Popular science
The phenomenon of day and night on Earth is caused by the combined effect of sunlight and the rotation of the Earth. This phenomenon causes every region of the planet to experience a day and a night in a 24-hour period. So, how does day and night change?
1. The rotation of the earth
The Earth rotates from west to east on its axis of rotation with a rotation period of about 24 hours. Since the Earth is an opaque, non-luminous sphere, sunlight can only illuminate half of the Earth. When sunlight hits an area, that area is daytime; When sunlight can't reach an area, it's night.
Second, the morning and dusk line
The rotation of the Earth leads to the creation of the morning and dusk lines. The morning and dusk line is the dividing line between day and night on Earth, also known as the day and night line. The morning and dusk line is located on the surface of the earth and circles the earth. On the morning and dusk line, the sun is just above the horizon.
On the morning and dusk lines, the transition between day and night is very rapid. As you move from east to west along the line of morning and dusk, you will first experience dawn, then day, then evening, and finally night. Conversely, when moving from west to east along the line of morning and dusk, you go through night first, then into evening, then dawn, and finally into day.
3. Seasonal variations
There is an angle between the Earth's axis of rotation and the Earth's orbital plane (ecliptic plane), which causes the direct point of the Sun to move on the Earth's surface during the Earth's revolution. This phenomenon leads to a change in the seasons and also affects the length of days and nights.
At the spring and autumn equinoxes, the sun shines directly on the equator, and day and night are equal in length around the world. During the summer solstice, the sun shines directly on the Tropic of Capricorn, and the Northern Hemisphere has the longest day hours and the shortest night hours; The Southern Hemisphere, on the other hand, has the longest nights and the shortest day. On the winter solstice, the sun shines directly on the Tropic of Capricorn, and the southern hemisphere has the longest day hours and the shortest night hours; The Northern Hemisphere, on the other hand, has the longest nights and the shortest day.
4. Polar day and polar night
In the polar regions of the Earth, the transition between day and night becomes more extreme. Within the Antarctic and Arctic Circles, polar day and night phenomena occur. During the polar day, the sun is always above the horizon and lasts for 24 hours during the day; During the polar night, the sun is always below the horizon and the night lasts for 24 hours.
The polar day and polar night phenomena are formed by the tilt of the Earth's axis of rotation causing the direct point of the Sun to move between the Tropic of Cancer. During the polar day, the solar altitude angle in the polar regions is very large, allowing the region to be exposed to sunlight for 24 hours. During the polar night, the solar altitude angle in the polar regions is very small, making the region unexposed to sunlight for 24 hours.
5. Summary
The transition of day to night is due to the combined action of the Earth's rotation and revolution. The Earth's rotation makes it possible for the sun's rays to illuminate only half of the Earth's, resulting in the phenomenon of day and night. The rotation of the Earth leads to a change of seasons, which in turn affects the length of days and nights. By studying the transition between day and night, we can better understand the Earth's movement in the solar system, as well as the Earth's climate and biological activity.