Spring equinox, summer solstice, autumn equinox, direct sunlight from winter solstice, polar day and night, the length of day and night in the northern and southern hemispheres,

Vernal equinox: the sun shines directly on the equator, the hard hemisphere is equally divided day and night, and there is no extreme day and night in the north and south poles.

Summer solstice: the days in the northern hemisphere are long and the nights are short, while the days in the southern hemisphere are short and the nights are long. Extreme day appears from the Arctic Circle to the North Pole, and extreme night appears from the Antarctic Circle to the South Pole.

Winter solstice: the days in the northern hemisphere are short and the nights are long, while the days in the southern hemisphere are long and the nights are short. Extreme night occurs from the Arctic Circle to the North Pole, and extreme day occurs from the Antarctic Circle to the South Pole.