Knot (astronomy)
Knots are called the intersections of the orbit of an astronomical object with a reference plane (in the solar system usually the ecliptic plane ):
- the point at which the orbit crosses the reference plane from south to north is called the ascending node
- the point at which the orbit crosses the reference plane from north to south is called the descending node .
North denotes the north direction of the earth's axis . If the reference plane is not related to the ecliptic, a different definition for ascent and descent is chosen.
The straight line connecting the two nodes is the node line .
The position of the ascending node in relation to the vernal equinox , i.e. the argument or the length of the node (denoted by ☊ in the graphic), and the position of the vertex of the ellipse to the node line, i.e. the argument of the periapsis (ω), form two of the six orbital elements , which are necessary for a complete description of an ideal Kepler orbit .
The period between two passes of the celestial body through the same knot is the Draconite period .
If the knot coincides with a conjunction , this leads to a covering , i.e. a transit or an occultation :
- If the new moon is near a lunar node , a solar eclipse occurs ; If the full moon is close to a lunar node, a lunar eclipse occurs .
- If Mercury and Venus are close to their orbital node during the lower conjunction , there will be a passage through Mercury or Venus .
- For the earth, the ecliptic itself forms the plane of the orbit, and no node can be defined. In pre- heliocentric times, the equinoxes were seen as the “nodes of the (apparent) solar path ”, the reference plane here was the celestial equator .
literature
- Andreas Guthmann: Introduction to celestial mechanics and ephemeris calculation , Spektrum Akademischer Verlag, 2nd edition 2000, ISBN 3-8274-0574-2 , p. 171
- Joachim Krautter et al .: Meyers Handbuch Weltall , Meyers Lexikonverlag, 7th edition 1994, ISBN 3-411-07757-3 , p. 24, 90ff