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Solstice

sun hemisphere northern tilted

The term solstice refers to the two dates of the year on which the Sun reaches its northernmost and southernmost declinations (declination is the celestial equivalent of latitude).

During the spring we frequently hear someone remark that "the days are getting longer," or during the fall that they are getting shorter. This phenomenon occurs because Earth's rotational axis is tilted with respect to the plane of its orbit around the Sun. As Earth revolves around the Sun, the latitude that is directly facing the Sun (which defines the Sun's declination) changes. At one point in Earth's orbit, the northern hemisphere is tilted toward the Sun, and the Sun appears higher in the sky for northern latitudes; six months later, when Earth has moved around to the other side of its orbit, the northern hemisphere is tilted away from the Sun, and the Sun appears higher for southern latitudes. The solstices refer to the days on which the Sun's apparent northward or southward motion reverses direction. The word solstice itself is derived from two Latin words meaning "Sun stands."

There are two solstices every year. One occurs on or around June 21, and it is the time of year when the days are long and hot in the United States; Americans call this the summer solstice. It is just the opposite for Australians, however. If the northern hemisphere is tilted toward the Sun, the southern hemisphere must be tilted away; and indeed, June and July are the coolest months of the year in Sydney. Conversely, on or about December 21, the northern hemisphere reaches the winter solstice, when the Sun appears to trace its lowest path across the sky. At the same time, it is high summer in Australia. For this reason, the 2000 Summer Olympics, in Sydney, Australia, were scheduled for September rather than July as they were for the 1996 Atlanta, Georgia, games; most of the world's countries are in the northern hemisphere, and it would hardly have been proper to ask the cyclists and marathoners to be completing their preparations in January.

Jeffrey Hall

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