- Earth (Wikipedia)
Earth is the third planet from the Sun and the only place known in the universe where life has originated and found habitability. Earth is the only planet known to sustain liquid surface water, with ocean water extending over 70.8% of the planet, making it an ocean world. Most of all other water is retained in Earth’s polar regions, with large sheets of ice covering ocean and land, dwarfing Earth’s groundwater, lakes, rivers and atmospheric water. The other 29.2% of the Earth’s surface is land, consisting of continents and islands, and is widely covered by vegetation. Below the planet’s surface lies the crust, consisting of several slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes. Inside the Earth’s crust is a liquid outer core that generates the magnetosphere, deflecting most of the destructive solar winds and cosmic radiation.
- In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location.
- Northern celestial hemisphere (Wikipedia)
The northern celestial hemisphere, also called the Northern Sky, is the northern half of the celestial sphere; that is, it lies north of the celestial equator. This arbitrary sphere appears to rotate westward around a polar axis due to Earth’s rotation.
- Southern celestial hemisphere (Wikipedia)
The southern celestial hemisphere, also called the Southern Sky, is the southern half of the celestial sphere; that is, it lies south of the celestial equator. This arbitrary sphere, on which seemingly fixed stars form constellations, appears to rotate westward around a polar axis due to Earth’s rotation.