Planet Earth. Photo courtesy of NASA
Perihelion: the point in Earth's orbit where it is closest to the Sun (in January)
Aphelion: the point in Earth's orbit where it is farthest from the Sun (in July)
Goldilocks Principle: "Venus is too hot, Mercury is too cold, and Earth is just right" for life to exist
Greenhouse Gases: Water vapor, carbon dioxide, methane, and nitrous oxide in the atmosphere that trap infrared radiation.
Greenhouse Effect: Heating of Earth's surface by certain atmospheric gases, which helps keep Earth's surface warm enough to sustain life.
Last modified: Tuesday, 27 April 2010, 4:57 PM