Day and night are caused by the movement of the sun. A day is the amount of time it takes the Earth to orbit the sun. A day is the time it takes the sun to orbit the Earth. When the moon obscures the sun, it is said to be night. When the sun is obscured by clouds, it is said to be night. However, when the moon is new or there is no moonlight, it is called a half-moon. The word "new" here means that the moon has not yet finished orbiting around the Earth after its last orbit (called an elliptical orbit). When the moon is full, it is called a moonlit night.
The sun appears to move across the sky because of how far we are from it. The sun appears to move slowly because it is so far away from us that even over a short period of time it covers a lot of ground. If the sun were as close as the moon, it would appear to move faster than the moon because all parts of its body would be visible from our location on the moon. As it is, only the part of the sun that faces us directly is visible even when it is daytime with no clouds in between us and the sun.
The sun appears to move around the earth because of where we are located in relation to the center of the planet. If we were anywhere else, the sun would still travel through space but at a different angle, causing different times of day and night throughout the year.
The positions of the different sites on Earth fluctuate in reference to the Sun as the Earth revolves on its axis. A city on Earth that faces the sun at midday will rotate 12 hours later to face away from the sun. Sunrise, sunset, day, and night are caused by the locations of the Earth and the Sun throughout the course of a 24-hour revolution.
Sunrise and sunset times vary according to where you are on Earth. If you're standing on the eastern shore of a continent about 30 degrees north of the equator, then sunrise this morning would have been around 5:00 AM and sunset last evening would have been about 7:00 PM. Eastern Daylight Time (EDT) starts early; if it's 5:00 AM now, then sunrise tomorrow will be around 2:00 PM.
If you're standing on the western shore of that same continent about 30 degrees south of the equator, then sunrise this morning would have been around 2:00 PM and sunset yesterday would have been around 7:00 PM. Western Daylight Time (WDT) starts late; if it's 2:00 PM now, then sunrise tomorrow will be around 5:00 AM.
The effect of direction on sunrise and sunset times is most noticeable near the poles, where days are either long or short depending on whether you're facing east or west. At the North Pole, daytime begins when the Sun is over the horizon and ends when it's below it.
Clearly, when the sun shines in the sky, it is day; when it does not, it is darkness. So the actual question is, "Why is the sun in the sky at times when it isn't?" The sun seems to travel across the sky in the morning, then toward the south around midday, and finally toward the west as the day finishes. But why do these movements occur?
The sun's path across the sky is called its ecliptic. The word means "scratch" in Greek. You can think of the Earth like a big turntable with our planet's axis pointing due north. When the Earth turns once on its axis, it will have traveled half of its orbit around the Sun, and that's how far north we would be if we could see out into space from where we stood. But since we can't, we would need to turn east or west to see farther distances.
However, there are other planets in the solar system that don't follow this rule. They always face the Sun no matter what angle they are at in their orbits. So if you were on one of these planets, you would always see the Sun rise in the east and set in the west, regardless of where it was in its yearly orbit.
These other planets are called axial planets because they have axes that are parallel to their orbital planes. The only planet that matches this description is Uranus. All the others rotate on an axis that is not perpendicular to their orbits.
The Sun looks to be in perpetual motion, rising on one side of the sky, travelling across it, and setting on the other. The apparent motion across the sky is caused by the Earth's rotation. Over the course of a year, these two motions impact the Sun's shifting location in the sky as well as the timings of daily sunrises and sunsets.
The true location of the Sun is actually quite constant. It rises about 49 degrees above the horizon in the east and sets about 48 degrees below the horizon in the west. Because of the curvature of the Earth, these angles are not linear distances but rather circles centered on the Earth with their axes pointing towards the North Star. By measuring the angle between the Sun and a fixed point such as a north star, we can determine its position in space.
By measuring the angle between the Sun and the Moon, we can also find out when the new Moon was formed. As we know, the full Moon is completely covered by Earth's shadow so it cannot reflect any sunlight back to us. However, if we look at pictures taken on either side of the lunar eclipse, then we can see that there is a dark region on one side of the Moon. This shows that some of the Moon is outside of Earth's shadow and so able to reflect light from the Sun back to us. Scientists use this fact to create maps of Earth's atmosphere at different times during a lunar eclipse. They do this by taking regular photographs of the eclipsed Moon and using computer software to analyze the images.
The motions of the Earth relative to the Sun, as well as the motions of the Moon and Sun relative to the Earth, impact several phenomena on Earth, such as day and night, seasons, tides, and Moon phases. These effects are called "Solar Physics."
Day and night happen because the angle between the Earth's axis and its orbit around the Sun is not constant: when the Earth is at its closest approach to the Sun (perihelion), it is tilted away from the Sun by about 23.5 degrees; at its farthest approach (aphelion), it is still tilted away from the Sun by about 23.5 degrees, but less strongly so.
This changing angle causes differences in sunlight across the planet: during perihelion, the amount of sunlight reaching a point on the Earth increases while during aphelion it decreases. This is why nights are longer when the Earth is closer to the Sun and days are shorter when it is farther away. The amount of daylight diminishes even more rapidly than this due to other factors including atmospheric absorption and reflection of light.
The interval between one new moon and the next is called a lunar month, and the average distance that the Moon travels through this orbit is called its monthly mean motion.