365 days a year, the earth rotates around the sun. When it rotates, different parts of the earth have different seasons. The tilt of the earth is exactly 23.5 degrees. If we did not have this tilt, we would have no seasons. Winter Solstice: Only December 21'rst in the Northern Hemisphere is the winter solstice. It is the Summer Solstice in the Southern Hemisphere when this happens. During this time the sun is pointed directly at the Tropic of Capricorn. That means that it's summer in the Southern Hemisphere and winter in the Northern Hemisphere.
Vernal Equinox: During the Vernal Equinox for the Northern Hemisphere, it is the Autumnal Equinox in the Southern Hemisphere. When it's the Vernal Equinox in the Northern Hemisphere, the sun is neither facing the top or the bottom of the earth; it is exactly lined up with the earth's midpoint, the equator.
Summer Solstice: During the Summer Solstice in the Northern Hemisphere, it's the Winter Solstice in the Southern Hemisphere. When it's Summer Solstice in the Northern Hemisphere, the sun is directly at the Tropic of Cancer. That means it's summer in the Northern Hemisphere, and winter in the Southern Hemisphere.
Autumnal Equinox: During the Autumnal Equinox for the Northern Hemisphere, it is the Vernal Equinox in the Southern Hemisphere. When this happens, the sun is lined up directly with the equator. This makes it autumn in the North, and spring in the South.
Those are the Reasons for the Seasons.