how can a moon produce a gap in a planatary ring system?
609788
I dunno, the Gap (tm) seems to be everywhere, don't it?
136889
Its an illusion.  Its because the moon is closer to objects to which you can compare it.
354795
It is a lot easier to demonstrate this with graphics than trying to explain it in text.  You can likely find a good diagram if you go to google it.\n\nFirst imagine the sun.  Now imagine the earth revolving around the sun.  Keep in mind that the earth and the sun are both extremely small compared to the distance that the earth is from the sun.  this means it's like two specks of dust revolving around each other in a basketball court, to give some perspective.\n\nNow imagine the moon revolving around the earth but ignoring gravitational effects of the sun (for simplicity).  Since the earth is so small compared to these revolutions, imagine that the earth very rarely blocks the light from the sun shining on the moon.\n\nNow when you observe the moon, if it is roughly between you and the sun, it is the backside of the moon that is lit.  This means the only part of the moon that you see is the shadow - or the new moon.  If when you observe the moon it is roughly behind the earth, you get to see the part of the moon that is being shined on by the sun - a full moon.  Since the moon revolves around the earth at a rate different from the rate at which the earth spins on its axis, the moon will change its orientation to give us our progression through the phases of the moon.\n\nJust an additional bit - eclipses are those rare times when the earth's shadow actually DOES hit the moon.. making what would've been a full moon into something else (a lunar eclipse).  The more commonly noticed (due to the drastic change in light during the day :D ) solar eclipses are when the moon directly blocks the sun's light from the earth.  These occur at regular intervals over series' of years.
987176
I can't say for certain, but I assume the length of the gaps does not increase.  The length of the tubes increase so that the time it takes to traverse the tube stays the same.  I think the only constraint on the gaps is how close the tubes can be to each other.  You'd want it to be as small as possible to get as much accelleration as possible.
335688
NOW FOR THE FULL ANSWER\n\nImagine a ping pong ball. Imagine it is the earth. Colour this ball in with a pencil, the thickness of that graphite coating is how thick the earth's crust is. The rest is liquid.\n\nWay back when earth version 1 was floating in space, moon v.1 come hurtling through space and hit earth v.1 on the edge. The two object tore through each other. They are liquid, not solid. The two mixed together and due to the dynamics of impacts, a blob about the size of moon version 1 blew out the other side. This is why earth and moon have similar composition. For  a few minutes they were one.\n\nNow we have earth v.2 and moon v.2. Remember both are liquid. All liquids have surface tension, that pulls them into a ball. There is also gravity, these balls are massive. This pulls everything to a central point. They are spinning, so this makes them a little out of round.
738089
Just as the moon can cause tides in the Earth's oceans due to gravity, the Earth can also cause tidal effects on the moon.  Over time, these effects have caused the moon to become "tidally locked".  This has the effect of causing the moon's rotation period to equal its revolution period.  You can see this effect by having someone walk around you but looking directly at you.  While they are facing you, the actual direction of their face changes as they move around you.\n\nHowever, the moon's orbit is not perfectly circular.  It is a slight ellipse so, sometimes, its rotation can get a little ahead or behind of its orbital position.  Also the orbit is tilted with respect to the Earth's equator.  These combine to allow us to see about 55 -60 % of the moon's surface during the orbit although, of course, we can only see half at any given time.
305090
The Moon's orbit around the Earth is elliptical, with a substantial eccentricity (as major Solar System bodies go) of 5.49%. In addition, the tidal effect of the Sun's gravitational field increases the eccentricity when the orbit's major axis is aligned with the Sun-Earth vector or, in other words, the Moon is full or new.\n\nAt Perigee (its closest Point to earth the Distance is  359861 km from earth measured from equator to equator.\n\nAt Apogee (its farthest point from eath the Distance is 405948 km from eath measured from equator to equator.\n\nAs it revolves around the earth in this EGG shaped orbit it cycles between getting further and closer from earth.\n\nTake a Boiled egg and cut it in half. Measure how THICK the white is from the yoke at any point on the FLAT cut surface of the egg. The moon would travel around the outside of the yoke. (the yoke is the earth.\n\nSee the below link for a great explanation and good photos and graphics.
