Zeenath PV asked:

why earth moves in an elliptical orbit instead of circular one?

Ans:

Please note that ELLIPSE is the more general form for an orbit and CIRCLE is a speacial case of ellipse.

So, it is more general way of saying that the orbits are elliptical. If it were circular, we would have more trouble explaining why they are perfect circles.

The situation where the Sun, for instance, would be exactly in the center of a perfectly circular planetary orbit couldn’t happen in our Solar System since there are other planets that would gravitationally affect the orbit and cause it to be immediately non-circular by pulling the object to one side or the other. Any other stable orbit type than a perfectly circular one turns out to be an elliptical orbit.

Read more at : http://www.astro-tom.com/technical_data/why_elliptical.htm