Why Is The Moon Not Spinning Then?

Log in or Sign up to hide all adverts.

Excellent and irrefutable OBSERVATIONAL evidence that the moon is spinning about it polar axis, but Ken is very fixed on his POV, so I will elaborate a little on your statement.

First however, here is a link that show this libration:

http://en.wikipedia.org/wiki/Libration

The first cycle or two is slow, then the it speed up more movie like - wait for that also.

The moon is nearly a constant distance from the earth so the libration is not large, but note when the moon appears larger in the wiki illustration (because it is closer to Earth) its spin rate is not keeping up with its more rapid orbital rate. And conversely, when smallest (most distant from earth) its spin rate is greater than the orbital rate.

Just so Ken will understand your and my term "orbital rate" that is the rate of angular advance around the Earth for a person at the North Pole. (I place the observer there as that removes the parallax associated with Earth's spin carrying an equatorial observer to different position.) The AVERAGE orbital rate is ~360degrees/ (~28) days but when the moon is near apogee the current orbital rate is slower and at perigee it is faster.

The moon is spinning around its polar axis at a constant rate and in the same direction that the Earth is spinning - for an observer far out in space over the North Pole this would be a counter-clockwise spin. (Why the sun appears to rise in the East) So at apogee with moon's polar spin rate higher than the now less than average orbit rate, the leading edge of the moon in its travels around the Earth spins a little around to be more towards the Earth, not exactly the leading edge.

Likewise at perigee the moon's orbital rate is higher than the average orbital rate of advance about the Earth and the moon is not spinning fast enough to keep exactly the same face to the Earth. So what was not quite the leading edge of the moon can become the leading edge.

This is of course the same thing that I explained long ago with a more extreme example. I.e. I assumed that a third body passed by the Earth / moon system and threw the moon in to a much more eccentric orbit with a 50 month orbit period but still a 28 day spin period (no net torques applied by force acting on center of gravity of moon with a symetric trajectory right over the back-side of the moon's bulge) and noted that then at the distant apogee the moon would spin completely around (as seen from Earth) perhaps a dozen times as its spin rate would still be one 360 turn in 28 days but it would be far from the Earth for about a dozen months with little orbital advance while making these dozen full turns.

See my earlier post 124 giving more details at.

http://www.sciforums.com/showpost.php?p=2012966&postcount=124

Here is some of the text from above wiki link:

"...Libration in longitude is a consequence of the Moon's orbit around Earth being somewhat eccentric, so that the Moon's rotation sometimes leads and sometimes lags its orbital position.
Libration in latitude is a consequence of the Moon's axis of rotation being slightly inclined to the normal to the plane of its orbit around Earth. Its origin is analogous to the way in which the seasons arise from Earth's revolution about the Sun. ..."

Ken can not explain any of this* with his erroneous POV.
--------------
*Not my imagined many full turns around with little orbital advance (an extreme libration case) nor the actual case of very modest libration that the moon currently has.

 

Log in or Sign up to hide all adverts.

”

Trippy, what is at issue here is NOT whether or not the moon is spinning around an axis (it does), ONLY the LOCATION of that axis!!!!

Some people here claim that the moon both rotates around its polar axis one time per each 27.3 day orbit (i.e., 2 spin axes), which I admit can be construed as consistent with the conventional definition of *Synchronous rotation*:

---Wikipedia---

~Synchronous rotation

In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting.
--/--

I say that our moon ONLY revolves around the Earth-moon common mass barycenter, a point located within the Earth, and that Earth-moon barycenter is the moon's ONLY remaining (relevant) axis of spin!

The moon and Earth are both revolving around the sun too, but the Earth-sun-moon barycenter is not relevant to this discussion of, whether or not, the moon is also rotating 360˚ around its internal polar axis as it orbits 360˚ around the Earth.

The classical definition that it takes the moon just as long to orbit the Earth (and barycenter) as it takes the moon to rotate around that same axis, is double-speak ... UNLESS, the 2-axes lunatic spin crowd are claiming that the moon spins 360˚ on two separate axes at the same time? I.e., does the moon spin 360˚ around the Earth at the same time the moon rotates 360˚ around its own polar axis?

OR, does the moon ONLY orbit (rotate, revolve, or spin) 360˚ around one (1) axis, which is located within the Earth?

I say the latter!

WHEREAS, the Earth still does rotate around its polar axis as it orbits the sun – so, the Earth still has two spin axes, (1) its own polar axis; and, (2) the sun.


In my POST 94 I gave a simple experiment you can easily do in your own home, which not only proves my one (1) spin axis claim, this demonstration will also show the moon's phases (shadows) that you seemed concerned about.

