why metiors hit earth and moon perpendicularly. Never seen elliptical crators ?
409171
Good question.  Meteors strike the moon at all angles, yet always leave round craters.  Why is that?  Well the answer lies in the way the craters are formed.  They are not formed by a plowing motion like a rock hitting a pile of sand, but rather they are formed by an explosion.\n\nA meteroite striking the surface of the moon is traveling at a very , very high speed (10-70km/s!), as a result it carries with it a great deal of kinetic energy.  When the impactor hits the surface it compresses and distorts the ground around the impact.  This compressed ground creates a shockwave and then rebounds upward, eploding up and out of the crater.  If you have ever seen a slow-motion movie of water or milk drops slashing, that is what the impact more resembles.  Remember that with this much energy being transfered you are really dealing with an explosion, just like one caused by a very large bomb.\n\nSince it is caused by this compression and then explosive decompression of the surface, you will get a round hole, regardless of what angle your impactor comes in from.  Almost.  If the impact is at a very, very steep, almost glancing angle you will leave more of an elliptical carter, you will also knock rocks off of the surface, as you really are just gouging the surface in this extream case.
811893
This phenomenon is caused by observing the earth/sun/moon system from within the system itself i.e. from the earth's surface. If one were to take a spaceship and travel far from earth and look back it the moon it would be obvious that its lit side always pointed directly at the sun, as one would think. It would be easy to see this as one could be assured that all the lines-of-sight were straight lines.\n\nFrom almost any viewpoint on the earth's surface, however, the sun appears to cross the sky in an arc. This is simply a consequence of living on a sphere: even if the earth's axis was not tilted, any point not at the equator will see the sun rise in the east and set in the west but because it would not pass directly overhead (it would only reach an altitude determined by the latitude of the observer) its path between east and west must be an arc. The same would be true for the moon, and the consequence of the sun and moon being constrained to cross the sky in an arc as opposed to a straight line is that, as the asker states, if you observe them at the same time the lit part of the moon will not appear to be pointing at the sun - you will tend to forget that the line-of-sight between them is actually curved as seen from the earth. However, if you took a photo of the sun setting and then one of the moon when it was about to set and compared them you would see that the lit part of the moon was indeed pointing to where the sun had set earlier: this is a valid comparison because they would both be on the same part of the arc. A similar effect is also very noticeable when the moon is a crescent - when it rises the "horns" of the crescent are pointing downwards, when it reaches maximum altitude they are pointing directly sideways and when it sets they are pointing upwards.\n\nThe fact that the earth's axis is indeed tilted does not affect the basic argument above, just complicates it a little because now even at the equator the path of the sun & moon will usually be an arc: it is only a straight line [overhead] at the equinoxes (and is so at one of the tropics on each solstice).\n\nThe slight tilt between the orbit of the moon and that of the earth is not the cause of this problem, as some answerers have stated, nor is the tilt of the earth's axis: these only affect the exact position of the moon in the sky relative to the sun i.e. which arc it traverses. Whether in a higher or lower arc, the moon will still appear to rotate as it crosses the sky in order to maintain its orientation relative to the sun and will still point directly at the sunset postion upon "moonset".
455077
The actual shape of the Earth today is an OBLATE SPHEROID. It's a kind of sphere but flatter on two opposite sides. These sides are found at the poles.\n\nThe reason why the world is shaped like so is because of the the Earth's rotation. This spinning occured from the time of the Earth's formation up to this day. Centrifugal force tends to throw more matter in its equator than its poles.
25792
I'm not sure what mean by "tilted," that typically refers to the axis of rotation. (The Earth's axis is tilted about 23.5 degrees from perpendicular with our Orbital plane.) The Moon is not tilted very much at all with respect to it's orbital plane around the Earth.\n\nDo you mean that the Moon always has the same side facing Earth? This is called tidal locking. Many of the satellites around Jupiter and Saturn are tidally locked. Mercury is almost tidally locked with the Sun.\n\nSolar tides (as opposed to the lunar tides) will determine the extent to which the Earth's rotation is slowed down so that Earth is tidally locked to the Sun. But Earth is big enough and far away enough that I don't think this will happen before the Sun enters the Red Giant phase and the Earth is (probably) destroyed.\n\nThe Earth's rotation is slowing down a bit, but that is due to the influence of the Moon.
