Moon ponderer

[cygnusx1]

I've often thought about the following questions

1. Does the :moon: follow the same orbit around the ecliptic continuously?

2. Why is it that some nights, esp towards full Moon it appears in the sky for much more than 12 hrs :scratch:

Surely it should only be visible for +- 12 hrs as the time it should take to traverse 180 degress albeit with retrograde motion?

3. Finally, why is that as it approaches full Moon it seems to reach almost the Zenith and infuriatingly wash out the entire hemisphere, and when only gibbous has a orbit nearer the horizon?

[barkis]

The moons orbit is a very complex motion Kevin. It's inclined to the plane of the ecliptic by 5deg. 8mins. It has an eccentricity of .054 which gives rise to the perturbations enabling us to see 59% of the surface. It's angular rotation is constant over a period of one sidereal month. I am not certain this information answers your question, although I do tend to think it is primarily due to It's orbital inclination as as the earths orbit follows the ecliptic, the moons orbit around the earth does not. This accounts for the different positions of the moon in the sky seasonally, as the earth orbits the sun.

I am sure a better explanation will be forthcoming, maybe Astroman will clarify my poor attempt.

Ron.

[James]

1. No, as Ron has pointed out its inclined to the ecliptic slightly. Right now the Moon's path around the sky takes it as far north as it can go as part of an 18 year cycle.

2. Same as the Sun; if the Moon is at a high point on the ecliptic in say Gemini its going to stay above the horizon for a long time, same way the Sun does in June.

3. The Moon reaches full moon in the constellation opposite the Sun so at this time of year when the Sun is low in constellations like Scorpius/Sagittarius/Capricorn the Moon will be full in the opposite constellations Gemini/Cancer/Leo and therefore much higher in the sky. Similarly in summer when the Sun is in Gemini we get full moons in Sagittarius therefore very low down.

[Astroman]

There's quite a lot to digest in the black art of Lunar orbital mechanics. In addition to the aforementioned inclination of its orbit with respect to Earth's rotational axis, there is the ellipsoidal orbit to contend with, as well. The orientation of this ellipse changes, or precesses over a period of 18 years, as James pointed out. As it changes, it influences its own orbit as well as Earth's, creating the perturbations mentioned by Ron.

So, answer no. 1 is No. The Moon doesn't orbit the ecliptic at all, in any meaningful way.

Answer no. 2 includes the precession, inclination, perturbed orbit AND the fact that the Moon orbits in the same direction as Earth spins, AND the fact that Earth and Moon orbit the Sun, so you need to add an additional distance traveled, (sidereal time/distance), to the equation. You may not notice it when the Moon is young, but you are sure to, (if you're paying attention, which you obviously are), when the Moon is full.

Answer 3 is more one of perception than orbital mechanics. The last two years, we've seen Luna hit maximum and minimum distances from both the ecliptic and the equator. We had successive full Moons where she appeared very far north and very high, and very far south and quite low. (Not to mention nearest and farthest away.)

Luna's orbit is quite complex. Due to the relatively slight difference in mass between the orbiting bodies, it makes long term prediction very difficult. Unless one is inclined to wander off into the minefield of chaos theory and fractals, it's best, imho, to just enjoy the Moon when she's up and hope she returns tomorrow. For if she doesn't, we'll all be in a world of hurt! :shock: 8)

[cygnusx1]

Thanks Macc + Astro for your replies.

I realised it was complex, but obviously the true orbit is really a 3 body dynamic problem ( Sun, Monn + Earth ) with so many factors + variables involved. Mathematically, i;m sure it has been modelled to a high degree of accuracy which is backed up by observation and tests.

Nevertheless i am a lot clearer thanks to your explanations.

[The Warthog]

Put in its simplest terms, the Full Moon is high in winter, and low in summer, and always rises about sunset, and sets about sunrise. As it does a complete orbit every lunation, different phases of the Moon take their position on the orbit each month. In April, the waning crescent through half Moon are fairly high, making April a good month for lunar observing.