I know that the earth is kept in orbit by the gravitational pull of the sun but what causes it to rotate about it's axis? Is it because it had some rotational inertia when it was created and it has just kept on spinning? What causes the moon (or any planet) to rotate?
201618
It goes back to when the solar system was formed.  The sun (and the planets, for that matter) were formed when a gigantic cloud of gas and dust collapsed.  When that cloud collapsed, it had something called 'angular momentum', which really just means that it was spinning a little bit.  When a large, slowly spinning thing collapses to something smaller, the rate of rotation speeds up (think about an ice skater that pulls her arms in...she spins up!).  Anyway, so now we have all this gas and dust in a disk that's orbiting the new Sun.  The dust eventually forms into planets.  As the dust collapses into new planets, the same thing happens, and the planets start spinning themselves.  So basically, the earth and moon and whatnot spin because of angular momentum.  I hope this made sense/helped!
91510
Orbital obliquity is probably influenced by the sun in some way during the time when the planets were forming, but I don't think we have any solid idea exactly what factors were involved.  It's clear that most of the planets' axis are more or less vertical, though Venus in fact rotates the opposite way (so its obliquity is considered to be nearly 180 degrees).  The big exception is Neptune, which is nearly lying on its side.\n\nIf the Earth was similarly lying on its side as you suggest, then yes, it could spin the same and have light and dark "sides" that only change as it moves around the Sun.  See Neptune for details of this exact mechanism.
473189
The sun rotates on its own axis that what causes the gavational pull to keep the planets moving around it.
12392
"The moon does not circle the Earth once a day. The Earth circles the sun once a day. The Moon orbits the Earth about twice every 24 hours. That's why we have tides (high and low)."\n\nSAY WHAT?  Not even close.  There are junior high science teachers weeping over this "answer".\n\nThe earth rotates once a day on its axis.  That is the definition of a day - the time it takes for a planetary body to rotate once on its axis. \nThe earth circles the sun once a year - actually 365.24 days (which is why we have leap years).  That is the definition of a year - the time it takes for a planet to orbit the sun.  As you can tell, planetary years are longer or shorter depending on the distance from the sun.\nOur earth's axis is tilted.  The northern hemisphere is closer to the sun in the summer and farther in the winter.  This is how we get seasons.\n\nThe moon orbits the earth once every twenty-eight days, give or take an afternoon. The phases of the moon are caused by it being in a slightly different position everyday relative to the sun.  \nThe moon does cause tides - its gravitational pull causes the oceans to bulge out towards the moon.  However, there is a matching bulge directly opposite the moon.  THAT is why we have two high tides and two low tides a day.  In addition, the sun exerts a lesser pull, which interacts with the moons pull.\n\nSorry about that - had to correct the flagrant misinformation there.\n\nAs for your question - it is a weird effect, isn't it?  Especially when the moon is closer to a new moon.\n\nWhat happens is that our atmosphere scatters the light from the sun so that our sky is quite bright.(Why is the sky blue? has been discussed here many times).  However, the moon is reflecting enough light so that it is brighter than the surrounding sky and we see it.\n\nWhen the moon is less than full, there is very little light reflected off of the dark side of the moon (what light is reflected is earthshine!)  This light is not bright enough to outshine the sky.\n\nThink of a white sheet (in a dark room) that images are being projected onto.  The sheet will look opaque until someone shines a light onto it from the back that is brighter than the image projected from the front.  That is the basic effect that we are getting - the atmosphere is a sheet, with the sun and the moon both shining on to it.
90311
Two objects will orbit one another indefinately unless disturbed.  The man made satellites that return to earth orbit to closely and are so small, atmospheric drag and solar wind eventually cause their orbits to decay.\n\nThe moon is large enough are far enough out, these factors don't come into play.  The primary force now is tidal friction, which causes the orbit to slow, and therefore expand, by a fractional amount each year.  \n\nDon't worry about losing the moon, though.  The sun will expand into a red giant, swallowing both earth and moon long before this happens.
21209
The sun does not orbit the Earth (LOL).  The reason we have night and day is because it takes the Earth about 24 hours to rotate (spin).  Each time a side is facing the Sun, it is day, and when it is turned away it is in night.
209464
The earth orbits the sun; the moon orbits the earth. The cycles of rotation are different meaning that the moon will orbit the earth many times every year. This means that it will very often be in a position to be seen during the day. The moon itself has no axial rotation so on earth we always see the same side of the moon.
177366
The earth's rotation does slow down, but not\ndue to the reason you are stating... gravitational\npull of the moon and sun causes more mass to move\ncloser to the surface over time. When an ice-skater\ntwirls around, if the pull in their arms to their\nsides they spin faster, when they push their arms\nout they slow down - in the same way, the earth\nis slowing down, but only by about 1 second per\nseveral years, which is adjusted for by adding\na leap-second from time to time to the atomic \nclock.\n\nThe earthquake in Sumatra on 26-Dec-04 resulted\nin a massive chunk of the ocean floor collapsing\ndown (closer to the earth's center), which caused\na measurable speeding up of the earth's rotation.\nThis was, however, not sufficient to compensate\nfor the otherwise slowing down trend, and only\nmade the adding of a leap-second get delayed...
506250
It turns one revolution for every orbit it makes.  This happens because of tidal locking.  Basically that means the part of the moon closest to the earth gets pulled slightly more then the further side.  This means a slight bulge forms on the moon facing earth.  This side has more mass then other part of the moon thus is "heavier."  The earth's gravity then pulls on the buldge more then the rest of the moon making the moon slowly slow down or speed up in its rotation until the bulge always points towards the earth, thus one side always faces us.
