The earth rotates once, and the day and night cycle once, forming a day and night. Due to the existence of the angle of intersection between the yellow and the red, except for the autumnal equinox and the vernal equinox on the equator, the day and night arcs are not equal in length everywhere. When the night arc is larger than the day arc, the night is longer and the day is shorter. Conversely, with the earth's revolution, the length of day and night changes constantly.
What is the reason for the change of the length of day and night
The earth is not only divided into day and night hemispheres in space, but also because of the rotation and revolution of the earth, the two hemispheres of day and night constantly alternate in time, so that each place is sometimes located in the day hemisphere, thus experiencing the day; Sometimes it is in the night hemisphere, so it experiences darkness. This is called alternation of day and night. The cycle of alternation of day and night is usually called the first day, which is different from the star day. The star day is a simple period of the earth's rotation, and the sun day is formed not only because of the earth's rotation, but also because of the earth's revolution, which is the convergence period of the two.
The length of the day and night depends on the situation that the twilight circle divides the weft. Generally, the latitude line is divided into two parts by the twilight circle: the part located in the day hemisphere is called the day arc; The part of the night hemisphere is called the night arc. The arc length of the day arc and the night arc determines the day length and night length of the place: the arc length is 15 °, which is equivalent to 1 hour.
The length of days and nights varies with the movement of the sun's direct point of view:
When the direct sunlight point falls on the equator (spring and autumn equinoxes), the twilight circle passes through the two poles (coincident with the meridian circle) and bisects all the latitude lines. Therefore, the day and night are equal all over the world.
What is the change rule of the length of day and night
1. The spring equinox points to the summer solstice
Northern Hemisphere: long days and short nights
The length of day and night at the spring equinox is equal, and the length of day continues to increase, and the longest day and the shortest night are at the summer solstice. Polar days begin to appear in the Arctic at the spring equinox, and extend to the entire Arctic Circle at the summer solstice.
Southern Hemisphere: short days and long nights
The length of day and night at the spring equinox is equal, and the length of day continues to shorten, and the shortest day and the longest night are at the summer solstice. Polar night began to appear at the Antarctic Pole at the spring equinox, and extended to the entire Antarctic Circle at the summer solstice.
2. The summer solstice points to the autumnal equinox
Northern Hemisphere: long days and short nights
The length of days continues to shorten until the autumnal equinox when the length of days and nights is equal. At the summer solstice, the polar day range of the Arctic began to narrow, and the autumnal equinox completely disappeared.
Southern Hemisphere: short days and long nights
The length of day continues to increase, and the length of day and night is equal until the autumnal equinox. At the summer solstice, the Antarctic polar day range began to narrow, and the autumnal equinox completely disappeared.
3. The autumnal equinox points to the winter solstice
Northern Hemisphere: short days and long nights
The day length continues to shorten, with the shortest day and the longest night until the winter solstice. Polar night began to appear in the Arctic at the autumnal equinox, and extended to the entire Arctic Circle at the winter solstice.
Southern Hemisphere: long days and short nights
The day length continues to increase, with the longest day and the shortest night until the winter solstice. Polar days begin to appear in the Antarctic at the autumnal equinox, and extend to the entire Antarctic circle at the winter solstice.
4. The winter solstice points to the spring equinox
Northern Hemisphere: short days and long nights
The length of day continues to increase, and the length of day and night is equal until the vernal equinox. At the winter solstice, the polar night range in the Arctic began to shrink, and at the vernal equinox, it completely disappeared.
Southern Hemisphere: long days and short nights
The length of the day continued to decrease until the vernal equinox, when the length of the day and the night were equal. At the winter solstice, the polar day range in the Antarctic began to narrow, and the autumnal equinox completely disappeared.
