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Earth's Moon is a Rarity
Smivs writes "Scientists have concluded that moons like the Earth's are actually quite rare. Only 5-10% of planetary systems are likely to contain moons formed by planetary collisions. 'By the time the Earth's moon formed, when the Sun was 30 million years old, the planet formation process in our Solar System should have been approaching its end. In the latest study, Dr Gorlova's team looked at the heat signature of stars using the infrared. This allows astronomers to predict how much of that heat comes from the star itself and how much is re-emitted by dusty material encircling it.'"
Isn't this rather old news? I thought that it was already pretty well known that Luna is rather rare, as shown in the Rare Earth hypothesis?
Not according to official definitions for binary systems, because the barycenter of the Earth-Moon system is less than the radius of the major body, in this case Earth. On the other hand, Pluto-Charon system has a barycenter above the surface of Pluto, hence it is a binary planet (well, dwarf planet) system.
http://en.wikipedia.org/wiki/Double_planet
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Well, a moon in geosynchronous orbit might be unusual, but it's certainly not ours (otherwise you'd never see the moon raising or sinking). Our moon just shows bound rotation (well, for now; at some time in the far future earth will indeed be slowed down enough by the moon's tidal forces to also show bound rotation in respect to the moon, and then the moon's orbit will indeed be geostationary).
The Tao of math: The numbers you can count are not the real numbers.
"Yes, I know this probably sounds paranoid and preposterous, but consider this. Despite what the revisionist historians tell you, there is no mention of the "moon" anywhere in literature or historical documents -- anywhere -- before 1950."
Yeah, I know you were joking, but 1950? I've got a copy of Jules Verne's 1865 "From Earth to the Moon", and 1870's "Around the Moon" both published prior to 1950.
I'm sure there are people with bibles published a couple hundred ago, all with a few dozen mentions of the moon.
And its well documented that Isaac Newton was considering the moon when he wrote 1686's Principia and its treatment of gravitational force.
Good nutter theories are supposed to be a little harder to disprove.
Earth and Pluto are similar in having a moon which is a decent fraction of their own mass. The two moons of Mars, and the moons of the four gas giant planets are minute in comparision to their primary bodies.
Earth and Pluto are sometimes called binary planets for this reason. And there is no easy way to show how they formed in this way, other than invoking chance impacts shortly after formation.
http://michaelsmith.id.au
While it's true that the moon is currently receding, it will eventually stop and begin approaching the Earth, then pass the Roche Limit, break up, and generally cause everyone to have a bad day. However, the universe might not last long enough for that to happen. As for the proto-earth/moon impactor, it was absorbed into the system, with the lighter materials of both it and proto-earth forming the moon, while the two cores sank to the bottom of what became Earth.
Since there are no -1: misinformation in the comment system, I instead reply to your post with wikipedia's article.
Antiope
Orbit of the Moon
Barycenter(Centre of mass)
They actually both revolve around a point that astronomers call the apotex, which lies about 187 million miles from Earth.
If I remember correctly, the center which they both orbit around is *inside* the Earth (but not at the center). Some have suggested that the difference between "moon" and "double planet" could be defined by whether the center of gravity is inside the larger body or on the outside (between them).
Table-ized A.I.
The entire Milky Way is made of milk.. thats why we call it the big whirlygig.
Because the moon is on the outer edge of the Milky Way it gets churned a lot, so it turned into cheese.
There's a book mostly intended as a resource for sf writers, What if the Moon didn't Exist? , which details many of the poor consequences for creatures like us given the absence of a large moon for earth. Its been years, but I remember two of the big ones.
First, without tidal interaction with a large moon, the earth would spin a great deal faster on its axis, resulting in much stronger and consistent winds. It would be hard for anything to be more than a few inches tall except in the windshadow of tall mountains, which themselves would erode much faster.
Second, without the tides, the transition from ocean life to land life would have been much more difficult, since there would be nothing like tidal pools to serve as temporary havens and opportunities for the creatures making that transition.
IMBO, the earth-moon system can still be called a binary planet as no other major body in the solar system except Pluto has a satellite with as large a mass fraction as the moon is to earth.
Are you serious, or are you trolling?
For this you need Newtons law of Universal Gravity. The formula you want is right there. As you see, there two masses, and one radius. Let's assume we have a mass of 1kg on the surface of earth (and later on the moon), that's our m2 and as such we can ignore it the whole calculation.
