Lunar node
The lunar nodes are the points where the orbit of the Moon crosses the ecliptic (which is the apparent path of the Sun across the heavens against the background stars). The North (or Ascending) Node is where the Moon crosses to the North of the ecliptic; the South (or Descending) Node where it crosses to the South.
Solar eclipses occur when the passage of the Moon through a node coincides with the New Moon; lunar eclipses occur when it coincides with the Full Moon. The lunar nodes precess rather quickly around the ecliptic, completing a revolution (called a draconitic or nodical period) in 6793.5 d or 18.5996 a.
Standstills
If one tracks the Moon in the sky on a continuous basis, one will notice its declination (celestial latitude) oscillate regularly between extreme values. Every half-synodical month (about 27 times per year), the Moon reaches a maximum of minimum declination value. This is called a standstill (the word has the same meaning as the "-stice" part of "solstice").
Because the Moon's orbit maintains a relatively constant inclination with respect to the ecliptic but precesses rather quickly about the ecliptic pole (completing a cycle in a little less than 19 years), the declination it can reach in the sky varies accordingly between limits of about 28.59° and 18.29°. This affects strongly the azimuth of moonrise and moonset. If an observer tracks the position on his horizon of successive moonrises, for example, it will appear to travel north, reach some maximum value, then travel south, reach a minimum value, and then head north again, over a period of nearly 19 years. When the Moon is nearing its maximum declination or moonrise/set azimuth, it will appear to rise or set in the same spot several nights in a row —this is the major standstill. When the Moon nears the minimum declination or moonrise/set azimuth, the same phenomenon occurs, and is dubbed the minor standstill. Prehistoric astronomers knew about this phenomenon and kept track of it.
See also
External links
- Sun and Moon Polar Applet, showing moonrise/moonset azimuths
- Astronomy Answers: What are the standstills of the Moon?
Categories: Celestial mechanics | Astronomy | Astrology