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This nice page from NASA has the possibility to create an astronomical events calendar for a certain period. I tried to do that for the year 2017 for Central Europe. What comes out puzzles me:

On July 8th the Moon is at it's south Declination. On July 12th the Moon goes through the descending node. On July 22nd it finds itself at the north Declination.

  1. I had always though the south Declination means the southernmost point of the Moon on its orbit. Furthermore I had always thought the descending node is the point the Moon passes by going "down" in its motion. How can it reach a southernmost point and later go even further downwards?
  2. I had been inclined to think that the north and south Declinations would be about half way between nodes (give or take due to slight changes during the motion). How can the Moon be in one of these south or north points and only 4 days later pass a node? And take again 9 more days to reach a south/north Declination again?

It would be nice if the answer could, besides clarifying these concepts (ascending and descending node, south and north Declination), point me to a reading where they as well as other Moon orbital points and parameters are clearly defined and explained.

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    $\begingroup$ North and south declinations means the moon is furthest north/south of the celestial equator. The ascending/descending nodes refer to ecliptic, not the equator. Example: if the moon's declination at 18h RA is -20 degrees, that's still 3.5 degrees above the ecliptic, which is -23.5 degrees at 18h RA. $\endgroup$ – barrycarter Jul 7 '17 at 13:19
  • $\begingroup$ Thank you for the answer, but I need to see a drawing in order to understand, and I haven't found one clear enough. I mean, I understand what you're saying, but I would have thought if the declination of, say, -20 degrees is it's minimum declination for the period being, then it starts going "up" with respect to the celestial equator, right? Therefore I would have expected it to start going "up" with respect to the ecliptic too, meaning that the +3.5 degrees above the ecliptic increase (become 4 and so on). Is this wrong? $\endgroup$ – Andyc Jul 7 '17 at 21:36
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enter image description here

The image above (click for full size) shows the moon's path for much of July. The celestial equator is in purple and the ecliptic is in green. Note that:

  • On July 8th (meaning between July 8th at 0000 UTC and July 9th at 0000 UTC), the moon does indeed reach it's southernmost declination, but is still north of the ecliptic.

  • On July 12th, the moon, whose declination is increasing slower than that of the ecliptic, crosses the ecliptic from north to south, it's descending node.

  • On July 22nd, the moon is at its northernmost declination, but still south of the ecliptic.

  • On July 25th, the moon, moving south slower than the ecliptic, crosses the ecliptic from south to north, it's ascending node.

As the page you reference notes, the moon's northern and southernmost declination this month are +-19.4 degrees, about 4 degrees less than the tilt of the ecliptic. Thus, the moon changes declination slower than the ecliptic, at least for this month.

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  • $\begingroup$ Wow, that picture makes the thing a lot clearer indeed, thank you. Where did you find it/how did you make it? I guess the point where the two lines cross around the center of the picture is the so called First Point of Aries, right? $\endgroup$ – Andyc Jul 9 '17 at 5:18
  • $\begingroup$ Yes, that's the first point of Aries (which is now in Pisces and precessing towards Aquarius, heralding the "age of Aquarius"). I spent a lot of time looking for a good image but couldn't find one, so I ended up making it: github.com/barrycarter/bcapps/blob/master/ASTRO/… -- I'm convinced someone's done a better job, but I wasn't able to find it. $\endgroup$ – barrycarter Jul 9 '17 at 14:38

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