414408
Yes, the radius of the moon's orbit is currently increasing by about 3.8 cm (about 1.5 inches) every year.  This happens because the moon raises tides on the Earth.  Because the Earth rotates faster than the moon orbits the Earth, the tidal "bulge" of the Earth on the side nearest the moon actually leads, or is in front of, the position of the moon in its orbit (see diagrams in sources given below). That "extra" mass of the Earth's tidal bulge acts to accelerate the moon in its orbit, increasing its angular momentum, and causing the moon's orbit to "spiral out" over time.  Because the angular momentum of the Earth-moon system must be conserved, the Earth's angular momentum must decrease to balance the increase in the moon's momentum, slowing the Earth's rotation rate by a bit (currently, about 15 microseconds/year).  \n\nThese rates have changed over geologic time because the position of the tidal bulge relative to the line connecting the Earth and moon depends on, among other things, the arrangement of continents and oceans on Earth.  These affect the friction affecting the tidal bulge, and therefore its velocity relative to the Earth's rotational velocity.\n\nThe energy of the Earth-moon system must also be conserved, and because the moon gets further from the Earth, its potential energy increases.  Consequently, it's kinetic energy must decrease by an amount exactly equal to the increase in potential energy.  Because the kinetic energy of the moon (or any other object) is proportional to the square of the velocity, the moon's velocity in its orbit is decreasing.\n\nThis process will continue until the moon's orbital period and the Earth's rotational period are equal, at which point the moon and Earth will be "tidally locked", with the same side of each body facing the other body at all times.\n\nThe story doesn't stop here, though.  The Earth-moon system loses angular momentum to the (Earth/moon)-sun system, resulting in an increase in the distance of the center of mass of the Earth+moon from the sun.  This is occurring today, but is a much smaller effect than the lunar tidal effect discussed above.  Once the Earth and moon are tidally locked, though, it will become the dominant effect. As a consequence, the moon's orbital radius will slowly *decrease* until the moon gets somewhere within the Roche limit, at which point (assuming the sun has not become a red giant and expanded beyond the orbit of the Earth by then!) the moon will break apart, most likely forming a ring system around the Earth.\n\nThe Roche limit is the distance from a parent body at which the difference between the gravitational force on a a satellite's near side and a satellite's far side exceeds the gravitational self attraction of the satellite.  Because the moon is made of rock, it has some cohesive strength (i.e., the moon is not just held together by self-gravitation) that will allow it to come within the true Roche limit before being disrupted.
224099
It's not that strange, why would you expect the Earth's orbit to be a perfect circle.\n\nEverything revolve in a elliptical orbit, because they can't all start moving in perfect circles, that's impossible, so when Earth got closer to the Sun, it basically falls and accelerates because of gravity, but because it's now faster, it can bypass the Sun so it won't crash into it.
897181
because they are mutually perpendicular and the ellipse is symmetrical about both
217587
First I want to clear up that clockwise is really a 2-dimensional description of rotation, and planet rotation can be viewed in 3D so you can't really describe planet rotation as clockwise or counterclockwise. If you define an "up" and a "down" for the solar system, and then say "when viewed from above the solar system rotates counterclockwise", then the "anticlockwise" description works. For my answer we will assume you have done all this and that the solar system rotates counterclockwise, so there is no confusion from the wording of the question.\n\nThe reason the solar system rotates counterclockwise has to do with the way the gas clouds that created the solar system collided. They didn't collide head on. Think of cars travelling in opposite direction down an interstate hitting each other not quite head on (i.e. the left headlights of both cars smack together). When they do this they will spin out sideways - they will effectively rotate around each other (in North America with the cars driving on the right side of the road, they will rotate counterclockwise when viewed from above if they collide like this). The gas clouds did the same thing and their collision exceeded the critical density needed for gravity to take over and start forming the solar system, and as this matter collapsed to form the sun and planets, the rotation sped up. The rotation these gas clouds got from their collision gave momentum to the rotation of most matter in the solar system; the sun rotates in the same direction as those clouds did when they collided and so does most other things, including all the planets.\n\nSo that is why.
1027284
because the sun's gravitational pull is much greater than the earth's...same reason the moon goes around the earth and not the other way around
195652
the earth is a oblate spheroid....we do not fall to the edges because of the gravitational pull...
214288
First, you must picture in your mind the 3 celestial bodies: sun, earth and moon. Picture the moon circling the earth: now it is between the earth and the sun, now it is at the other side of the earth, now it comes back again between earth and sun... As you know it takes about 4 weeks for the moon to make an entire circle around the earth.\nWhat happens when the moon is between us and the sun? well, it receives light from the sun on the face that is turned that way, so  we can't see it at the moment. That's why we can't see the new moon! It's there is the sky, but not lighted in the side facing us!\nThen 2 weeks later the moon is on the opposite side. The face that is facing the sun - and reflecting all that sunlight - is also facing the earth! So we see it fully - full moon.\nBut the path of the moon is not entirely on the same plane as us and the sun, there is a slight angle!!  Only sometimes it happens that the shadow of the earth catches the moon when it's full (and when it is perfectly aligned with the earth and the sun). THAT's your eclipse!!
