Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continents and islands. The remaining 71% is covered with water, mostly by oceans but also by lakes, rivers and other fresh water, which together constitute the hydrosphere.

The Moon is Earth's only proper natural satellite. It is one quarter the diameter of Earth making it the largest natural satellite in the Solar System relative to the size of its planet. It is the fifth largest satellite in the Solar System and is larger than any dwarf planet.

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as visible light and infrared radiation. It is by far the most important source of energy for life on Earth.