Why are the days short in winter and long in summer?

The sun is in the Tropic of Cancer in summer, the Tropic of Cancer in winter and the equator in spring and summer.

The heat and cold in summer and winter are not determined by the distance between the earth and the sun, because the earth's orbit is almost circular, mainly because of the length of the sun's exposure time.

The autobiographical axis of the earth is not perpendicular to the orbital plane around the sun, and there is an included angle of 66.5 degrees. In this way, there is a period of illumination time at different positions and latitudes in the earth's orbit. In summer, the northern hemisphere has a long illumination time, while the southern hemisphere has a winter illumination time, so there are summer solstice and winter solstice. Only in the vernal equinox and the autumnal equinox, the illumination time in the northern hemisphere is the same.

Specifically, on the summer solstice, the Tropic of Cancer becomes the equator (the Tropic of Cancer is a line with 23.5 degrees north latitude, plus the autobiographical axis of 66.5 degrees, which is exactly 90 degrees perpendicular to the orbital plane), while on the winter solstice, the Tropic of Cancer becomes the actual equator, and the southern hemisphere becomes summer.

As for the long days in summer and short days in winter, it is directly related to latitude. The closer you get to the poles, the more obvious it is, and it is not obvious near the equator. This is related to the sphere. In the Arctic Circle, from the vernal equinox to the autumnal equinox, there is no night, on the contrary, there is half a year's night, because the sun shines for a long time in summer.