20309
I want to add more:\n1) Due to the distance to the Moon relative to the size of our planet, it doesn't really matter from which point on Earth you are looking at our natural satellite. You will see virtually the same part of the surface.\n2) The reason the Moon rotates around its axis at about the same time it rotates around the Earth is because it is tidally locked with the Earth. Tidal lock worked for billions of years to achieve the present effect, but it's still not perfect: the Moon slightly advances or gets behind in its axial rotation relative to its revolution around the Earth. So, over time, we are actually able to see around 57% of its surface, rather than just 50%.\n3) The Earth is not close enought to the Sun to be tidally locked by its gravity (even Mercury, the closest planet to the Sun, isn't). So that's why the Sun-Earth relation is different.
1071120
Stars and planets form as a result of the gravitational collapse of accreting material. Any net translational motion of that initial material is accelerated as its radius of rotation decreases. This is due to the same conservation of angular momentum principle that makes skaters and divers spin more rapidly when they bring their arms closer to their bodies. In its extreme, this effect can make collapsed neutron starts spin many times each second, creating the regular beat of pulsars as observed from Earth. \n\nOnce an initial spin was established for the Earth, the same conservation principle says that its angular momentum will continue unless it can be transferred to another object. With no significant frictional forces to allow that to happen, the Earth can continue to spin indefinitely without any further prodding. (More on that later.) \n\nIt is incorrect to say that the Moon does not spin. It rotates on its axis in synch with its revolution around the Earth, keeping the same face always pointed toward us. If it did not rotate at all, we would see all of its surface over the course of one month's orbit. This condition was created by tidal forces between the Earth and Moon, gradually slowing its initial rotation until the current more stable situation evolved. This same 'tidal lock' condition has been observed for satellite closely orbiting other planets. Because of its elliptical orbit, a slight 'wobble' allows us to actually see slightly more than half of the Moon's surface over the course of a month. \n\nTidal forces, by the way, continue to affect the Earth/Moon system. They exert enough frictional force on the Earth so that some of its angular momentum IS being transferred slowly to the Moon. The net effect of that is to gradually slow down Earth's rotation and lengthen the day. Where is that energy and momentum going? The moon is gradually receding from the Earth. Both of these effects are small. The Moon is receding at a rate of less than 2 inches per year and the day is getting longer by one second every 67,000 years or so.
882017
No one who has answered so far got it right.  The answer is TRUE, and the reason is because the moon rotates on its axis in exactly the same amount of time it takes it to make one revolution around the earth.  No one seems to get this.
370080
Finally a good question.\n\nThe Moon's apparent size being very similar to the Sun's apparent size is a coincidence. But the Moon doesn't always cover the Sun during an eclipse, look up annular eclipse (the looks smaller), and when it does cover it, it is usually a bit larger.\n\nBut now let's talk about the main question.\n\nWe always see the same side of the Moon because its during its full rotation around the Earth it also makes a full rotation around its axis. This is not a coincidence. There are several satellites in the solar system whose rotation is synchronized with their rotation around a planet (eg. Io around Jupiter, Charon around Pluto but also Pluto around Charon :-) etc. ).\n\nThe Earth's gravitation decreases with the distance from its center. So the Moon's opposite side (which is further away from the Earth) is attracted less by the Earth than the side which is closer. This results in the Moon being a little bit deformed along the Earth-Moon line.\nThe Moon is a little bit shaped like an egg: if it would rotate faster, it would not be elongated towards the Earth anymore, and the Earth's gravitation would try to turn it back. This is very difficult to explain without the opportunity to draw figures ... but anyway, search for tidal locking on the net, I hope you'll find a good, illustrated explanation.\n\n\nEDIT: In one answer someone said that Mercury is in a similar relation with the Sun ... this is not exactly true, as the ratio of period of its rotation and the period of its orbiting is 3/2.\n\nMercury also has an elongated shape: due to this 3:2 resonance, at its perihelion, the axis of its elongation is always parallel with the Sun--Mercury axis.\n\nEDIT2: As I said, the apparent size of the Moon is just a concidence. Other planents do get total eclipses. What does a total eclipse mean? It means that a moon casts a shadow on the planet: http://www.c-science.com/txt/images/1999/990428unssx2.jpg\n\nThere is a nice program calles Celestia, which can show you the eclipses caused by various moons on various planets (3D animations): http://www.shatters.net/celestia/
1040803
Explain how a 600 yard wide rock could cause the 2,160 mile wide Moon to wobble, without touching it? It'd be like throwing a golf ball at a bus and expecting it to tip it over....\n\nIt doesn't matter what the moon does as far as wobbling and spinning, it's gravity isnt affected (unless it breaks up) and therefore the end result means zip to us. We may just get to see the far side!
465729
Two possibilities not previously mentioned: first, phone your various friends around the world, and ask them the time! You will then find out that the sun is at a different height in the sky at different places. So at least the world is not flat, since the sun is very much farther out than the distances between any two points on earth.\n\nSecond, wait for a lunar eclipse. This is the Earth's shadow projected on the moon. It is round. The Earth is not a disk (see first experiment) but casts a round shadow: a sphere seems indicated. In fact, it is a sliughtly flattened sphere (ellipsoid, in fancy language)