Inner Planet and Outer Planet
Inner Planet
Planets that their average distance to the sun are shorter than the average distance of Earth-Sun are grouped as inner planet. The planets that is included into this group are Mercury and Venus.
Because of circulatory velocity of the planets are different, the position of inner planet seen from the Earth changes. The change of position is described in the change of angle formed by line that connects Earth-Sun with Earth line inner planet. The angle called elongation of inner planet its size changes.
Inner Planet as Morning Star and Night Star
Inner planet that is making west elongation will formerly rise compared to the Sun. It will reach east horizon when the sun still lies under horizon. At that time, the planet has opportunity reflects Sunlight directly to the Earth. Planet is seen as star in the morning (morning star)
Inner planet that is making east elongation will reach east horizon in the morning, after the sun lies above the horizon. Therefore, in the position of east elongation, the inner planet cannot be seen above east horizon. It has opportunity reflects sunlight to the Earth and it will be seen as night star, several times after the sun sets.
How long is the inner planet seen as morning star or night star? This condition very depends on the magnitude of elongation of the inner planet. The larger elongation, the longer that planet is seen from the Earth. Based on its magnitude of maximum elongation can be calculated how long Mercury and Venus are seen from the Earth.
Outer Planet
Planets that their average distance to the Sun is longer compared with the average distance of Earth-Sun are grouped into outer planet. The planet included into this group are planets of Mars, Jupiter, Saturn, Uranus, and Neptune.
The position of outer planet seen from the Earth, as also the inner planet, they are always changes. The transformation is reflected in the changes of elongation of outer planets.
