Lunar node


A lunar node is either of the two orbital nodes of the Moon, that is, the two points at which the orbit of the Moon intersects the ecliptic. The ascending node is where the Moon moves into the northern ecliptic hemisphere, while the descending node is where the Moon enters the southern ecliptic hemisphere.
A lunar eclipse can occur only when the full Moon is near either lunar node, while a solar eclipse can occur only when the new Moon is near either lunar node.
Because the orbital plane of the Moon precesses in space, the lunar nodes also precess around the ecliptic, completing one revolution in. The same cycle measured against an inertial frame of reference, such as International Celestial Reference System, is 18.599525 years.
Both solar eclipses of July 2000 occurred around the time when the Moon was at its ascending node. Ascending-node eclipses recur after one draconic year on average, which is about 0.94901 Gregorian year, as do descending-node eclipses.

Names and symbols

The nodes are called by different names in different areas of the world.
In medieval texts, the nodes are referred to as ras wa dhanav al-tinnîn in Arabic. rosh ha-tely u-zenavo in Hebrew, and caput draconis or cauda draconis in Latin. The ascending node is referred to as the dragon's head with the astronomical or astrological symbol of ☊ and the descending node is known as the dragon's tail with the symbol ☋.
In Hindu astronomy, the nodes are considered with the seven planets among the nine Navagrahas; the ascending node ☊ is called Rahu and the descending node ☋ is called Ketu. In Tibetan Astrology the descending node is named Kalagni.

Declination extremes

The Moon's orbit is inclined about 5.14° to the ecliptic; hence, the Moon can be up to about 5° north or south of the ecliptic. The ecliptic is inclined about 23.44° to the celestial equator, whose plane is perpendicular to the rotational axis of Earth. As a result, once during the 18.6-year nodal period, the Moon's declination reaches a maximum and minimum : about 28.6° from the celestial equator. Therefore, the moonrise or moonset azimuth has its northern- and southern-most points on the horizon; the Moon at culmination has its lowest and highest altitude ; and first sightings of the new moon potentially have their latest times. Furthermore, occultations by the Moon of the Pleiades star cluster, which is over 4° north of the ecliptic, occur during a comparatively brief period once every nodal period.

Effect on tides

The precession of the lunar nodes has a small effect on Earth's tides – atmospheric, oceanic, or crustal.
The U.S. National Oceanic and Atmospheric Administration determines mean lower low water at a location by averaging the height of the lowest tide recorded at that location each day during a 19-year recording period, known as the National Tidal Datum Epoch. The 19-year recording period is the nearest full-year count to the 18.6-year cycle of the lunar nodes.