Why is Lunar Orbital path so far off the Ecliptic?

slackjr · Feb 27, 2005

I took an Astronomy course about 8 years ago, after years of personal study and sky watching, and during one of my late-night observations in mid-winter, I was surprised to note that the moon was nearly straight overhead even in Massachusetts. Since I was under the impression that the moon/planets/sun all rode in generally the same area across the sky (Ecliptic), I couldn't figure out the disparity of paths. I asked my professor (a fellow at the Cambridge Center for Astrophysics, slumming a the community college for penance or some such), and he brightened straight up, stopped the class, and said, "Did everyone hear that! This is really a beautiful question." Then he launched into Harvardspeak that lost me completely, and it isn't as though I know nothing about the sky and movements of all those bodies. I'm no expert, but not a novice either. Is there a simple answer to why the moon's path can diverge from the Ecliptic by 20-plus degrees at times?

Saiph · Feb 27, 2005

well, it's only titled 5 degrees to earths orbital plane (which defines the ecliptic). http://en.wikipedia.org/wiki/Moon I too have been suprised at how far it seems to get from the ecliptic. One thing to remember, is that it doesn't follow the celestial equator, but the ecliptic (my typical mistake). So from here the moon can get to: 90-lat = Cel. Eqator ~50 +23.5+5 = 78.5 degrees up. here's an odd thing I found though: http://www.math.nus.edu.sg/aslaksen/teaching/convex.html <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

vogon13 · Feb 27, 2005

Isaac Asimov stated that very characteristic of lunar orbit in his tug-of-war essay! Sure miss his science essays. Tremendous ability to 'splain things. <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

slackjr · Feb 27, 2005

Thanks for those responses. I'm not any further ahead in my understanding, but that's okay, a fella can't know everything.

newtonian · Feb 27, 2005

slackjr - Frankly, I also do not understand why the moon seems to be directly overhead sometimes, while the sun never is. Of course, one basic difference is that it is earth's rotation and axis tilt which changes the sun angle. However, this seasonal effect would not effect the moon's angle, would it? Put another way, the sun angle is caused by earth's revolving around the sun, while the moon's angle is caused by the moon's revolving around the earth. Still, it does seem that the moon's angle changes with the seasons. I don't know why, so I will also be following this thread to learn. Perhaps it is because of this complicating fact: the moon is revolving around a spinning earth - earth's spin is independent of the moons orbit, though there is tidal interaction - it is not a lock. Nevers would probably know, btw. And then there is star angles; and planet angles; etc.

newtonian · Feb 27, 2005

Saiph - Lincoln, Nebraska? Here in south Louisiana it does appear sometimes to be almost directly overhead. Could that just be a perception, that is actually closer to 80 degrees than 90 degrees? My latitude is in the low 30's north, if I remember correctly.

tony873004 · Feb 28, 2005

It's more of an illusion than anything. The Moon has a 5 degree tilt to its axis compared to the ecliptic. So it can get 5 degrees higher than the Sun, and 5 degrees lower too. Earth has a tilt of 24 degrees. So the Moon can be overhead for any latitude between 29N and 29S. So in Mass., at 42 degrees, the Moon can come within 13 degrees of overhead, which looks very much like directly overhead. The next time you see it "overhead", measure how long of a shadow it is casting, and measure the height of object casting the shadow. Then use the formula inverse tan (height / shadow length) to compute your angle. You'll find that it is at least 13 degrees away from being directly overhead.

Saiph · Feb 28, 2005

the moon is on same plane as the solar system. It is not along the plane of the earths equator. If you're at 30 degrees north, thats only 10 degrees south of me so your altitudes are all 10 degrees higher. If it exceed's 90 (straight up) you're actually heading down the other side. So the moon, to you, can get to be ~88 degrees up (just shy of straight up). <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

nexium · Feb 28, 2005

Everyone gave a good answer. 5 degrees is about average for our solar system, for moons and planet tilts with respect to each other and the planes of their orbits. Neil

Leovinus · Feb 28, 2005

No matter what, there is always some place on Earth where the Moon is directly overhead. <div class="Discussion_UserSignature"> </div>

nexium · Feb 28, 2005

Hi shuttle_guy: I think you should reconsider. Every star, planet and asteroid is directly over head somewhere on Earth's surface, So why not the Moon? Neil

spacester · Feb 28, 2005

There is one place on Earth where the north star is directly overhead all the time. What Leo is saying that there is always an imaginary straight line between the two orb's centers, and as the moon orbits the Earth, the intersection of this line and the Earth's surface always exists. <div class="Discussion_UserSignature"> </div>

Leovinus · Mar 1, 2005

Now that would be a neat trick. <div class="Discussion_UserSignature"> </div>

Saiph · Mar 1, 2005

you know...if I blow it up it should be overhead everywhere...for a bit that is. After that it'd settle into a ring system. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

newtonian · Mar 1, 2005

shuttle_guy - Is north up? I know I've been up too long!

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Why is Lunar Orbital path so far off the Ecliptic?

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