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perihelion = the point in its orbit where a planet is closest to the Sun. when referring to objects orbiting the Earth the term perigee is used; the term periapsis is used for orbits around other bodies. (opposite of aphelion)
aphelion = the point in its orbit where a planet is farthest from the Sun; when refering to objects orbiting the Earth the term apogee is used; the term apoapsis is used for orbits around other bodies. (opposite of perihelion)
greenhouse effect = the solar rays enter the atmosphere, and thanks to several reflections on this atmosphere, warm the planet many times instead of only one. This is the Greenhouse effect.
hydrosphere = there are water, ice and steam on the planet.
features = here, it means caracteristics of the face of the moon.
unobtainable = we can't find.
volumic mass = the mass of a planet on a specific volume, here it is in kg/litre.
ecliptic orbit = most of the planet have them orbits more or less in the same plane, that is called the ecliptic.