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The Latin meaning of equinox is "equal night," the times of the year when day and night are equal in length. In astronomy, the equinox is the point at which the Sun appears to cross the equator as a result of Earth's rotation around the Sun. The vernal equinox, which occurs as the Sun moves from south to north across the equator, takes place around March 21 and marks the beginning of spring. On about September 23, the Sun moves from north to south across the equator, marking the autumnal equinox and beginning of autumn. It is important to realize that the Sun does not actually move; its apparent path is a reflection of Earth's orbital rotation about the sun, and the tilt of Earth's axis.

When you stand on Earth and gaze upward on a clear night, you see the sky as part of a giant sphere that surrounds Earth. Although we know it is Earth that rotates, it appears as though this star-bearing dome turns about us. Early astronomers thought the stars were attached to this giant sphere. Today, astronomers still find it useful to imagine a celestial sphere that surrounds Earth. The extension of Earth's north and south poles extend to the north and south celestial poles, and Earth's equator can be projected outward to the celestial equator. Time and horizontal angles are measured eastward from the vernal equinox—the point where the Sun crosses the celestial equator in March—and vertical angles are measured north or south of the celestial equator.

Earth's axis of rotation is tilted 23.5° to the plane of its orbit. This tilt causes the seasons, and (from our frame of reference) it makes the Sun and the planets, which have orbital planes parallel to Earth's appear to move north and south during the course of the year along a path called the ecliptic. Because the ecliptic is tipped relative to Earth's equator, it is also tipped relative to the celestial equator. The two points where the ecliptic intercepts the celestial sphere are the equinoxes. When the Sun reaches either equinox, it rises in a direction that is due east everywhere on Earth. After the vernal equinox, the sun continues to move northward along the ecliptic and rise a little farther north of east each day until it reaches the summer solstice—a point 23.5° above the equator—around June 22. The summer solstice marks the beginning of summer, after which, the Sun begins to move southward. It crosses the celestial equator (the autumnal equinox) and continues to move southward and rise a little farther south of east each day until it is 23.5° south of the celestial equator at the winter solstice around December 22. It then begins its northward movement back to the vernal equinox.

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Science EncyclopediaScience & Philosophy: Ephemeris to Evolution - Historical Background