Hi Guys, I am new here so please don't hammer me out .. I always wondered why is the shape or all the planets and starts round. Why don't we see a Square shaped asteroid or a planet flat as believed in Middle Ages... Why do we have an elliptical path around the sun where as Moon makes almost a perfect circle around the earth... My branching question is if the shapes of most of the planets and stars are circular ...why do we consider Time which is part of our universe to have a linear property and not a circular property.. I am not a science student so please pardon me if I am asking Astronomy 101 Question. Peace:m:
Star could not be anything but approximately spheirical. They are composed af gas and plasma (which is gas-like). Gravity will always tend to force such material into a spherical shape. Rotational effects can cause a flattening along the axis of rotation and a bulge at the equator. Planets are similarly shaped by gravity. Since planets are composed of more rigid materials, it is possible for them to be nonspherical. Some asteroids are not spherical.
The sphericle shape has something to do with size, or density, or mass I think. Smaller asteroids like Gaspra are irregular, but you don't find sphericle asteroids unless they are larger, like Ceres and Pallas as examples. Phobos is irregular but somewhat sphericle while Diemos is very odd looking.
This essay might give some idea of the causes of sphericity http://www.bitnik.com/mp/archive/spherical.html
actually this topic should have it's own thread there is a theory that both moons are remains of a grand explosion/impact 4 billion years ago we still know far too little about Mars moons. they are quite fascinating really here are some articles about them http://www.space.com/scienceastronomy/forgotten_moons_010313-1.html
Why Gravity makes them round I would like to take a step back and rephrase my question...why does Gravity make it spherical in shape ..why not flat .. Also are all the planets in the Universe revolving around stars ..can't there be planets not revolving which might not make them spherical in shape.. I have not read about gravity too much so I might be sounding Naive...please be easy on me Please Register or Log in to view the hidden image! PS: Also I have seen a Documentary on Phobos which says that the moon is hollow and shaped like a peanut ...
Re: Why Gravity makes them round Let's try and put it this way: Gravity is essentially due to the fact that every bit of matter is attracted to every other bit of matter in the universe. The closer the bits of matter are to each other and the more matter there is, the stronger the attraction. When you get alot of matter together,(like with a planet) the attraction the matter trys the squeeze everything as close to the center as it can. The shape that allows this best is a sphere, so the matter tends to clump as a sphere. With smaller objects, (people, small asteroids etc.) the attraction of gravity still tries to pull it into a sphere, but since there is a smaller amount of mass, the pull isn't strong enough to overcome the strength of the object and it retains its shape. Whether or not a planet revolves around a star or not has nothing to do with its shape. The only factor is whether it is large enough or not. (And generally, any body that does not have enough mass to pull itself into a spherical shape is not classed as a planet, so all planets are spherical.)
Guru: If you think a little bit about how gravity acts, you might be able to answer your own question. If you have a tub full of water or more viscous jelly. and pour it on a flat surface, the Earth’s gravity flattens it out on the surface. Your intuition has been conditioned by what you have seen happen on the surface of the earth. You must ignore what you have learned from your intuition and think about a tub full of water (without the tub) thousands of light years away from any planet or star. Far from other gravitational influences, only its own gravity will affect that volume of water. If it helps, think of that water in an otherwise empty universe. Far from other gravitation influences or in an empty universe, only its own gravity would have any effect. Why would its own gravity make it flat like a pancake? What direction would be preferred for the thin flat direction and what directions would be preferred for the wide parts? With no other forces acting, does it not seem reasonable to assume that the water would take a spherical shape? Solar systems are created from vast rotating clouds of interstellar gas which condense due to self gravitational attraction. The rotation results in a disk shaped object with a large blob in the center. The large blob in the center becomes the star like our sun. The planets form from the disk. The dynamics are not fully understood. Planets rarely (if ever) form independently of the formation of a solar system. Some systems form with two stars. It is possible for a planet to get ejected from its solar system and become an isolated object. I do not know of any evidence for this happening. At any rate a solitary planet is at best a very rare object. Furthermore, the influence of the star does not cause the spherical shape of a planet. The rotaion of a planet or star does not cause the spherical shape. The self gravitational forces cause the spherical (or nearly spherical) shape. BTW: Are you aware that the Earth is not a sphere? The diameter from North Pole to South Pole is about 7901 miles (12,715 km), while the diameter at the equator is about 7928 miles (12,759 kilometers)?
Gravity tends to pull things into the "lowest" position possible, because this minimises the gravitational potential energy. Imagine a planet in the shape of a cube. The cube's corners would be further away from the centre of the cube than the middle of each face. Since not all parts are in the lowest possible position (closest to the centre), the higher parts will tend to collapse. So, the corners would spread out into the centres of the cube's faces, making them a little higher and the corners lower. This process would continue until all points on the surface were the same distance from the centre - and then the planet would be a sphere.
Thanks Guys I thank you guys for clearing the Shape of the "heavenly Bodies" for me ...I also wanted to know if you guys can answer the side question of why does not time get curved in to a spherical shape rather than being linear....I mean does time also take the shape of the Star because of the Gravity Thanks
I don't think there is anything in the universe that is "round" or circular is there? Even the earth has a "bulge"? I think that time is curved. But, alas, my brain cells are not well enough equipped to debate it. Something to do with Einstein but isn't it something about solid bodies in space form an "indentation" in surrounding space and therefore bend both light and time? Forgive my ignorance, my brain power is equal to the power of 10 - NOT!
