Why Does the Size of the Sun Appear to Change?
ES2603  How Eccentric!

Johannes Kepler, a famous 17th century astronomer, discovered the basic laws of planetary orbits. His first law states that planets move in ellipses, with the sun at one focus.

Scientists use the term eccentricity to describe the shape of an ellipse. (Eccentricity literally means "off center"—exactly what the sun is compared to Earth's orbit!) A circular orbit has an eccentricity of zero. The more stretched out the orbit, the higher its eccentricity.

  !   Move your cursor over each planet to display the eccentricity of its orbit. Use the information to answer the question below.


Viewed from a planet with a low eccentricity, the sun would barely appear to change size through a complete orbit. From planets with higher eccentricity, the change in the sun's apparent size would be more noticeable.

7. Which planet has the most circular orbit? Which planet has the least circular orbit?

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