The second-closet planet to the sun, it is also classified as a terrestrial planet and it is sometimes called earth's "sister planet" because the two are similar in size, gravity, and composition.

In an experiment examining the effects tai chi on arthritis pain, callahan (2010) selected a large sample of individuals with doctor-diagnosed arthritis. half of the participants immediately began a tai chi course and the other half (the control group) waited 8 weeks before beginning. at the end of 8 weeks, the individuals who had experienced tai chi had less arthritis pain that those who had not participated in the course.