Alternate day and night

The alternation of day and night is 24 hours, which is a solar day and a flat solar day. This is not the real period of the sun's rotation. 23 hours, 56 minutes and 4 seconds is a sidereal day, which is the real cycle of the earth's rotation.

Summary:

At any moment, the day and night state of the earth can be expressed by the height of the sun. The height of the sun is the abbreviation of the solar altitude angle, which indicates the inclination of the sun's rays relative to the local ground level. In all parts of the hemisphere during the day, the height of the sun is always greater than 0 degrees, that is, the sun is above the horizon; In the whole termination line, the height of the sun is equal to 2 degrees, which means that the sun is just on the horizon.

Throughout the night hemisphere, the height of the sun is always less than 0 degrees, that is, the sun is below the horizon. Because the earth keeps turning and alternating day and night. The cycle of alternating day and night, or the daily variation cycle of solar height is 24 hours, which is called a solar day. Solar day restricts human daily life, so it is the basic unit of time.

In addition, the solar day is not long, so that the heating and cooling of the whole earth surface will not be too intense, thus ensuring the survival and development of life on earth.

Because of the rotation of the earth, the situation of day and night is different in different parts of the earth. Some are at noon, some are at midnight, and some are experiencing morning, dawn or dusk alternately. When the sun rises to the highest position in a day, the sun only shines on the meridian where the place is located. At this point, it is already local noon. The time so determined is called local time. For the longitude difference of 15 degrees, the local time difference is 1 hour.

Because the earth's axis is inclined, the length of day and night varies in different parts of the earth. At the north and south poles of the earth, the sun is oblique all year round, and the length of day and night changes the most. In the high latitudes of the northern and southern hemispheres, there will be a phenomenon that the sun never sets or stays out all day, that is, 24 hours a day is day or night, which is the polar "extreme day" and "extreme night" phenomenon. In the north and south poles, there are extreme days and nights for half a year.

Baidu encyclopedia-alternating day and night