Some Moon Questions

[zigaretten]

OK…I know that the moon has equal periods of rotation and revolution, so that we are always looking at the same face. I also know that this is not just a coincidence, but is the result of some gradual process which minimizes the energy of the system. Now some questions…

1…Are the periods of rotation and revolution exactly equal or is there some tiny difference which will cause people 50,000 years from now to see a “different” moon than what we see today?

2…Before the the moon reached its present state of equilirium, do we know whether its rotation was faster than its revolution or vice versa, and if so which was the case?

3…How far back would we have to go to reach a time when the periods of rotation and revolution were not equal? (If known)

Thanks, Ron

[Triskadecamus]

Still a bit of controversy over the origin of the Moon, so the answers are not authoritative. One theory, the so called “Mars sized impactor” theory is moderately well received, so I will base the answers on that one.

A thing the size of mars hit the earth, during the first half billion years or so of the Earth’s existence. It was a glancing blow, probably on roughly similar orbital trajectories. Off it goes, into the billiard game of planetary development, leaving behind the Earth, and a great big gout of vaporized crust, and mantle from the Earth, and from the impactor. Some liquid, and a small percentage of solid material is ejected as well. Over millions of years the big gout of stuff settles down, and begins to coalesce into a discrete body. It is in an orbit that isn’t in the equatorial plane of the planet, and it is a bit far from its primary as such things go.

Tidal lock is a gradual process, especially at the distances of the Moon, given the relative size of the two. But, the Moon was still molten for a fairly long time, and it probably was at least very close to tidal lock before it entirely solidified. During the same couple of hundred million years the rest of the stuff that was ejected either fell onto the moon, or fell onto the Earth. Some small amount was tidally launched away from Earth by the Moon, as well, and some tidally deorbited onto the Earth.

So, if this theory is at all truthful, the Moon probably solidified as a tidally locked body. Always the same face toward the Earth. That dynamic involved having the moon gradually slow, or increase it’s rotation, as it gradually increased it’s orbital distance, and period. The most stable condition is where they match.

Tris

“Error of opinion may be tolerated where reason is left free to combat it.” ~ Thomas Jefferson ~

[Hari_Seldon]

To answer at least one of the questions: tidal lock is a stable equilibrium and will not change. Owing, I imagine, to the eccentricity of the moon’s orbit around the earth, the lock is not perfect and the moon appears to oscillate a bit. I think we see something like 55% of the moon. But that doesn’t change the fact that the lock is perfect over time, in the sense that we will never see the opposite side, except from a spaceship.

[zigaretten]

Drat…I was under the impression that astronomers had the moon pretty much figured out by now. Thanks for your replies and thanks for the term “tidal lock.” I knew there was a phrase for it, but I couldn’t remember it to save my life.

[Chronos]

Owing, I imagine, to the eccentricity of the moon’s orbit around the earth, the lock is not perfect and the moon appears to oscillate a bit. I think we see something like 55% of the moon.

Correct. Like all objects on an elliptical orbit, the Moon is moving faster when it’s closer to the Earth, and slower when it’s further out. It’s not very elliptical, so the effect is small, but it’s present and measureable. On the other hand, there is no mechanism to change the speed at which the Moon rotates over the course of a month, so the speeds can’t always match perfectly. The result is that the visible portion of the Moon varies slightly over the course of a month, in a process called libration.

We do have the Moon well figured out, in terms of what it’s doing currently, but there’s still room for speculation concerning its origin. To really pin that down, we would need to see a variety of similar objects in various stages of formation, and our moon is currently the only object of its type known (other planets have moons, but they all seem to have formed differently than ours).

[sailor]

And the earth is returning the favor and will lock step with the moon over time. Then only one side of the Earth will see the moon constantly while the other side never sees it.

[zigaretten]

OK…now I’m really getting interested. I hope you folks won’t mind if I tack on a couple more questions.

4…given the moons elliptical orbit…how are its apogee and perigee oriented with respect to the Earth/Sun? e.g…the apogee is not constant and always lies on the sun-earth radius…or the apogee is constant and lies 36 degrees from the Earths apogee.

5…I understand about bodies moving faster as they approach perigee…does the moon have to go through a second acceleration/deceleration cycle to compensate for the motion of the Earth? In other words, sometimes it’s trying to overtake the earth while other times it’s moving opposite the path of the Earth, does this have an effect on the moons speed? I hope you can make sense of this one.

6…do the moons of the other planets all seem to have been formed in the same way? Is our moon totally unique or are all moons fairly unique?

