The lower conjunction is the celestial event in which an inner planet moves through its orbit between the observer and the sun . It then shows the observer its unlit side (similar to a new moon ) and passes, mostly unobservable, north or south of the sun.
With respect to the earth, there are only two inner planets - Mercury and Venus . Because of the different orbital inclinations to Earth, the planet rarely passes directly in front of the solar disk: Mercury passes about every decade, Venus passes a maximum of two per century.
The astronomers define the lower conjunction as the point in time at which the planet has the same ecliptical length as the sun, which roughly corresponds to the moment of the smallest angular distance.
In the case of the closest neighboring planet, Venus, the passage can take place up to 9.5 degrees north or south of the sun. Theoretically, the planet then shows an extremely narrow sickle shape, which is broadened by the refraction of light in the dense Venusian atmosphere (see also dichotomy ). Because the side of Venus facing the earth is not completely unlit, it can be seen in a good telescope for a few days both in the morning and in the evening sky .
If a lower conjunction takes place near the orbit of the orbit of Mercury or Venus, Mercury or Venus can pass in front of the solar disk during this event; there is a so-called passage of Mercury or Venus . It is possible to observe them with suitable solar observation instruments as small, deep black discs that wander past the sun from east to west.
- Andreas Guthmann: Introduction to celestial mechanics and ephemeris calculus . 2nd Edition. Spektrum Akademischer Verlag, Heidelberg, Berlin 2000, ISBN 3-8274-0574-2 , pp. 191 .
- Joachim Krautter et al .: Meyers Handbuch Weltall . 7th edition. Meyers Lexikonverlag, Mannheim, Leipzig, Vienna, Zurich 1994, ISBN 3-411-07757-3 , pp. 25 .