Geographical image of the main view overlooking the dusk line of winter solstice and summer solstice.

Geographical images of the main view overlooking the winter solstice and summer solstice at dusk are as follows.

The dividing line between the daytime hemisphere and the night hemisphere is called the terminator line, also known as the twilight zone. Because the earth is a non-luminous and opaque sphere, the sun can only illuminate half the earth at the same time. The hemisphere facing away from the sun is daytime (day hemisphere), and the hemisphere facing away from the sun is night (night hemisphere).

On the second solstice, the maximum included angle is 23 26'. When the termination line intersects the meridian, the included angle is equal to the latitude value of the direct point of the sun at this time. If the terminal line passes through the pole and coincides with a meridian, the solar term is the vernal equinox and the sun shines directly on the equator.

If the termination line is tangent to the polar circle, that is, the angle of intersection with the earth axis is the largest, it can be divided into two situations: first, the arctic circle and its north are extremely daytime, then the summer solstice, and the sun is directly on the tropic of cancer; The second is the polar night in the Arctic Circle and the north, which is the winter solstice. The sun shines directly on the tropic of Capricorn.

From summer solstice, the sun shines directly on the Tropic of Cancer, and at noon, the height of the sun decreases from the Tropic of Cancer to the north and south sides. (3) On the solstice in summer, the days are short and the nights are long everywhere in the southern hemisphere. Point A is located in the Antarctic circle, so a day is 0 hours. Point B is located at the equator, and a day is 12 hours.