Lunar Orbit Precession Tutorial?
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Topic authorrbroberts
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Lunar Orbit Precession Tutorial?
I'm looking for an explanation of the cause of the lunar orbit precession. Something like a force diagram showing the origin of the torques that are the origin of the draconic month. Anyone know of such a place? Even a textbook I could get my hands on?
Roland B. Roberts, PhD
6818 Madeline Court
Brooklyn, NY 11220
6818 Madeline Court
Brooklyn, NY 11220
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revent
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Re: Lunar Orbit Precession Tutorial?
Draconic month? (ruffles around Wikipedia) Nodical month sounds more familiar. 
The Nodical month is just another way of measuring the lunar orbit. It's the average length of time between crossings of the ascending node, i.e. between times the moon crosses the ecliptic while moving northward. I think what you're asking is more why the length of the nodical month is different from the length of the sidereal month, since the answer is ....lunar orbit precession.
Lunar orbit precession is intimately related to the nutation of the Earth's pole (they both have a period of 18.6 years), and you're probably more likely to find an article about nutation that gives a good explanation that one specifically about lunar orbit precession.
Essentially, the effect is due to the fact that the plane of the lunar orbit is relative to inertial space, as opposed to solar space. Since the moon is accelerated by solar gravity when approaching the sun, and decelerated when receding, there is an asymmetric effect on the lunar orbit over the duration of a /solar/ month. If you drew this out on a sheet of paper as of a specific moment in time, it would look like the moon would be seeing each effect half of the time, and in the sense of a /solar/ month it does. However, the moon's 'physical' orbit (i.e., the location of the line of nodes and such) does not have the same length as a solar month, and so the time during which the moon sees each effect is different during each sidereal month, and the effect occurs as a different place w.r.t. the line of nodes during each orbit. It is the change in the location of the 'impulses' from the sun along the length of the lunar orbit each month that produces the torque on the moon's orbit.
Don't know how clear that was, but hopefully it helps. It'd be hard to draw a diagram that really explains this, since it's more 'dynamic' than the tides, but maybe someone out there has managed it.
The Nodical month is just another way of measuring the lunar orbit. It's the average length of time between crossings of the ascending node, i.e. between times the moon crosses the ecliptic while moving northward. I think what you're asking is more why the length of the nodical month is different from the length of the sidereal month, since the answer is ....lunar orbit precession.
Lunar orbit precession is intimately related to the nutation of the Earth's pole (they both have a period of 18.6 years), and you're probably more likely to find an article about nutation that gives a good explanation that one specifically about lunar orbit precession.
Essentially, the effect is due to the fact that the plane of the lunar orbit is relative to inertial space, as opposed to solar space. Since the moon is accelerated by solar gravity when approaching the sun, and decelerated when receding, there is an asymmetric effect on the lunar orbit over the duration of a /solar/ month. If you drew this out on a sheet of paper as of a specific moment in time, it would look like the moon would be seeing each effect half of the time, and in the sense of a /solar/ month it does. However, the moon's 'physical' orbit (i.e., the location of the line of nodes and such) does not have the same length as a solar month, and so the time during which the moon sees each effect is different during each sidereal month, and the effect occurs as a different place w.r.t. the line of nodes during each orbit. It is the change in the location of the 'impulses' from the sun along the length of the lunar orbit each month that produces the torque on the moon's orbit.
Don't know how clear that was, but hopefully it helps. It'd be hard to draw a diagram that really explains this, since it's more 'dynamic' than the tides, but maybe someone out there has managed it.