Thanks again, Ron

[Hari_Seldon]

Although there is doubtless some precession, I assume that there are an apogee and a perigee once a month. Since there are (approximately) 12 7/19 lunar orbits a year the apogee and perigee can not lock into the aphelions and perihelions of the earth. The result is that tides differ a lot during the year. The tides are caused by both the moon and the sun, although the lunar contribution is about 6 times that of the sun. But the moon’s distance varies by about 10% and the tides will vary by about 30 or 35%. The earth’s distance from the sun varies by about 3% which will cause another variation of nearly 10%. So if the earth is perihelion, the moon at perigee and the moon is full (or new), there ought to be a tide 45% larger than at full moon, aphelion and apogee. Then when the moon is half, the sun and moon are pulling in perpendicular directions for yet smaller tides.

[Triskadecamus]

Let’s see how odd or how ordinary our moon is.

Earth
Radius (km): 6,378.14
Mass (x10[sup]22[/sup]kg): 597.42

Moon
Radius (km): 1,738 (27.25 percent of primary)
Mass (x10[sup]22[/sup]kg): 7.35 (1.23 percent of primary)
Distance (in planetary radii) from primary: 60.2
Eccentricity: 0.0549
Orbital inclination (degrees): 18.2 to 28.6

Mars
Radius at equator (km): 3,397
Mass (x10[sup]22[/sup]kg): 64.191

Phobos
Radius (km): 14 by 10 (not round) (0.41 percent of primary)
Mass (x10[sup]22[/sup]kg): 0.00000096 (0.000000015 percent of primary)
Distance (in planetary radii) from primary: 2.76
Eccentricity: 0.015
Orbital inclination (degrees): 1.1

Deimos
Radius (km): 8 by 6 (not round) ( 0.23 percent of primary)
Mass (x10[sup]22[/sup]kg): 0.0000002 (0.000000003 percent of primary)
Distance (in planetary radii) from primary): 6.9
Eccentricity: 0.0008
Orbital inclination (degrees): 0.9 to 2.7

Jupiter

Radius at equator (km): 71,492
Mass (x10[sup]22[/sup]kg): 189,920

Ganymede
Radius (km): 2,631
Mass (x10[sup]22[/sup]kg): 14.9 ( 0.000078454 percent of primary)
Distance from planet (km): 1,070,000
Distance (in planetary radii) from primary:15.1
Eccentricity: 0.001
Orbital inclination (degrees): 0.2

Io
Radius (km): 1,815
Mass (x10[sup]22[/sup]kg): 8.92 ( 0.00004697 percent of primary)
Distance (in planetary radii) from primary:5.95
Eccentricity: 0.004
Orbital inclination (degrees): 0.0

Europa
Parent planet: Jupiter
Radius (km): 1,569
Mass (x10[sup]22[/sup]kg): 4.87 ( 0.000025642 percent of primary)
Distance (in planetary radii) from primary:9.47
Eccentricity: 0.00
Orbital inclination (degrees): 0.5
Saturn
Radius at equator (km): 60,268
Radius (Earth = 1): 9.46
Mass (x10[sup]22[/sup]kg): 56,865

Titan
Radius (km): 2,575
Mass (x10[sup]22[/sup]kg): 13.46 ( 0.0002367 percent of primary)
Distance (in planetary radii) from primary: 20.3
Eccentricity: 0.029
Orbital inclination (degrees): 0.3

Rhea
Radius (km): 765
Mass (x10[sup]22[/sup]kg): 0.249 (0,000004378 percent of primary)
Distance (in planetary radii) from primary:8.73
Eccentricity: 0.001
Orbital inclination (degrees): 0.4

Iapetus
Radius (km): 730
Mass (x10[sup]22[/sup]kg): 0.188 ( 0.0000033 percent of primary)
Distance (in planetary radii) from primary: 59
Eccentricity: 0.028
Orbital inclination (degrees): 14.7

Dione
Radius (km): 560
Mass (x10[sup]22[/sup]kg): 0.1052 ( 0.00000185 percent of primary)
Distance (in planetary radii) from primary: 6.26
Orbital period (Earth days): 2.737
Eccentricity: 0.002
Orbital inclination (degrees): 0

Tethys
Radius (km): 530
Mass (x10[sup]22[/sup]kg): 0.0755 (0.00000133 percent of primary)
Distance (in planetary radii) from primary:4.88
Eccentricity: 0
Orbital inclination (degrees): 1.1

Enceladus
Radius (km): 250
Mass (x10[sup]22[/sup]kg): 0.0074 ( 0.00000013 percent of primary)
Distance (in planetary radii) from primary:3.95
Eccentricity: 0.004
Orbital inclination (degrees): 0

Mimas
Radius (km): 196
Mass (x10[sup]22[/sup]kg): 0.00455 ( 0.00000079 percent of primary)
Distance (in planetary radii) from primary:3.08
Eccentricity: 0.02
Orbital inclination (degrees): 1.5

