Why Does the Size of the Sun Appear to Change?
ES2603  A Year of the Sun

This animation shows twelve images of the sun, each taken through a telescope on Earth, one month apart. The dates range from August 2000 to July 2001. The dark markings on the sun are sunspots—regions where the surface is slightly cooler than the surrounding area. Notice how the size of the sun appears to change in a regular pattern.

Big Bear Solar Observatory
A year of the sun.

Consider these two possible explanations for the changing size of the sun:

  • either the sun actually grows and shrinks over a period of one year, or
  • the distance from Earth to the sun changes, causing the sun to appear to change size.


All stars change size over their lifetimes, but the sun is currently very stable. Evidence indicates the sun has a constant diameter of about 1.4 million kilometers. The only reasonable explanation for the change in the sun's apparent size is that Earth's distance from the sun changes in a regular pattern.

1. How would the sun appear in the animation if Earth orbited it in a perfect circle?

2. Sketch the shape of Earth's orbit that you think explains the sun's appearance in the animation.

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