Take a few minutes of your time and try this:

---snip from my Post 94---
A simple demonstration - draw a happy-face on an orange, then go into a darkened room and take the shade off of one table lamp (that will be the sun), then hold the orange on your outstretched palm with the happy-face facing you, then spin 360 around counter-clockwise on your heels.

If you do that, then you'll see all phases of the moon likewise pass across that orange.

The orange will NEVER spin in your hand, yet, its happy-face will always remain pointed towards your spinning body!

You can even see more than 50% of the orange's surface (libration) by raising the orange/moon model above and below your eye level. Viewing the orange/moon first with one eye and then the other eye will also cause some libration (caused by parallax.)

When you show a person that demonstration to prove that the moon doesn't spin on its polar axis (only around your body's axis), they will either grok it, or they'll dig their heels in and claim your arm isn't the same thing as gravity.

Of course an arm isn't gravity, but in an accurate model your arm can serve the same purpose.

Men have been making sun-Earth-moon models (called Orreries) for hundreds of years now, and no orrery ever constructed has any gearing to rotate the moon-model on its internal axis! BECAUSE, rotating the moon-model on its spindle just isn't necessary to do!!!

E.g., here's a good example of one (add the http & www to it):

.abhirjoshi.com/models/sme_model_abhir.jpg
--/--

Ken

 

Log in or Sign up to hide all adverts.

Ken. I need a good laugh. Please explain libration*, either the observed one the moon has or the one I imagined with the moon thrown into a much more exccentric orbit with 50 month orbit period.

using only your uniform spins about the barycenter axis.
----------------
*see post 162 or 163

 

but that keeping the SAME hemisphere turned is only possible IFF the spin about the polar axis is exactly perpendicular to the orbit plane.* This is not true of the moon. Moon's is spinning about an axis about 7 degrees tilted from the orbit plane, as I recall. So even if it were in a perfectly circular orbit (which it is not) more than one hemisphere would be visible from Earth.

If the moon's polar spin axis were 90 degrees tilted then all of themoon's surface would be visible from Earth - that was what Janu58 was trying to show you in his drawings. Surely even you would admit the moon spining at the 28 day per orbit rate was spinning about a polar axis if that axis were 90 degrees tilted. (Spin axis in the plane of the orbit.) What about if 80 degrees tilted - is it only spinning about the barycenter yet? 70 tilted, etc ...10degrees tilted? 9, 8 or down to the current 7?

As Janus tried to get thru your thick skull: At what tilt (greater than 7 degrees) did the moon stop spinning about its polar axis and begin to spin only about the barycenter?

It is a simple question with a simple numerical answer, but you will still ignore it, I bet.
------------------
*Other things also required. E.g tidal locked and exactly circular orbit to avoid longitudinal librations.

 

Ken, you are being an idiot.

How do we determine if something is rotating about its axis? Try this:

As described, stand with an orange in your outstretched hand, with your head representing the earth. Now, gently drive a spike through your head into the ground and affix an arrow to the top of the spike (like a weather vane). The arrow points at a fixed mark on a distant wall. As you rotate, you keep the arrow fixed on this relatively non-co-rotating reference. Its free to translate in any direction, but not rotate.

Now, drive a similar spike through the orange and your hand. It too is free to translate but not rotate. Keep the arrow on this spike also pointed at the distant reference mark on the wall. Now, as you spin (being careful not to slip on the blood) you will indeed see that the orange rotates about it's own axis.

 

You say, "therefore it must have rotated about its own axis."

Geeze ... how many times do I need to ask you guys to SPECIFY the EXACT location of your spin axes that you're talking about!!

You do understand the difference between *REVOLUTION* and *ROTATION*, right?

CLUE: "The Earth *REVOLVES* around the sun as it *ROTATES* around its polar axis."

Words do have meanings ... unless, you're an astronomer heavy in math and light on English!

You confusedly state that, "...the orange spun once on its axis for each turn of me. If it hadn't, it would have appeared to rotate as I did..."

So then, was your orange's *spin axis* the common-mass of the Vkothii-orange barycenter (i.e., you and the orange's common mass?)

Do you understand what a barycenter is? If not:

BARYCENTER: "Of or relating to the center of gravity."

THUS, when considering the combined mass of two (or more) objects, their combined mass will lie on a point between their respective centers.

OK then, Vkothii (YOU) and the orange had a common center of gravity, which was likely very close to your own body's center (perhaps, a hair's thickness off?)

In any event, I claim that the orange REVOLVED around the Vkothii-orange common-mass barycenter, AND FURTHER, that your orange NEVER *ROTATED* around its own center of mass in the process of that demonstration. YES, the orange revolved around your combined center of masses (which was in your body), but the orange did NOT spin or rotate around any axis internal to the orange itself.

