Astronomy discussion board---

Discussion Topic:

Planets move around the sun in the elliptic orbits (roughly: circular), under the influence of the gravitational force between the planet and the sun. Because planets move in the uniform motion approximately, the gravitational force must be counterbalanced by another force. What force is that? Insert formulae along with interpretations for the two forces to your answer.  Introduce your opinion on: "why the orbits are elliptical (circular)?" 

Writing Your Response:

1. Students still need to use proper grammar and writing techniques. Responses should be complete, well thought out, and presented in a logical manner. 
2. A minimum of one paragraph is required for this posting. 
3. You are not required to support your post with references, but you are encouraged to include images, graphics, links to videos, and so on to create a more dynamic submission.