Kids Research Express

At any given time, there are two high tides on the earth, the direct tide on the side facing the moon and the indirect tide on the opposite side. As the earth rotates on its axis, the location of the two diametrically opposed tidal bulges varies on the earth's surface. The earth's rotation and the moon's revolution, which have the same direction, bring each point on the earth opposite the moon once every 24 hr and 50 min. Therefore, the average interval between direct and indirect high tides is about 12 hr and 25 min. In many places along the Atlantic coasts of N America and Europe, the two daily low tides are of nearly equal duration and magnitude, called semidiurnal tides. In certain shallow seas and narrow estuaries, the tides differ from this simple pattern. For example, in certain regions such as the Pacific coast of N America, one of the two daily tides is appreciably higher than the other or the interval between successive tides is unequal; these are called mixed ti

Tide, alternate and regular rise and fall of sea level in oceans and other large bodies of water. These changes are caused by the gravitational attraction of the moon and, to a lesser extent, of the sun on the earth. More generally, tides are the deformations of celestial bodies from a perfectly spherical shape that result from stresses created by their mutual gravitational attraction (see gravitation ). Another way of viewing the tide is as the longest possible ocean wave, one which stretches all the way around the earth. The tide regarded as a wave is sometimes referred to as a tidal wave , although this term has been commonly applied to the shock wave propagated by an underwater earthquake. (To avoid confusion, such shock waves are now called tsunamis , their Japanese name, or seismic sea waves.) Numerous schemes have been proposed to harness the earth's tides, especially in various estuaries, as a practical source of power, but none as yet have proved economically or technolog

Sea level, the level of the sea, which serves as the datum used for measurement of land elevations and ocean depths. Theoretically, one would expect sea level to be a fixed and permanent horizontal surface on the face of the earth, and as a starting approximation, this is true. However, a number of factors operate to cause variations in sea level ranging up to several meters from place to place and to cause long-term global variations, often severe enough to cause flooding and damage to coastal zones. Sea levels vary greatly from one location to another, i.e., between Nova Scotia and Florida sea-level heights differ at about 16 in (40 cm). Locally the levels of the surface of the world's oceans are disturbed by wind-driven waves and tides . Sea level therefore fluctuates in periods ranging from seconds to a year as a result of these factors. Thus for some purposes it is necessary to know the mean sea level (MSL) in a particular area, determined by averaging the elevations of the se

Front, in meteorology, zone of transition between adjacent air masses . If a cold air mass is advancing to replace a warmer one, their mutual boundary is termed a cold front; if the reverse, then the boundary is termed a warm front, whereas a stationary front indicates that no relative advance of either air mass is occurring. An occluded front is one in which a warm front has been completely undermined by cold air and is therefore positioned aloft. Since warmer air always overrides colder, denser air, the frontal boundary is sloped closer to the horizontal than the vertical. A mature cyclone usually involves all of the frontal types. The recognition of atmospheric fronts and their relative importance to weather forecasting came about only at the beginning of the 20th cent. as a result of publications by the meteorologists Vilhelm and Jakob Bjerknes .

Natural selection, process that results in adaptation of an organism to its environment by means of selectively reproducing changes in its genotype. Variations that increase an organism's chances of survival and procreation are preserved and multiplied from generation to generation at the expense of less advantageous variations. As proposed by Charles Darwin, natural selection is the mechanism by which evolution occurs. It may arise from differences in survival, fertility, rate of development, mating success, or any other aspect of the life cycle. Mutation, gene flow, and genetic drift, all of which are random processes, also alter gene abundance. Natural selection moderates the effects of these processes because it multiplies the incidence of beneficial mutations over generations and eliminates harmful ones, since the organisms that carry them leave few or no descendants.

Amber, fossilized tree resin. Amber can vary in color from yellow to red to green and blue. The best commercial amber is transparent, but some varieties are cloudy. To be called amber, the resin must be several million years old; recently hardened resins are called copals. The tree species that produced amber are now extinct. They included cedars and other conifers and broadleaved trees. The most famous source of the world's amber is the Baltic coast of Germany. Amber is also found off the coasts of Sicily and England and in Myanmar (Burma). In the Western Hemisphere, there are rich deposits in the Dominican Republic, Mexico, and the state of New Jersey. Amber is of interest both for its decorative value and for the ancient, once-living inclusions that it preserves. Capable of being highly polished, it is the oldest decorative substance known. It was familiar to Paleolithic peoples and to the Greeks and Romans, who used it extensively in jewelry. Pliny recounts several instances

Sedimentary rock, rock formed at or near the Earth's surface by the accumulation and lithification of fragments of preexisting rocks or by precipitation from solution at normal surface temperatures. Sedimentary rocks can be formed only where sediments are deposited long enough to become compacted and cemented into hard beds or strata. They are the most common rocks exposed on the Earth's surface but are only a minor constituent of the entire crust. Their defining characteristic is that they are formed in layers. Each layer has features that reflect the conditions during deposition, the nature of the source material (and, often, the organisms present), and the means of transport.