---Wikipedia---
"A rotation is a movement of an object in a circular motion.

"A two-dimensional object rotates around a center (or point) of rotation. A three-dimensional object rotates around a line called an axis.

"If the axis of rotation is within the body, the body is said to rotate upon itself, or spin—which implies relative speed and perhaps free-movement with angular momentum.

"A circular motion about an external point, e.g. the Earth about the Sun, is called an orbit or more properly an orbital revolution."
--/--

Armed with those definitions, since you were brave enough to actually get off your tush and spin around with a dumb orange in your hand (I hope no one saw you), can you please specify EXACTLY where you believe the orange's spin axis was located at?

AGAIN, was the spin axis within you, or within the orange?

Location, Location, Location!!!!!

Ken

 

Ken,
when are you going to answer the libration question?

 

It's interesting to be called an "idiot" by someone that doesn't know the correct forms of "IT!" Don't they teach that in jr. High?

*it's* is the contraction of the two words, *it is*.

For example, when you wrote, "you will indeed see that the orange rotates about It's own axis."

You actually wrote:

"...you will indeed see that the orange rotates about it is own axis.

I'm not a grammar Nazi and I don't normally care about typos and grammar as long as ideas are clearly expressed (I make errors too), but calling someone an "idiot" while expressing an incomprehensible and a bloody example, means you're fair game.

Not to mention, this grammar post gets me closer to 20 and I'll soon lose my newbie wings and be able to post URLs and images! So, thanks for that!

Please Register or Log in to view the hidden image!

At least, now you can say an *idiot* once taught you something!

Seriously, if you can politely express a clear non-bloody statement or question (even with typos or other errors), then I'll honestly try and respond to your posting.

Ken
(aka the idiot)

 

It also spins around its own axis (a polar axis) as it rotates (orbits) the sun. Both are called angular momentum.

No, it was the orange's axis, the one parallel to my vertical axis of rotation.

 

I FORGOT - since *it's* is the contraction of the two words *it is*, *ITS* (without the apostrophe) is the possessive form of *IT*

Likewise, there are other possessive words that don't use apostrophes, such as *HIS* *HERS* and *YOURS* since the words themselves are possessive.

*her's* or *your's* may not look too funny at first blush, but *Hi's* is definitely funny looking!

Another post closer to 20!

Ken
(aka the idiot)

 

If I had a perfect bearing tied to a rope with something inside it, and started to spin the rope around, the object would not spin just because it is "orbiting" me.

 

Ken Dine --- You do understand the difference between *REVOLUTION* and *ROTATION*, right?

====Evidently not.

 

Ken,
when are you going to answer the libration question?

If you do, then you'll (short for you will) be only one post away from twenty.

 

Here's one more puzzle for you, Ken, my man.

A solid cylindrical bar of metal 0.5m in length, weighing 400gm, and with a diameter of 2cm is tack-welded to a small sleeve bearing, and the assembly is press-fitted at the bearing end to a stub-axle, which is attached to a plate bolted to an upright frame, so the bar swings freely from the bearing through a vertical arc:

i) estimate the period of the swinging bar, for small displacements.

ii) write a formula for the relationship between the angular momentum of a cross-section of the bar near the free-swinging end, and a cross-section near the bearing end.

iii) if the plate with the attached cylindrical bar on its bearing, is bolted to a horizontal surface, what happens to the relationship in the previous question?

 

Well ken, if you're capable of just a bit more than correcting silly late-night grammar mistakes, can you address the essence of my post? I.e. that the moon or any orbiting body rotates about its(!) own axis whether you like it or not?

Remember, idiocy can be a temporary condition. You've rejected several mathematical and intuitive explanations that show clearly how the moon is rotating about it's(!) axis, yet you still blindly insist it is not. Why? Do you enjoy coming across as an idiot?

 

Spin with respect to what? Are you going to argue that the earth is not rotating about it's own axis? Clearly it's not, right? It's the cosmos that's rotating about us, yes? Unfortunately for you though, theres a distinct centripetal acceleration at the equator that demonstrates axial rotation (same for the moon).

Your object-in-a bearing is not spinning with respect to the bearing because they share the exact same rotation! Reference the axis of the bearing and that of your body to a common fixed point and see that they are both rotating about their axes!

It's called choosing a common reference. You can't just decide to choose one reference for one part of a system and a diffrent one for another part and then argue based on this. Well, you can, if you want to look like an idiot.

 

The pot calls the toilet black.

 

Huh? :bugeye: