Because the Earth revolves on its axis, we have day and night. The Earth's axis, on the other hand, is inclined at 23.5 degrees (the angle is measured between Earth's equatorial plane and the plane in which it circles our Sun). This means that the north pole is tilted toward the Sun during part of the year and away from it during another part, while the south pole faces away from the sun all the time.
As the northern hemisphere experiences spring and summer, the north pole is tilted away from the Sun, allowing sunlight to shine on it for more hours each day. As autumn approaches and winter sets in, the north pole begins to tilt back toward the Sun, bringing nighttime events once again.
This seasonal change is what causes seasons in the Northern Hemisphere. During a season, certain regions of the planet experience light days and nights because of their location with respect to the Sun. For example, the far north and far south poles are always dark because they are too far away from the Sun to experience any significant amount of sunlight during most years. Regions in between have orbits that bring them closer or further from the Sun throughout the year. These areas will eventually experience spring and fall, but not until after several months have passed.
In the Southern Hemisphere, things work slightly differently because the south pole is tilted toward the Sun all year round.
Because of this tilt and the Earth's journey around the Sun, there are times when the north pole of the Earth is inclined 23.5 degrees toward the Sun. This is called the winter solstice. At other times the north pole is tilted away from the Sun. These are called the summer solstice.
The amount that the north pole is tilted toward the Sun at a given time of year is called the latitude of the location. The farther north you go, the longer the days are. The closer you go to the Arctic Circle, the shorter the days are. Near the Antarctic Circle, the days are long enough for nautical matters!
At the winter solstice, the sun is at its farthest south position and gets lower in the sky every day after that. At the summer solstice, it is at its farthest north position and stays up later at night than any other time of year.
These changes in the angle of the north pole with respect to the Sun lead to differences in how much daylight is present at different times of year. In addition, because the Earth goes through these changes more rapidly near the poles, there are also differences between day and night temperatures. In general, the farther from the equator you go, the greater the difference between daytime and nighttime temperatures.
Day and darkness are caused by the earth's rotation on its axis. As the planet rotates, so too do all of its parts, including the earth's surface. The axial tilt results in the formation of both the Arctic and Antarctic circles.
Earth's rotation is driven by its mass: it takes 24 hours to complete one rotation because it takes that long for all of earth to spin once on its axis. If earth did not have any atmosphere or water, it would still take 24 hours to rotate completely due to the presence of its mass. However, because it does have an atmosphere and oceans, some scientists say this effect is not fully realized because air and water resist change, while gravity always acts on everything within it.
The result is that the northern half of the planet is exposed to sunlight for a larger part of the year than the southern half, which results in the formation of ice caps in the north and deserts in the south. This is why winter is cold where it is dark and summer is hot where it is light.
If earth's axis were perfectly perpendicular to its orbital plane, then there would be no seasonal variation in climate.
Every twenty-four hours, the world rotates on its axis. When one side of the earth is facing the sun, it is daylight on that side and darkness on the opposing side. The sun seems to rise in the east in the morning and set in the west in the evening because to the way the globe spins. During the day, light travels through space until it reaches the dark side of the earth, where it is reflected back into space. At night, light travels through space until it reaches the bright side of the earth, where it is absorbed. The result is that the eastern part of the earth is illuminated during the day and the western part at night.
The sun's rays are called "photons" which means "light particles." Photons have energy, but they can't do anything by themselves. They need something to trigger them off. For example, if you drop a book, it will break because the force of gravity does this all by itself, but photons from the sun trigger off molecules in the book, causing them to vibrate, which in turn causes the book cover to split open.
At night, when there is no sunlight, we don't see any stars because starlight only makes it out as far as the atmosphere allows. So, for example, if you were on a plane and looked down, you would see the earth with no starlight above it because there are no stars that are close enough to light up over the surface of the earth at night.