965145
It's called a "captured orbit".  Use two oranges or balls. Hold one as the Earth in your left hand on a table.  It's going to stay still for demonstration purposes.  Hold the other orange with your right hand like the moon nearby and stick a toothpick in it so that the toothpick faces the "Earth" orange.  Now, just to see how the moon can be rotating AND keeping the same face toward the Earth, slowly begin to orbit the moon counter-clockwise around the Earth with the toothpick facing the Earth. Take the moon all the way around the earth, keeping the toothpick facing the Earth.  Do it again, slowly, and notice that the moon actually turns all the way around once, or, rotates, everytime it orbits all the way around the Earth. It takes the real moon 28 days to orbit or go all the way around the Earth, and 28 days to rotate all the way around it's own axis. Cool, huh?  : )  Took me like fifty years to figure that out.
335545
Because when it's closer to the horizon, we have other objects (trees, houses, or even mountains) to compare it to, and the moon (or sun) looks very large, taking up 2% of the horizon.\n\nWhen it's farther up in the sky, there's nothing around it, so it tend to disappear in all that empty space.  Even though it still takes up 2% of any given arc, there are wide open arcs to the right, to the left, to the top and to the bottom of the moon that are totally devoid of objects.  Relative to all that open sky, it doesn't look so big.
895142
full moon.  a lunar eclipse occurs when the earth blocks the light from the sun, so the earth is positioned between the sun and moon - the full moon phase.
14526
The bright ring is called a 'halo'\n\nIt's a fairly common sight and occurs when high thin clouds containing millions of tiny ice crystals cover much of the sky. Each ice crystal acts like a miniature lens. Because most of the crystals have a similar elongated hexagonal shape, light entering one crystal face and exiting through the opposing face refracts 22 degrees, which corresponds to the radius of the Moon Halo.\n\nSee a nice picture of one here:\nhttp://antwrp.gsfc.nasa.gov/apod/ap000515.html
12525
There is a bulge of water on the side of the earth facing the moon because of lunar gravity. The previous explinations for the second bulge (on the opposite side of the moon) are incorrect.  The mass of the earth is constantly accelerating toward the moon, as they orbit eachother.  The smaller second bulge is a result of the earth accelerating away from the water, toward the moon
1031159
They are called moonquakes.  If the moon had an atmosphere it probably would not be able to keep it or hold it at the moon's surface due to little gravity.  (only 1/6 of earth's)
246091
the moon doesnt have a strong enough graviational field to allow it to retain the necessary atoms/molecules to form an atmosphere
1059943
This is a mystery and no one really knows. It could be that the rings are relatively recent and the result of the gravitational destruction of a small moon and the debris has not yet totally circumscribed the planet. An even bigger mystery is why an object so cold can have winds of 1200 kms/hour! Where's the energy coming from? What's driving these colossal winds? Jules, Australia.
871535
The distance between the two systems vary because neither the moon orbiting the earth nor the earth orbiting the sun are in circular orbits; they are in elliptical orbits. The furthest distance is called the apogee, the closest, the perigee.\n\nYou can get the variation in distances by referring to Wikipedia:\nhttp://en.wikipedia.org/wiki/Earth\nand\nhttp://en.wikipedia.org/wiki/Moon\n\nHope this helps!
758796
First of all it is a significant body.  At one time it was geologically active therefore had a molten interior and rotation similar to the earth.  Over time due to the combination of gravity, rotation and geological activity the moon developed a round shape.  The moon has long since been "dead" and its rotation slowed due to the earths gravitonal pull so now its rotational period precisely matches it revolution around the Earth.