Perihelion, << `pehr` uh HEE lee uhn, >> is the position of a planet or comet when it is closest to the sun. In the Northern Hemisphere, the earth is at perihelion at midwinter. At this time, the sun shows a larger diameter, but this difference is only 1.7 percent of the sun’s average size. A person cannot see the difference without instruments. When a planet or comet’s distance from the sun is greatest, it is at aphelion.