Some data about Earth:
Some data about The Moon:
The gravitational constant G = 6.67x10^-11 N m^2 kg^2. The formula we are going to use is F = G * m1 *m2 / r^2. We calculate the force exerced on an object on the surface of the stellar body, so we need to take its radius, because the center of gravity of the stellar body is well, in it's center.
So, first for Earth: F = ( 6.67x10^-11 * 5.9736x10^24 * 1 ) / ( 6371000 ^2 ) = 9.81 N
Now, the moon Earth: F = ( 6.67x10^-11 * 7.3477x10^22 * 1 ) / ( 1737100 ^2 ) = 1.62 N
So, as you can see the force exerced on a body of 1kg on the surface of the Moon is only 1/6 of the same object on the surface of Earth. Yet, as you can see, Earth has 80x the mass of the Moon. What did you ignore? The radius! That one has an even greater effect on gravity than the mass: it is a inverse square law.
You do realise that this is middle-school physics, don't you?
Ahhh...the great dumpster continuum. Many a free computer will be found there. -- sowth (748135)
--
Reverse outsourcing: it's the future
Except for
the snakes and some lizards, who lost their limbs.
and the birds, who lost their teeth,
and the dolphins and whales, who also lost (some) limbs, and their fur,
and the bats and moles who lost their sight,
and ungulates, who lost some of their toes,
etc.
The fact that Earth's moon subtends roughly almost exactly the same angle as the sun does (as seen from Earth's surface) is merely the way it appears now. You know that the moon always points the same face toward Earth, but do you know how that comes to be? You need to understand an effect called tidal locking:
(1 of 4: description, and explanation of a familiar example):
Suppose the moon's day used to be longer than its orbital period around Earth (which it almost certainly was when the Earth-moon system formed). Since the moon is not a perfectly balanced body, there is a "heaviest part" and a "lightest part" subjected to Earth's gravity. Earth's gravity exerted a torque on the moon each lunar day, lengthening lunar days until the heaviest part of the moon always faced Earth, reaching a stable equilibrium. It's a bit like an ice cube in a drink: Earth's gravity rolls it around until either something stops its roll or the heaviest part faces Earth. Even if the moon "rolls" (spins) through unstable equilibrium, it still has rotational energy to carry it through to the stable equilibrium.
(2 of 4: generalize the example to an unfamiliar, but understandable one):
Just as the moon can be tidally locked to Earth, Earth can be tidally locked to the moon. Earth too has a "heaviest part" and a "lightest part", upon which the moon's gravitational field acts. Eventually, Earth's heaviest part will point toward the moon just as the moon's heaviest part points toward the Earth. The moon is locked because it is much less massive than the thing it [closely] orbits; the same is true with every other moon in the solar system. The same tidal locking happens to planets that orbit close to their stars.
(3 of 4: orbital energy and altitude)
But even if the moon is always over one part of Earth's surface (as Earth is always over one part of the lunar surface), that doesn't explain how the moon's apparent size would change as viewed from Earth. Remember how the moon's days got longer until the moon became tidally locked with Earth? That's how Earth's days are still growing longer, and will do so until Earth becomes tidally locked with the moon. Earth's rotational energy is being transfered to the moon's orbital energy. The moon must either destroy mass and remain at its current orbital altitude, or since it can't get rid of mass instead stay the same mass but increase its orbital altitude. The effect is that as Earth spins slower (and Earth days get longer), the moon gets farther away.
(4 of 4: Earth's oceans and ocean tides):
We already have all the basic concepts so we can basically understand what's going on, but we should look at another big effect before looking at the big picture: Earth has oceans. The combination of the moon, the spinning Earth, and Earth's oceans literally stretch Earth's mass resulting in a tidal bulge each on the side of Earth closest and farthest away from the moon, but not quite exactly toward or away from the moon. The net effect is that the moon collects Earth's rotational energy faster. Ask the internet if you want the gory details or more clarification.
As it turns out, Earth will not become tidally locked to the moon until after the moon is so far away that it subtends a smaller angle of the sky than the sun. After one last total solar eclipse, the most complete solar eclipses on Earth will be annular eclipses.
But the real coincidence is this: Over the literally billions of years of geological time, the portion during which the sun and moon subtend approximately the same solid angle of sky is small. For most of Earth's history, the moon appeared noticeably larger, and for most of Earth's future, the moon will appear noticeably smaller. We're viewing a single, small timeslice in a ~10 billion year continuum of time.
In response to the apparent coincidence of the lunar and human menstrual cycles, there are fruitful explanations from evolutionary biology. Perhaps you'd also like to consider that humans are the only creature