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The lunar nodes are the orbital nodes of the Moon, that is, the points where the orbit of the Moon crosses the ecliptic. The ascending node is where the moon crosses from south of the ecliptic to north of the ecliptic. The descending node is where it crosses from north of the ecliptic to south of the ecliptic. Eclipses occur only near the lunar nodes: Solar eclipses occur when the passage of the Moon through a node coincides with the new moon; lunar eclipses occur when passage coincides with the full moon. A lunar eclipse may occur if there is a full moon within 11° 38' (Celestial Longitude), of a node, and a solar eclipse may occur if there is a new moon within 17° 25' of a node. The plane of the lunar orbit precesses in space and hence the lunar nodes precess around the ecliptic, completing a revolution (called a ''draconic'' or ''nodal period'', the period of nutation) in 6798.3835 days or 18.612958 years (note that this is not the same length as a saros). The same cycle, measured against an inertial frame of reference such as ICRS (relative to the stars) is 18.599525 years. ==Names and symbols== The nodes are called by different names in different areas of the world. Since the ascending node is the point of intersection between the ecliptic and the plane of the lunar orbit where the Moon is ascending from the South to the North, it is sometimes called the North node. In ancient European texts, it is referred to as the dragon's head (''Caput Draconis'', or ''Anabibazon''). The symbol of the ascending node is , the astronomical and astrological symbol for the Dragon's head. Similarly the descending node is the point where the Moon is descending from North to South, and is sometimes referred to as South node. It is also known as the dragon's tail (''Cauda Draconis'', or ''Catabibazon''), and its symbol is the inversion of that of the ascending node: . Note that the so-called North node may in fact lie South of the South node in the course of the nodal cycle. In Hindu astronomy, the ascending node ☊ is called ''Rahu'' and the descending node ☋ is called ''Ketu''. In Tibetan Astrology (partially based on the Kalachakra Tantra) the southern node is named Kalagni. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lunar node」の詳細全文を読む スポンサード リンク
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