Expert: James Gort - 7/20/2005

Question
Since I was a child I've noticed that the moon rises & sets pretty much between the same positions the sun rises and sets at its solstice extremes.  While I do understand the basics of the sun's progression between solstices, I don't understand this tilting of the moon's orbit.

I live at latitude 41°38'S in Fingal, Tasmania. A couple of weeks ago I'm pretty sure the crescent moon set near its furthest northwesterly set point - where our winter solstice sun set a month ago - yet the full or near full moon set this morning near its furthest southwesterly point, where our summer solstice sun sets. Last evening the moon rose where our summer solstice sun rises.

Why does this progression of moon-rise and moon-set points occur relatively quickly at times, relatively slowly at other times?  Or does it?  I realize it's predictable from the existence of tables for moon and sun eclipses and moon-rise and moon-set times.  Also, would this change in tilt have an impact on tides?  Thank you, and I'm looking forward to your response, Eden

ps, I grew up with Northern Hemisphere night skies (in California then Alaska) and miss them from time to time.  Still, it's great fun having the Southern Cross circle its pole fully viewable all year, also seeing the constellations backwards and upside down, so to speak.  Winters here are a joy with the Milky Way visible all night (generally too low on the horizon & not so easily viewable in summer night skies) and we have brilliant night skies here in rural Tasmania.  Thanks again for your attention to my query.

Answer
Hello Eden,

Good observation!  As you know, the ecliptic is the path the sun appears to follow.  Since it's inclined at 23.5 degrees to the earth's equator, the sun sets at a more northerly or southerly location depending on the season.  But I'm sure you know all that!  What about the moon's orbit?

Well, the moon is inclined at about 5 degrees to the ecliptic (crossing the ecliptic in two nodes - the ascending node - crosses to N. side of ecliptic, and descending node - crosses to S. side of ecliptic).  The time it takes to go from ascending node to the next ascending node is called the Dragonic Month - about 27.21 days.  The time from new moon to new moon is called the Synodic Month - about 29.53 days (longer, since the earth moves in its orbit in a month's time).

Now to your question.  If you observe the moon at its ascending node (say, it's a new moon), then it will rise/set close to the location of the sun (since it's on the ecliptic).  In fact, that's when eclipses of the sun might occur!  But now you wait two weeks and observe the full moon.  Well, it's now at the descending node (also on the ecliptic).  So the full moon will also rise/set close to the sun's position.

But say you observe the new (or crescent) moon when it's at it most northerly position (5 degrees) above the ecliptic.  Then you wait 2 weeks and observe the full moon - it will be at its most southerly position (5 degrees) below the ecliptic.  So the rise/set positions of these two moons will be quite different from the sun's rise/set positions (which are always on the ecliptic).

Draw a little picture, with the ecliptic circling the earth, and the moon's orbit inclined to it at 5 degrees.  It will become crystal clear.

Hope that helps!  Keep enjoying those dark Tasmanian nights - I envy you!

Prof. James Gort