solar cycle Period in which several important kinds of solar activity repeat, discovered in 1843 by Samuel Heinrich Schwabe (1789-1875). Lasting about 22 years on average, it includes two 11 -year cycles of sun¬ spots, whose magnetic polarities alternate between the Sun’s northern and southern hemispheres, and two peaks and two declines in the phenomena (e.g., solar prominences, auroras) that vary in the same period. Attempts have been made to connect the solar cycle to various other phenomena, including possible slight variations in the diameter of the Sun, sequences of annual growth rings in trees, and even the stock market’s rise and fall.
solar energy Radiation from the Sun that can produce heat, generate electricity, or cause chemical reactions. Solar collectors collect solar radiation and transfer it as heat to a carrier fluid. It can then be used for heating. Solar cells convert solar radiation directly into electricity by means of the photovoltaic effect. Solar energy is inexhaustible and non¬ polluting, but converting solar radiation to electricity is not yet commer¬ cially competitive, because of the high cost of producing large-scale solar cell arrays and the inherent inefficiency in converting light to electricity.
solar flare Sudden intense brightening of a small part of the Sun’s sur¬ face, often near a sunspot group. Flares develop in a few minutes and may last several hours, releasing intense X rays and streams of energetic par¬ ticles. They appear to be connected with changes in the Sun’s magnetic fields during the solar cycle. The ejected particles take a day or two to reach the vicinity of Earth, where they can disrupt radio communications and cause auroras, and may pose a radiation hazard to astronauts.
solar heating Use of solar radiation to heat water or air in buildings. There are two types: passive and active. Passive heating relies on archi¬ tectural design; the building’s siting, orientation, layout, materials, and construction are utilized to maximize the heating effect of sunlight fall-
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When sunlight strikes a solar cell, an electron is freed by the photoelectric effect. The two dissimilar semiconductors possess a natural difference in electric potential (voltage), which causes the electrons to flow through the external circuit, supplying power to the load. The flow of electricity results from the characteristics of the semi¬ conductors, and is powered entirely by light striking the cell.
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ing on it. A well-insulated building with a large south-facing window, for instance, can trap heat on sunny days and reduce reliance on gas, oil, or electricity. Brick, stone, or tile capacity walls are often incorporated to absorb the sun’s energy and radiate it into the interior, usually after a time lag of several hours. In active solar heating, mechanical means are used to collect, store, and distribute solar energy. In liquid-based systems, a blackened metal plate on the exterior absorbs sunlight and traps heat, which is transferred to a carrier fluid. Alternatively, fluid may be pumped through a glass tube or volume of space onto which sunlight has been focused by mirrors. After picking up heat from the collector, the warm fluid is pumped to an insulated storage tank. The system can supply a home with hot water from the tank or provide space heating with the warmed water flowing through tubes in floors and ceilings.
solar nebula Gaseous cloud from which, in the nebular hypothesis of the origin of the solar system, the Sun and planets formed by condensa¬ tion. In 1755 Immanuel Kant suggested that a nebula gradually pulled together by its own gravity developed into the Sun and planets. Pierre- Simon, marquis de Laplace, in 1796 proposed a similar model, in which a rotating and contracting cloud of gas—the young Sun—shed concentric rings of matter that condensed into the planets. But James Clerk Maxwell showed that, if all the matter in the known planets had once been distrib¬ uted this way, shearing forces would have prevented such condensation. Another objection was that the Sun has less anguar momentum than the theory seems to require. In the early 20th century most astronomers pre¬ ferred the collision theory: that the planets formed as a result of a close approach to the Sun by another star. Eventually, however, stronger objec¬ tions were mounted to the collision theory than to the nebular hypothesis, and a modified version of the latter—in which a rotating disk of matter gave rise to the planets through successively larger agglomerations, from dust grains through panetesimals and protoplanets—became the prevailing theory of the solar system’s origin.
solar neutrino problem See neutrino problem, soar
solar prominence Arched stream of hot gas projecting from the Sun’s surface into the chromosphere or corona. Prominences can be hundreds of thousands of miles long and can be seen with the unaided eye during a total eclipse. They appear to lie along and are supported by loops in the Sun’s magnetic field, where they may remain for days to months.
solar system The Sun, its panets, and the small bodies (see asteroid, Centaur object, comet, Kuiper belt, meteorite, and Oort cloud) interplanetary dust and gas under the Sun’s gravitational control. Another component of the solar system is the soar wind. The Sun contains more than 99% of the mass of the solar system; most of the rest is distributed among the nine planets, with Jupiter containing about 70%. According to the prevail¬ ing theory, the solar system originated from the soar nebua. See also Earth, Mars, Mercury, Neptune, Pluto, Saturn, Uranus, Venus.
solar wind Flux of particles, chiefly protons, electrons, and helium nuclei accelerated by the hot solar corona’s high temperatures to speeds high enough to allow them to escape the Sun. Soar fares increase its intensity. The solar wind deflects planets’ magnetospheres and the ion tails of comets away from the Sun. The uninterrupted portion of the solar wind continues to travel to a distance of about 20 astronomical units, where it cools and eventually diffuses into interstellar space. See also heliopause.
soldering \'sa-d3r-iq\ Process that uses metal alloys with low melting points to join metallic surfaces without melting them. Tin-lead solders, once widely used in the electrical and plumbing industries, are now replaced by lead-free alloys. Such alloys are also used to solder brass and copper automobile radiators. Solders are supplied in wire, bar, or premixed-paste form, depending on the application. Soldering can be car¬ ried out using a torch, a soldering iron, a flame heater, or an induction heater. See also brazing, flux.
sole Any of several fatfishes, especially about 100 species in the family Soleidae. Those found from Europe to Australia and Japan are marine; some New World species live in freshwater. The eyes are on the right side of the head. The Dover sole (Solea soled), found from estuaries to offshore waters in the eastern Atlantic and Mediterranean, grows to 20 in. (50 cm) long. The hogchoker (Trinectes maculatus), seldom over 10 in. (25 cm) long, is found in shallow coastal waters from New England to Central America and far inland in habitats associated with large rivers.
Solemn League and Covenant (1643) Agreement between the English and Scots in which the Scots agreed to support the English Parlia¬ mentarians in their disputes with the Royalists, and both countries agreed to work for a civil and religious union of England, Scotland, and Ireland under a presbyterian-parliamentary system. The Scots sent an army to England in 1644, and Charles I surrendered to them in 1646. He later agreed to the covenant and received Scottish military assistance (1647). Neither Oliver Cromwell’s Commonwealth nor Charles II (after the 1660 Restora¬ tion) honoured the covenant, and it was not renewed. See also Covenanter.
Solent Vso-lontV The Strait of the English Channel. It extends 15 mi (24 km) between mainland England and the Isle of Wight and varies in width from 2 to 5 mi (3 to 8 km). The submerged valley of a former eastward¬ flowing river, it is the scene of yacht races and is famous for the naval reviews off Spithead.