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Earth's orbit and solstices and equinoxes, illustration. The Earth is shown in its orbit (red ellipse) around the Sun (sizes and distances not to scale), with the Earth's axis of rotation (red) and axial tilt shown. The equator and axis of rotation are tilted at an angle of just over 23 degrees. This axial tilt gives rise to the Earth's seasons as it orbits the Sun during the year. The four points of the year shown here, and named from the perspective of the northern hemisphere, are (anti-clockwise from far left): the summer solstice, the autumnal equinox, the winter solstice, and the vernal equinox (in spring). The solstices are the longest and shortest days of the year. The equinoxes are the two points of the year where the day and night are about the same length. (KEYSTONE/SCIENCE PHOTO LIBRARY/TIM BROWN/SCIENCE PHOTO LIBRARY)