This article discusses the effects of El Niño, a serious dislocation of the world’s weather system that occurs every few years. In 1982 and 1983, El Niño was particularly severe, and this account describes the damage it caused in many … Continue reading
The geological evolution of Africa has been hinged on stability. Africa contains three major cratons, or areas of basement-complex rock that have been geologically stable for hundreds of millions of years. The Kalahari craton is located in southern Africa, the Congo craton is in Central Africa, and the northwest African craton, forming the core of West Africa, is centered in the Western Sahara. Areas between the cratons contain somewhat younger rocks. These areas have undergone more extensive and continuing geological change since the late Precambrian Period, caused by processes such as faulting, volcanism, folding, and crustal displacement. The stability of the areas of basement-complex rock has helped define the geological evolution of Africa.
Acid rain and the dry deposition of acidic particles damage buildings, statues, automobiles, and other structures made of stone, metal, or any other material exposed to weather for long periods. The corrosive damage can be expensive and, in cities with very historic buildings, tragic. Both the Parthenon in Athens, Greece, and the Taj Mahal in Agra, India, is deteriorating due to acid pollution.
Alpha Centauri is the closest star system to Earth and third brightest star in the sky. Alpha Centauri is located in the constellation Centaurus and is sometimes called Rigil Kentaurus, which literally means “foot of the centaur.” Alpha Centauri is actually a triple star system that appears as a single point of light because its two largest and brightest members, Alpha Centauri A and Alpha Centauri B, are too close together for the naked eye to tell them apart and its third member, Alpha Centauri C, is too small and dim to be seen at all. The Alpha Centauri system is only visible from the Southern Hemisphere and the southernmost portion of the Northern Hemisphere. In Texas, Louisiana, and Florida, for example, it appears very low in the southern sky and is most easily visible in May.
Alpha Centauri A and Alpha Centauri B lie about 4.35 light-years from Earth. A light-year is the distance light travels in a year, equal to about 9,460 billion km (5,880 billion mi). The two stars circle their common center of gravity, a point in space between them, about once every 80 years. The average distance between A and B is about 3.6 billion km (2.2 billion mi), which is a bit more than the average distance between the Sun and the planet Uranus. Alpha Centauri C orbits A and B at a tremendous distance—about 1,500 billion km (930 billion mi)—so far out that C takes millions of years to circle the two larger stars. Alpha Centauri C is also called Proxima Centauri; in the current portion of its orbit it is the closest star to the solar system, at a distance of 4.2 light-years.
Alpha Centauri A is a yellow star, slightly larger and brighter than the Sun, of spectral type G2 and apparent magnitude +0.01. Spectral type indicates a star’s surface temperature and the predominant color of the light it gives off. Apparent magnitude is a measure of how bright stars appear in the sky—a small, nearby star may appear just as bright as a much larger star that is farther away. The lower the apparent magnitude, the brighter the star appears. Alpha Centauri B is a yellow-orange star somewhat smaller and cooler than the Sun of spectral type K1 and apparent magnitude +1.34. Proxima Centauri is a red dwarf star of spectral type M5, much smaller and cooler than the Sun.