Try this: Take two coins of the same size. Place them on a table, touching each other. Now, hold one of them in place with your finger (that's the Earth) and roll the other one around the first one with your other hand (that's the Moon). Notice which way the rolling coin faces as it goes around. Look at which way it faces compared to the centre of the Earth, and compared to a fixed point somewhere else on the table.
A theory of expanding space needs a really good explanation why space cannot expand within solar space. I will accept the idea provisionally if someone claims to have calculated it out with some degree of certainty, but even the idea that space has stretched since the moment of creation is not certain. On the other hand a sane and reasonable person could hypothesize that the pulls of the various planets and stars can stretch out space over time. It it is elastic that is how it is pulled on.
Space overall can get bigger and the distance between galaxy clusters grow and grow, but small objects held together by gravity, like the Earth and moon, Earth and sun, all stars in this one galaxy, the few galaxys in out local group - stay the same distance apart. If you put 2 rocks on an elastic sheet and tied them together with string, they would stay at the same separation even when the sheet is stretched. Thats not hard to grasp, right? I think that if the barycentre is still inside Earth though not at its core, the moon can still be called our satelite because it orbits a point within the Earth. If the moon was 2 or 3 times heavier the barycentre would be OUTSIDE Earth, and both bodys would orbit a point in space between them. Then you could call us a binary planet. Isnt Pluto and Caron like that?
Over time the sheet might stretch, the way that a pair of blue jeans gets loser as you wear it. That's not too hard to grasp, is it?
actually the earth is getting further and further away from the sun because the sun is losing mass (due to the solar wind) if you think that's insignificant take in account that the sun has shined for around 5 billion years and that it already has lost abouth a 100 earth masses and proberly will lose a equal amount before it goes to it's second stage. Then again the earth has also have a natural cycle in wich it's migrates a bit. http://en.wikipedia.org/wiki/Sun#Life_cycle
Would you like to show us the math you are using, Billy T ? Seems to me that if the Sun's gravity were stronger than the Earth's, the Moon would always have the same side turned towards the Sun.
If the same side is always pointing towards the Earth, wouldn't the same side always be pointed towards the sun?
Hard to follow your mental processes, Orleander! When the same side is facing the Sun as is facing the Earth is when we see a Full Moon.
sorry, I confuse easily. Please Register or Log in to view the hidden image! lol I'm still not understanding all this, so I kinda gave up. Are meteors still hitting the moon?
Naw. The Moon has been in its present orbit for over 4 billion years. The meteors have learned to go around.
There is a limit, of course. http://en.wikipedia.org/wiki/Hill_sphere#Further_examples An astronaut could not orbit the Space Shuttle (mass = 104 tonnes), where the orbit is 300 km above the Earth, since the Hill sphere is only 120 cm in radius, much smaller than the shuttle itself. Lunar orbits are problematic not only because of the Moon's relatively weak gravity but also because of the Moon's lumpy gravity. The orbit of PFS-2 about the moon is particularly bizarre. http://science.nasa.gov/headlines/y2006/06nov_loworbit.htm The orbit of PFS-2 rapidly changed shape and distance from the Moon. In 2-1/2 weeks the satellite was swooping to within a hair-raising 6 miles (10 km) of the lunar surface at closest approach. As the orbit kept changing, PFS-2 backed off again, until it seemed to be a safe 30 miles away. But not for long: inexorably, the subsatellite's orbit carried it back toward the Moon. And on May 29, 1972—only 35 days and 425 orbits after its release—PFS-2 crashed.
This is merely bad logic. The same bad logic applies to most moons, including Pluto and Charon. The same bad logic applies to our orbit about the galaxy (we aren't orbiting the sun!) It is not geocentricism that makes us think the Moon is a satellite of the Earth. It is math and physics. Astronomers use the Hill sphere as the metric for determining whether some body is a bound satellite of a some other object that is orbiting the Sun. They use the location of the barycenter to distinguish between satellites and double planets. If the moon were not a satellite of the Earth it would behave more like 2002 AA29.
The moon is in synchronous orbit around the earth but all of its surface still gets lit by the sun(so faces the sun) thats why we get half moons, quarter moons, full moons etc... despite us basically only being able to see one face of the moon from our earthly home.
No, coz thats just what I was saying myself mate. The sheet may stretch, but the rocks sat on dont move away from each other if theyre tied together! The lock on the moon's rotation was done by TIDES, not gravity total. Tides are caused by the pull on 1 side of the moon being stronger than on the other side - the more the diference, the more the tide. The sun's gravity is so strong and the moon (and Earth) are so far from it that the diference felt across teh moon's width is tiny. But the moon is lots closer to Earth, and Earth's gravity drops off much faster than the sun's. So the diference, and tide, caused by Earth is much stronger, though the total gravity is less. Its the same reason why the moon causes bigger tides on Earth than the sun, although the moons gravity is much weaker . If the moon's pull on all parts of Earth was the same, it would cause no tides at all.
You are correct. I can only add: Gravity falls off as the inverse square (of separation between mass centers) and the gradient og gravity (tide producer) as the inverse cube.
Which means: if the Moon were twice as far from Earth as it is, its tides would be only one eighth of their current strength. The Solar tides would then be the stronger, and there woulld be no ambiguity among us forumites about which body exerts the most gravity on our globe. I wonder what the ocean tides on Earth were like in the distant past, when the Moon was much closer (and the Earth's rotation faster?)
This would never happen, and not just because the Sun will expand into a red giant and likely envelop the Earth before it can occur. Assuming that such a fate is avoided, the following would happen: The moon will continue to recede from the Earth and the Earth will continue to slow its rate of rotation. Eventually, the period of the Moon's orbit and the Earth's rotation will match, and the Earth will present the same to the Moon, just as the Moon presents the same face to us. at this point the moon will stop receding. (it was the difference between the two periods that caused the transfer of angular momentum, that led to the recession) If these were the only two bodies we had to consider, this would be the final stable configeration, but it isn't. The Sun also exerts an tidal influnence on the Earth, and will continue to slow the Earth's rotation so that it will become tidally locked with the Sun. As the Earth begins to slow in its rotation, the Moon will now orbit in a shorter period than the Earth rotates. Once again there will be a difference between the Moon's orbital period and the Earth's rotational period, which sets up a new tidal interaction. Under these new conditions(moon faster, Earth slower), the moon will start to approach the Earth rather than recede. It will continue to move in, orbiting closer and closer to the Earth, until it passes the Roche limit and is torn apart by tidal forces. Earth becomes a ringed world. This would take billions upon billions of years to occur, and as I said, the Earth and Moon aren't likely to last that long. In fact, I do believe that the Sun is due to swell to red giant stage before the Earth can even become tidally locked to the Moon.
I have read that the open ocean tides may have been much higher. You are correct about the 1/8 th amplitude of the open ocean tides but those near shore and in inlets are more complex. I do not know if the 1/8 tide is less than that of the sun.
