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I know earth is spherical in shape.
Does this means earths atmosphere is spherical in shape or the ground/(land+water combined) is in spherical shape.
I have some doubts if the second is true.
I'll add my other doubts once some one answers my question.
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The form of the Earth is a approximately (to a very good approximation) an oblate spheroid (difference between the polar diameter and equatorial diameter is ~20km which is ~1/300 of the mean diameter). The shape is distorted by tides (both in the solid body as well as the sea) and by topography. Topographical variation is about +10 to -10km (height of Everest to depth of the Challenger Deep).
Up and down are defined by the local gravity vector, which to a good approximation is pointing into the Earth towards its centre. Hence wherever the sun is, down is into the Earth's surface (in the direction colloquially know as down)
As noted by Conrad Turner, the approximate shape of the Earth is an oblate spheroid, though it is so close to spherical that you would be pretty hard-pressed to see the difference without precise scientific equipment. This web page has some pretty good information on this topic that you might be interested in.
When we talk about this shape, we are generally speaking of the land and sea. Gas in general has the property of not only having fluid shape, like a liquid, but also having indefinite volume. Gas expands to fill the container it is in. The atmosphere is not contained per se. What keeps it from simply expanding all the way out to the moon is Earth's gravity. But this means that faster moving air particles get further away from the surface than slower ones before being pulled back by gravity. So where exactly is the edge of the atmosphere? The outer-most layer of the atmosphere, the ionosphere, is very thin, and the further out toward space you go, the thinner it is. Where exactly its edge is, and what shape it has is... complicated. Anyone who wants to make any authoritative statement on the shape or height of the edge of our atmosphere will have a few other authoritative statements to argue against.
While the exact shape of the atmosphere is difficult to define, it's pretty safe to say that it is not exactly spherical due mostly to the planet's rotation, and solar winds. The Earth's surface is also not exactly spherical due mostly to the planet's rotation, and tidal forces. Even if the entire surface was covered in water, rotation would flatten the planet's shape ever so slightly. And even if our rotation became tidally locked with the sun, and the moon... um... went away somehow, the tidal forces of the sun, would distort the spherical perfection ever so slightly. But laying all of that aside, the Earth is still probably closer to a sphere than most ball-shaped objects you have ever handled, whether you measure the "edge" of the atmosphere, the land and sea, or whatever.
