Within the solar system, all the planets together with the Earth orbit in different paths around the Sun. We cannot observe this planetary movement from a completely stationary observation point, rather from the Earth, which is itself rotating, thus making the individual movements appear complicated.
Whether a planet can be seen from the Earth at all depends on the respective positions of the Earth, the planet and the Sun at a given time. If a planet, seen from the Earth, is situated exactly opposite the Sun, i.e. Sun-Earth-planet, the position is called the 'opposition' position. The planet can then be seen right through the night. It rises when the Sun sets and disappears under the western horizon when the Sun rises again .
The two inner planets, Venus and Mercury can never be in opposition. In their case, therefore, a distinction is made between an upper and a lower conjunction, in which the planets remain invisible. However, they can be observed when they are in an eastern or a western " elongation " to the Sun, i.e. they are at a certain angle with respect to the Sun. If an "outer" planet, as viewed from the Sun, is behind the Sun, i.e. planet-Sun-Earth, it cannot be seen. This position is called conjunction. The planet rises and sets with the Sun.
Certain co-ordinates are used to determine the position of the stars. One of these is the so-called declination. It designates the angular distance of a celestial body from the celestial_equator and is measured in radians. The "right ascension" corresponds to the longitude in geography. The celestial equator is the projection of the terrestrial equator onto an imaginary celestial hemisphere, which is separated into two hemispheres by it.
