One revolution of the Earth around its axis takes around 24 hours. It takes the Earth 365 days and 6 hours to complete one rotation, so each hour of daylight is due to sunlight being visible because the Earth is rotating once during every minute of that hour.
In fact, if you were to walk around the world in a straight line, you would end up where you started after exactly 24 hours have passed since you began your journey.
The reason why we see only part of the day's rotation is because we are on the surface of the planet. If you were floating on a balloon over London, you would see the entire city from beginning to end in just under 24 hours.
This means that there are times when London isn't seen completely by anyone on Earth! The times come about twice a year, when the Earth blocks out all of the sun for a few minutes at a time from one side of its orbit to the other.
These periods of darkness occur at both the north and south poles during mid-winter and mid-summer respectively.
The Earth rotates once on its axis in only 23 hours, 56 minutes, and 4.0916 seconds. This is called one rotation. It takes a little over 24 hours for the Earth to make one complete rotation.
There are two ways that the Earth's rotation can be measured. The first is by using a clock. If we watch the clock during a rotation we see that it gets rotated 180 degrees along with everything else. So if we start at 12:00 midnight and follow the clock around its circle we see that it reads 3:00 am when we return to 12:00 midnight. The second method uses geodetic techniques where measurements are made from fixed objects such as buildings or radio telescopes that are not affected by weather or time changes. These methods will yield the same result within about 1 minute per million years.
The average distance between the Earth and the Sun is about 93 million miles (150 million km). Because of this distance, the Earth orbits the Sun once every 365.256 days on average. But due to gravitational forces from planets outside of our own, the Earth's orbit isn't exactly circular. It's an ellipse with the Sun at one of its foci.
The axis of the Earth goes from the North Pole to the South Pole. One complete revolution of the Earth around this invisible line takes 24 hours, or one day. Each portion of the Earth's surface gets a chance to face and be warmed by the sun as it spins. The part that is turned toward the sun will experience heat gain, while the part that is facing away from the sun will lose heat.
When the Earth spins, it also rotates about its own axis. The northern half of the planet turns more slowly than the southern half because it is not free to move as much during rotation. This effect is called "Northern Hemi-Sphere Geography." The result is that the north pole is closer to the center of mass of the Earth than is the south pole.
This imbalance in mass causes the northern hemisphere to rise when the earth spins, which is why most people see clouds going from west to east instead of east to west. If there were no hemispheres, everything would spin with the planet, and there would be no way to tell which direction was up unless something else was pointing up (such as a tree or mountain).
The axis of the Earth is at 90 degrees to the orbit plane of the Earth. When the Earth orbits the Sun, it does so in an almost flat plane.
This is critical for all life on Earth. Without this rotation, there would be no days and nights, and thus no solar power for any plants to use. Also without rotation, water would not circulate, and most living things would die.
As well as rotating, the Earth also revolves around its own axis, which is perpendicular to its orbit around the Sun. The plane of the Earth's equator moves through space with respect to the stars. As it does so, part of Antarctica faces the sun, another part faces away from it. Winter and summer cover these regions differently - the Antarctic winter contains four seasons, while the tropical summer only has two. The point at which the Earth's axis crosses its orbit is called the Equator. It is here that we find the greatest distance between the Earth's surface and its center - the average radius of the Earth is 6,371 miles (10,000 km).
At the Equator, all points are given equal weighting in relation to gravity. This is because there is no difference in height above sea level at the Equator compared to anywhere else on Earth.
A sidereal day is the length of time it takes the Earth to spin once on its axis, which is 23.9344696 hours. The hour hand on your watch or clock points to the same position in the sky during a sidereal night. It takes about 96 sidereal nights for the Earth to rotate completely around itself.
The angle made by the upper arm and forearm when they are extended horizontally, 0 degrees. At 90 degrees is a straight-outstretched position.
At a right angle to the wrist is the radial margin. This is the edge of the palm where the fingers begin. The palmar surface faces up towards the heart. The radial surface faces down towards the elbow.
The term "radial" comes from the Latin word radius, meaning "ray". Because the artery that supplies blood to the hand travels along the radial side of the wrist, doctors can use this vein as a guide to locate these vessels.
The term "ulnar" comes from the Latin word usus, meaning "use". Because the nerve that controls feeling in the hand runs along the ulnar side of the wrist, doctors can use this nerve to find it during surgery.