Solstice

Solstice, in astronomy, refers to the two points in the ecliptic for which the sun is the farthest distance from the celestial equator. Thus, it also refers to the two dates of the year on which the sun reaches its northernmost (summer solstice) and southernmost (winter solstice) declinations (declination is the celestial equivalent of latitude; these seasons apply only to the northern hemisphere—being opposites for the southern hemisphere). These two dates are, respectively, June 21st or 22nd, and December 21st or 22nd for persons living in the northern hemisphere. Thus, the solstices and the equinoxes (when the sun is directly above the equator) are associated to the seasons on Earth. The word solstice is derived from the Latin words sol sistere for sun stands still.

During the spring humans living in the northern hemisphere 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; that is, it is not perpendicular to its orbital plane. The rotational axis makes an angle of about 23.44° to its orbital plane (what is called the obliquity of the ecliptic). Although Earth varies in its distance to the sun throughout the year, this variation does not contribute to the change of seasons as much as the tilt of Earth.

As the 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.

There are two solstices every year. One occurs on or around June 21, and it is the time of year when the daylight hours are usually 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, Australia. 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. In 2004, the Summer Olympics were held in Athens, Greece, which is located fairly close to the equator. Thus, its location made it more mild for its summer events.

Earth’s solstice will change over the course of thousands of years. This change is due to the variability of the tilt. Although it is now about 23.44°, Earth’s tilt varies from 22.1 to 24.5°. This variation will cause the seasons to be opposite from what they are now in about 10,000 years—winter will occur beginning in June for the northern hemisphere instead of December.

Jeffrey Hall

Solstice

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

sol·stice / ˈsōlstis/ • n. either of the two times in the year, thesummer solstice and thewinter solstice, when the sun reaches its highest or lowest point in the sky at noon, marked by the longest and shortest days.DERIVATIVES: sol·sti·tial / sōlˈstishəl/ adj.

solstice Either of the two days each year when the Sun is at its greatest angular distance from the celestial equator, leading to the longest day and shortest night (summer solstice) in one hemisphere of the Earth, and the shortest day and longest night (winter solstice) in the other hemisphere. In the Northern Hemisphere, the summer solstice occurs around June 21, and the winter solstice around December 22.

solstice either of the two times in the year, the summer solstice and the winter solstice, when the sun reaches its highest or lowest point in the sky at noon, marked by the longest and shortest days. Recorded from Middle English, the word comes via Old French from Latin solstitium, from sol ‘sun’ + stit- ‘stopped, stationary’.