
791325
Most are actually more ellipsoid in shape (i.e. not perfect spheres).\n\nI'm no astronomy/physics expeert, but I think it has something to do with gravity and rotational forces.
285187
The are round, in short, because it takes the least enegry to do it.\n\nGravity pulls the matter that will make up the planets/sun/... together.  All the forces are pointed towards to the objects center of mass, everything is being pulled inward toward the center.  With everything trying to get as close as possible to the same center of mass, the object develops a round/sphereical shape.  This shape minimizes the amount of potential energy the matter has above the center of mass.\n\nThe reason why some objects (like the Earth for example) as not totally sphereical is because they are roating around an axis.  Since the Earth is rotating on its axis (causing our days and nights), there is a bulge around the equator as the matter located there is rotating faster [tangential speed] than the matter located at the poles.  This faster tangential speed requires more centripetal force [not to be confused with the fictional "centrifugal" force] or order to keep it rotating.  In order to lessen the required force, the matter bulges out a little bid, increasing its radius away from the axis.\nCentripetal force = m(v^2/r),\nyou can see that a larger radius (r) away from the center means a smaller centripetal force is needed.
44602
Why are any of the planets round?