I've read a lot about the Lunar Nodes online (on Wikipedia, astro.com and a bunch of other websites), but there is one thing I cannot figure out and the sources don't seem to clearly explain: why are the nodes moving on a daily basis? As the node is a point of intersection of Moon's orbit and the Ecliptic I understand that this point does not fall in the same place in space every time, but I would expect that once it's passed by the Moon it stays where the crossing occurred and its position gets updated on the next crossing of the Ecliptic, so in ephemeris I would expect to see the same values of the coords between the crossings, but instead I can see they're changing on the daily basis (ie. this ephemeris has 2 columns showing true and mean nodes' positions: http://www.astro.com/swisseph/ae/2000/ae_2016.pdf). I'm sure the ephemeris cannot be wrong, so I must be missing some details. I thought that finding the Moon's nodes would be as easy as finding the dates/times/longitudes when the Moon's latitude equals 0, but apparently this way I can only find the times and static points of the crossings of the Ecliptic, not the moving nodes as they're shown in ephemeris tables.
1 Answer
The nodes are the two points on the orbit of the moon at which the plane of the orbit crosses the elliptic. The position of the nodes change in a predictable way due to the perturbation by the sun (and the other planets). Since the orbit is constantly changing, the position of the longitude. The node is not the position where the crossing last occurred. Rather it is the intersection of two imaginary lines in the sky, the ecliptic and the orbit of the moon.
The position of the nodes are measured as an angle, clockwise from the first point of Aries, thus it is a longitude, or right ascension. On the first of September 2016 the longitude of the ascending node of the moon is 162.484 degrees, and decreasing slowly.
The linked pdf indicates a value of 12♍41, apparently this means 12 degrees 41 minutes within Virgo. The notation is not used in astronomy. If you are interested in the scientific study of the sky you should be aware that astro.com is an astrology website. The presentation of ephemeris may not represent scientific practice. I consider modern astrology to be pseudoscience and fraudulent.
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$\begingroup$ Thanks, James. I'm using pyephem for astro calculations. Do you think it would be a good method of tracing Moon's nodes: 1. get dates where Moon is at 0 lat; 2. find longitudes of the crossing points on those dates; 3. calculate average daily rate of motion using longitudes of two consecutive crossing points, ie. (lon2 - lon1)/days. ??? $\endgroup$– mac13kAug 27, 2016 at 6:06
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$\begingroup$ pyephem knows ascending node.
m=ephem.Moon();m.compute('2016/9/1');print(m._raan)$\endgroup$– James KAug 27, 2016 at 7:09 -
$\begingroup$ Yes, I knew about _raan, but unfortunately it doesn't work for me for some reason giving AttributeError: 'ephem.Moon' object has no attribute '_raan', but that's a separate issue I'm about to investigate (upgrading pyephem package didn't help...). $\endgroup$– mac13kAug 27, 2016 at 9:07
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$\begingroup$ In pyephem _raan is the attribute of EarthSatelites, but not the Moon, see github.com/brandon-rhodes/pyephem/issues/110 . $\endgroup$– mac13kAug 27, 2016 at 19:34
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$\begingroup$ @mac13k Did my comment on that github thread help? $\endgroup$– user21Aug 30, 2016 at 12:54