Our planet's travels through space around the sun create night and day, the four seasons, and the passage of the years. As a result, each night the stars appear to rise, span the sky, and set four minutes early. This equates to waking up an hour sooner after 15 days and two hours earlier after 30 days. After one month on Mars, the earth will appear to be rising earlier than it does now, but since Mars is farther from the sun, daytime will last for about half as long -- 3.4 days instead of 7.
The Earth's axis is not fixed in place, but rather spins around like a spinning top. This means that the appearance of the sky changes whenever we look at it from a location on Earth. If you were flying over Africa this morning at 10,000 feet, you would have seen something like this: When you look out the window now, you are looking down on eastern North America. In a few minutes, you will see western North America come into view.
In addition to daily and seasonal changes, there are also longer term changes to the night sky. Over time, the Earth's atmosphere gets thinner because of natural processes like wind erosion and human activity like pollution. This makes it easier for radio waves to reach space from the ground, so astronomers use satellites to study distant objects. Also, light bulbs emit mostly blue light these days because they are used widely for lighting.
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 next morning, the process begins again.
The sun's rays are called "ultraviolet" because they have a frequency higher than visible light but lower than X-rays or gamma radiation. Therefore, they can penetrate some materials such as glass but not others such as skin. Even though we are mostly made up of water, which itself is made up of two hydrogen atoms and one oxygen atom, most of the time our bodies are protected from the sun's ultraviolet rays because those rays cause cancer of skin cells. However, there are products in the environment that are capable of breaking down some of these rays before they reach the ground or ocean floor. One example is lightning. When clouds pass over a region with a high concentration of ozone molecules (such as an ozone hole) some of the ultraviolet rays are broken down before they reach the ground. Another example is volcanic eruptions. Volcanoes often emit gases during periods of activity that contain various concentrations of ozone-destroying chemicals.
The rotation of the Earth on its axis causes the variation between day and night. The durations of days and nights vary depending on where you are on Earth and the season. In addition, the tilt of the Earth's axis and its route around the sun influence daylight hours. The result is that there are two different lengths of night in any given month.
Daylight saving time (summer time) has been used by many countries for over a hundred years as an effective way of reducing energy consumption and thus reducing costs. During summer time, which starts at 2am on April 30th and ends at 2am on October 31st, clocks are put back one hour, thereby creating more daytime electricity. This saves electricity and oil resources that would otherwise be needed to produce light after sunset. It also reduces heat-related deaths from power outages caused by lack of air conditioning. Finally, it makes going to sleep later without losing daytime productivity possible.
During winter time, which starts at 2am on November 3rd and ends at 2am on March 31st, clocks are set forward one hour, thereby creating less nighttime electricity. This has the same benefits as summer time except that it saves energy instead of wasting it. Winter time can only be implemented during certain months due to astronomical calculations needed to determine when to start switching off lights at night.
We have day and night because the Earth spins (or rotates) on an imaginary line known as its axis, and various areas of the world face the Sun or away from it. It takes 24 hours for the Earth to complete one full rotation, which we call a day. Summer days are longer while winter days are shorter. The amount of time it takes for the Earth to rotate completely around itself is called a year. Spring and autumn have four months each, while summer and winter have five.
As the Earth orbits the sun, it passes through different parts of the solar system every month. The plane of the Earth's orbit around the sun is called its ecliptic, and it forms a straight line with the Earth's equator. The part of the ecliptic that we can see from Earth is called the celestial equator, and it forms a right angle with the horizon at any given location on Earth.
The position of the Earth in relation to the Sun changes throughout the year, so there are different lengths of day and night at each point on the planet. In spring and early summer, the Northern Hemisphere is tilted towards the Sun, so people there experience more daytime heat than people in the Southern Hemisphere who are tilted away from the Sun. In late summer and early fall, the situation is reversed: people in the Southern Hemisphere experience more daylight than people in the North who are now in darkness.