Earth's orbit is nearly ________.

• A. Circular
• B. Elliptical
• C. Spiral
• D. Oblong

Answer: (A)

Shapes of orbits are described by eccentricity. A circle is a special kind of ellipse with eccentricity zero. Earth’s orbit is an ellipse with only a very small eccentricity (about 0.0167), so the distance to the Sun changes only a few percent over the year. That tiny variation keeps the orbit looking and acting almost like a circle, with the orbital speed nudging a bit faster near closest approach and a little slower near farthest, but overall staying very regular. If it were a perfect circle, the distance would stay constant and the speed would be the same everywhere; because the ellipse is so shallow, “circular” is the best way to describe its nearly uniform shape. The other terms don’t fit a bound, nearly circular path as well as circular does.