178768
This situation is the natural result of one body orbiting another.  It is called tidal locking.  All of the moons of Jupiter are tidally locked to it, and Pluto and it's moon, Charon are tidally locked to each other for example.\n\nWhat happens is that an object that is rotating faster than it is orbiting feels a torque from the tidal force (differential gravity) exerted across it by the object it is orbiting.  Over time this will cause the rotation period to slow down to match the orbital period.  The time this takes depends on the distance between the two bodies and the reletive masses, as well as a few other factors.
4454
****Edit based on further details from asker *****\nSailing around the world is sailing within the sperhical confines of our planet.  If there were no athmosphere, or gravity, then we would simply fall off the earth.  However, as gravity is pushing us down, and is affecting the winds, ect within the athmosphere, we are bound to the surface of the water.  \n\nEven though a sphere is included in the define of spheroid, it is a type of spheroid, not the inclusive answer.  The fact that gravity pushes down on us from all around, mainly because the earth is spinning so fast, and other more complicated reasons; the sphereoid holds whatever is on it's surface down to adhere to the confines and shape of such a surface.\n\nEven if the world were flat, or a flattned sphereoid, we woudln't fall off, instead, we'd loop around to the otherside becasue gravity wouldn't allow us to actually 'drop' off the side of the cliff.  The inverse gravity on the other side of the plane would pull us to it's confine, or surface.\n\nSo, there's your answer.  Sorry i didn't get it more clear the first time.\n\nMathamatically you can say that gravity pushes us from all around using a tangent direction.  That means that no matter the point you are on this earth, gravity is pushing down on you within the same confines as any other like region.  As to falling off the earth, if gravity didn't exist, we wouldn't 'fall' exactly; we would more likely be thrown or float off the earth (thrown if the world is spinning).\n\n****End Edit.******\n\nThe world indeed is a sphere, but it is not a perfect sphere, due to the rotation of the earth around it's axis, as well as the rotation of the earth around the sun (central axis), the spinning and speed of the planet make it more of a spehere that jiggles, sometimes squeezed, and sometimes elongated.  But, not by much :) Just a bit :)
531805
The Earth orbits the sun in an ellipse.\n\nEvery ellipse has two foci. It is at these two points where the orbited body is normally located.\n\nPlease see the wiki source that has further information on how to get the numerical data if you need it.
351873
probably because it takes the same time for the Moon to rotate around its axis (a day) and orbit the Earth (a year) \nit is always facing the same side because it rotates in conjunction with its orbit (bit hard to explain)\nbecause it is also rotating at the same time, we always see the same side of the Moon
168726
The First law of Kepler is: \nPlanets move along elliptical orbits with the Sun\nat a focus.\n\nSo, all i can say is that the sun is relatively stagnant in our immediate universe.\nThe earth rotates along an elliptic orbit around the sun.\nThe moon rotates along an elliptic orbit around the earth.\n\nWell, all "movements" are relative to some reference point.
480896
Electromagnetic fields don't cause the earth to wobble. The earth processes like a top and wobbles. As the earth processes the northern and souther hemispheres get different amounts of heat. This drives the weather. This motion of the earth is due to angular momentum from left over from its formation and the procession probably results from collisions with large objects, including the one that formed our moon. To a much smaller extent, some of the wabble is probably due to plate techtonics.  The moon, by the way, acts to stabilize the wabble of the earth.
12201
Approximately one rotation every 24 hours [grin].\nBut, seriously though, the Earth's radius (at the equator) is\n6380 km so the equatorial circumference is about 4 x 10^4 km,\nso the speed of the earth's surface (at the equator) due to the earth's\nrotation is 4 x 10^4 / 24 = 1700 km/hr.\nAs an exercise for the reader, compute the speed for your particular\nlatitude.\n\n1. What is the Sun's rotation speed around center of the galaxy? \n\nThe disk of our galaxy is flattened by rotation (just as the planet Saturn is flattened, and protostellar disks are flattened.) Individual stars in the disk are on nearly circular orbits in the plane of the disk, all revolving in the same direction around the center of our galaxy. (By contrast, stars in the halo are on elongated orbits, at random orientations, some traveling clockwise, and others counterclockwise.) \nThe Sun, for instance, a fairly typical disk star, is orbiting with a speed\nv = 220 km/sec = 0.000225 parsecs/year.\nThe radius of the Sun's orbit around the galactic center is\na = 8000 parsecs.\nThe circumference of the Sun's orbit is then\n2 pi a = 50,300 parsecs.\nThe orbital period of the Sun thus turns out to be\nP = 2 pi a / v = (50,300)/(0.000225) = 220,000,000 years. \n\n2. What is the speed of center of our galaxy?\nThe sun is about 26,000 light years from the centre of the milky way
349067
It doesn't wobble exactly, though if appears that way from Earth. The main cause of this libration, as the wobble is called, is that the moon's orbit is slightly elliptical. This makes it move faster in its obit at some times (when it's closer to Earth). Its axial rotation, however, is at a constant speed, so the axial rotation and orbital revolution don't quite perfectly synchronize. So when the moon is moving fast, we see a little more of the trailing edge, and when its slow, a little more of the leading edge. \n\nSome of the wobble is also due to the tilt of the moon's axis and the inclination of its orbit relative to ours.