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The Seasons
The Earth has seasons because its axis of rotation is tilted with respect to the plane of its orbit. During the summer months in the Northern Hemisphere, the axis leans towards the Sun resulting in long days and short nights. The first day of summer occurs about June 21 each year; this is the summer solstice. At the same time in the Southern Hemisphere, the axis leans away from the Sun. The winter days are short and the nights are long. June 21 is the winter solstice in the Southern Hemisphere.
In six months the Earth is on the opposite side of its orbit. The axis has remained fixed in space, but from this position in the orbit, the northern tip of the axis now leans away from the Sun. This is the winter solstice. At this same time in the Southern Hemisphere, the axis tilts toward the Sun. Summer begins and the days grow long.
About March 21 the Earth is midway between winter and summer. The Sun lies on the celestial equator, and day and night are of equal length all over the Earth. This is the spring equinox (meaning “equal night”) when spring begins in the north and fall begins in the south. Around September 21 is the fall equinox when spring begins in the south and fall begins in the north.
The Seasons.
Throughout the year, the Sun appears alternately above and below the celestial equator for six month intervals. When the Sun is above the celestial equator and high in the sky in the Northern Hemisphere, it is low in the sky in the Southern Hemisphere. Seasons alternate between the hemispheres every six months. Summer in Australia and South America is winter in North America and Europe.