Precession of the equinoxes

Precession of the equinoxes, westward motion of the equinoxes along the ecliptic. This motion was first noted by Hipparchus c.120 B.C. The precession is due to the gravitational attraction of the moon and sun on the equatorial bulge of the earth, which causes the earth's axis to describe a cone in somewhat the same fashion as a spinning top. As a result, the celestial equator (see equatorial coordinate system), which lies in the plane of the earth's equator, moves on the celestial sphere, while the ecliptic, which lies in the plane of the earth's orbit around the sun, is not affected by this motion. The equinoxes, which lie at the intersections of the celestial equator and the ecliptic, thus move on the celestial sphere. Similarly, the celestial poles move in circles on the celestial sphere, so that there is a continual change in the star at or near one of these poles (see Polaris). After a period of about 26,000 years the equinoxes and poles lie once again at nearly the same points on the celestial sphere. Because the gravitational effects of the sun and moon are not always the same, there is some wobble in the motion of the earth's axis; this wobble, called nutation, causes the celestial poles to move, not in perfect circles, but in a series of S-shaped curves with a period of 18.6 years. There is some further precession caused by the gravitational influences of the other planets; this precession affects the earth's orbit around the sun and thus causes a shift of the ecliptic on the celestial sphere. The precession of the earth's orbital plane is sometimes called planetary precession, and that of the earth's equatorial plane (caused by the sun and moon) is called luni-solar precession; the combined effect of the moon, the sun, and the planets is called general precession. Planetary precession is much less than luni-solar precession. The precession of the equinoxes was first explained by Isaac Newton in 1687.

Equinox

Equinox (ē`kwĭnŏks), either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. The vernal equinox, also known as "the first point of Aries," is the point at which the sun appears to cross the celestial equator from south to north. This occurs about Mar. 21, marking the beginning of spring in the Northern Hemisphere. At the autumnal equinox, about Sept. 23, the sun again appears to cross the celestial equator, this time from north to south; this marks the beginning of autumn in the Northern Hemisphere. On the date of either equinox, night and day are of equal length (12 hr each) in all parts of the world; the word equinox is often used to refer to either of these dates. The equinoxes are not fixed points on the celestial sphere but move westward along the ecliptic, passing through all the constellations of the zodiac in 26,000 years. This motion is called the precession of the equinoxes. The vernal equinox is a reference point in the equatorial coordinate system.

Solstice

Solstice (sŏl`stĭs) [Lat.,=sun stands still], in astronomy, either of the two points on the ecliptic that lie midway between the equinoxes (separated from them by an angular distance of 90°). At the solstices the sun's apparent position on the celestial sphere reaches its greatest distance above or below the celestial equator (see equatorial coordinate system), about 23 1-2° of arc. At the time of summer solstice, about June 22, the sun is directly overhead at noon at the Tropic of Cancer (see tropics). In the Northern Hemisphere the longest day and shortest night of the year occur on this date, marking the beginning of summer. At winter solstice, about Dec. 22, the sun is overhead at noon at the Tropic of Capricorn; this marks the beginning of winter in the Northern Hemisphere. For several days before and after each solstice the sun appears to stand still in the sky, i.e., its noontime elevation does not seem to change from day to day.

Tropics

Tropics, also called tropical zone or torrid zone, all the land and water of the earth situated between the Tropic of Cancer at lat. 23 1-2°N and the Tropic of Capricorn at lat. 23 1-2°S. Every point within the tropics receives the perpendicular rays of the sun at noon on at least one day of the year. The sun is directly overhead at lat. 23 1-2°N on June 21 or 22, the summer solstice, and at lat. 23 1-2°S on Dec. 21 or 22, the winter solstice. Since the entire tropical zone receives the rays of the sun more directly than areas in higher latitudes, the average annual temperature of the tropics is higher and the seasonal change of temperature is less than in other zones. The seasons in the tropics are not marked by temperature but by the combination of trade winds taking water from the oceans and creating seasonal rains called monsoons over the eastern coasts. Several different climatic types can be distinguished within the tropical belt, since latitude is only one of the many factors determining climate in the tropics. Distance from the ocean, prevailing wind conditions, and elevation are all contributing elements. The tropics contain the world's largest regions of tropical rain-forest climate (Amazon and Congo basins). These lush rain-forest regions, whose immense vegetation growth is attributed to monsoon rains, contain some of the most prolific and widely speciated regions on earth for a wide variety of flora and fauna. Toward the northern and southern limits are low-latitude savanna, steppe, and desert climates (with decreasing seasonal rainfall). Tropical highland climates, which have the characteristics of temperate climates, also occur where high mountain ranges lie in the zone. High temperatures and rainfall make rubber, tea, coffee, cocoa, spices, bananas, pineapples, oils and nuts, and lumber the leading agricultural exports of the countries in the tropical zone. Progress in tropical medicine, advancing technology, and the pressure of increasing populations have led in recent years to the cultivation and settlement of some rain-forest areas. Such population growth has led to deforestation of the tropical forest, which is thought to contribute to the greenhouse effect and global warming, and to the elimination of numerous unique species.

extrasolar planet

Extrasolar planet (also called exoplanet), planet that orbits a star other than the Sun. The existence of extrasolar planets, many light-years from Earth, was confirmed in 1992 with the detection of three bodies circling a pulsar. The first planet revolving around a more sunlike star, 51 Pegasi, was reported in 1995. Over 200 stars with one or more planets are known. Current detection methods, based on the planets' gravitational effects on the stars they orbit, have revealed only planets much more massive than Earth; some are several times the size of Jupiter. A number of them have highly elliptical orbits, and many are closer to their stars than Mercury is to the Sun. These findings have raised questions regarding astronomers' ideas of how Earth's solar system formed and how typical it is.