Uranus
Radius at equator (km): 25,559
Mass (x10[sup]22[/sup]kg): 8,684.90

Titania
Radius (km): 790
Mass (x10[sup]22[/sup]kg): 0.348 ( 0.0000136 percent of primary)
Distance (in planetary radii) from primary:17.07
Eccentricity: 0.0024
Orbital inclination (degrees): 0.14

Oberon
Radius (km): 762
Mass (x10[sup]22[/sup]kg): 0.292 ( 0.00001143 percent of primary)
Distance (in planetary radii) from primary:22.82
Eccentricity: 0.0007
Orbital inclination (degrees): 0.1

Umbriel
Radius (km): 586
Mass (x10[sup]22[/sup]kg): 0.127 ( 0.00000497 percent of primary)
Distance (in planetary radii) from primary:10.41
Eccentricity: 0.0035
Orbital inclination (degrees): 0.36

Ariel
Radius (km): 579
Mass (x10[sup]22[/sup]kg): 0.135 ( 0.00000528 percent of primary)
Distance (in planetary radii) from primary:7.47
Eccentricity: 0.0028
Orbital inclination (degrees): 0.31

Miranda
Radius (km): 236
Mass (x10[sup]22[/sup]kg): 0.008 ( 0.000000313 percent of primary)
Distance (in planetary radii) from primary:5.08
Eccentricity: 0.017
Orbital inclination (degrees): 3.4

Neptune
Radius at equator (km): 24,760
Mass (x10[sup]22[/sup]kg): 10,235

Triton
Radius (km): 1,360
Mass (x10[sup]22[/sup]kg): 2.16 ( 0.000211 percent of primary)
Distance (in planetary radii) from primary:14.6
Eccentricity: 0
Orbital inclination (degrees): 160
So, a perusal of the data shows that only two moons are larger, and they orbit the two largest planets. As a percentage of the primary, nothing is even in the same order of magnitude. It is also much farther out, expressed in planetary radii, and it has a very large eccentricity, and is inclined to the planetary equator by a much greater factor.

It is in fact not just a bit odd. In our system, it is unique.

Tris

Man, that was a lot of work.

[A_wizard_song_for_thee]

Erm, okay, astronomy isn’t really my cuppa, but what about Pluto and Charon? I’m not sure where you got the data you used before, or I’d look it up. So, do you either have a cite or the data about Pluto? I only ask because I was under the impression that the masses of the two bodies are so similar, they’re practically a binary planet system. But I could be completely wrong.

[Irishman]

http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html

That site will give you info on the planets.

Pluto:
orbit: 5,913,520,000 km (39.5 AU) from the Sun (average)
diameter: 2274 km
mass: 1.27e22 kg

Charon ( “KAIR en” ) is Pluto’s only known satellite:
orbit: 19,640 km from Pluto
diameter: 1172 km
mass: 1.90e21

So Charon is about half the diameter and 15% the mass of Pluto.

Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held
by Earth’s Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.

And interestingly,

Charon’s composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to Saturn’s icy moons (i.e. Rhea). Its surface seems to be covered with water ice. Interestingly, this is quite different from Pluto.

Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved.

It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth’s Moon.

zigaretten asked:

5…I understand about bodies moving faster as they approach perigee…does the moon have to go through a second acceleration/deceleration cycle to compensate for the motion of the Earth? In other words, sometimes it’s trying to overtake the earth while other times it’s moving opposite the path of the Earth, does this have an effect on the moons speed? I hope you can make sense of this one.

I think I understand what you’re asking, but let me see if I can help clarify the situation. From the vantage point of the Sun, the moon never travels in a retrograde direction. You could map the orbits of the Earth and the Moon and overlay them as two sinusoidal curves that overlay the same orbital region and entertwine, each causing the other to wobble but otherwise orbiting the Sun directly. This is a tricky way to look at it. Essentially, from that view the Moon’s speed does vary, and so does Earth’s.

[zigaretten]

Irishman…

That’s exactly what I’m looking for! I’ve been trying to build a mental picture of this whole process but I’ve been stuck in an Earth reference frame. From the Suns perspective I can picture it perfectly. (Or at least as perfectly as I require.)

Thanks everyone, you’ve convinced me that the Moon is pretty unique. Any idea how unlikely this is? (As in very/not very etc.) Or do we need to wait until we can see a lot more solar systems to find out?

[vibrotronica]

The creatures who live in the inner core of the Moon are called Mooninites. Their ways are far superior to ours, and they possess advanced technology like the QuadLazer. Don’t try to understand them, they are far too advanced for your puny mind to comprehend.

They smoke while they shoot the bird.

(I watch far too many cartoons.)