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Science (from the Latin word scientia, meaning "knowledge") is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe.

The earliest roots of science can be traced to Ancient Egypt and Mesopotamia in around 3500 to 3000 BCE. Their contributions to mathematics, astronomy, and medicine entered and shaped Greek natural philosophy of classical antiquity, whereby formal attempts were made to explain events of the physical world based on natural causes. After the fall of the Western Roman Empire, knowledge of Greek conceptions of the world deteriorated in Western Europe during the early centuries (400 to 1000 CE) of the Middle Ages but was preserved in the Muslim world during the Islamic Golden Age. The recovery and assimilation of Greek works and Islamic inquiries into Western Europe from the 10th to 13th century revived natural philosophy, which was later transformed by the Scientific Revolution that began in the 16th century as new ideas and discoveries departed from previous Greek conceptions and traditions. The scientific method soon played a greater role in knowledge creation and it was not until the 19th century that many of the institutional and professional features of science began to take shape.

Modern science is typically divided into three major branches that consist of the natural sciences (e.g., biology, chemistry, and physics), which study nature in the broadest sense; the social sciences (e.g., economics, psychology, and sociology), which study individuals and societies; and the formal sciences (e.g., logic, mathematics, and theoretical computer science), which study abstract concepts. There is disagreement, however, on whether the formal sciences actually constitute a science as they do not rely on empirical evidence. Disciplines that utilize existing scientific knowledge for practical purposes, such as engineering and medicine, are described as applied sciences.

Science is based on research, which is commonly conducted in academic and research institutions as well as in government agencies and companies. The practical impact of scientific research has led to the emergence of science policies that seek to influence the scientific enterprise by prioritizing the development of commercial products, armaments, health care, and environmental protection.

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A potassium Faraday filter designed, built and photographed by Jonas Hedin for making daytime LIDAR measurements at Arecibo Observatory.
An atomic line filter (ALF) is an advanced optical band-pass filter used in the physical sciences for filtering electromagnetic radiation with precision, accuracy, and minimal signal strength loss. Atomic line filters work via the absorption or resonance lines of atomic vapors and so may also be designated an atomic resonance filter (ARF).

The three major types of atomic line filters are absorption-re-emission ALFs, Faraday filters and Voigt filters. Absorption-re-emission filters were the first type developed, and so are commonly called simply "atomic line filters"; the other two types are usually referred to specifically as "Faraday filters" or "Voigt filters". Atomic line filters use different mechanisms and designs for different applications, but the same basic strategy is always employed: by taking advantage of the narrow lines of absorption or resonance in a metallic vapor, a specific frequency of light bypasses a series of filters that block all other light.

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A Persian astrolabe, used for determining the time at both day and night.
Cr: Andrew Dunn

An 18th Century Persian astrolabe used for determining the time at both day and night. The points of the curved spikes on the front rete plate mark the positions of the brightest stars, the name of each star being labeled at the base of each spike. The back plate, or mater, is engraved with projected coordinate lines. From the Whipple Museum of the History of Science collection.

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Gregor Mendel (1822–1884) was an Austrian monk who is often called the "father of genetics" for his study of the inheritance of traits in pea plants. Mendel showed that there was particular inheritance of traits according to his laws of inheritance.

It was not until the early 20th century that the importance of his ideas was realized. In 1900, his work was rediscovered by Hugo de Vries, Carl Correns, and Erich von Tschermak. His results were quickly replicated, and genetic linkage quickly worked out. Biologists flocked to the theory, as while it was not yet applicable to many phenomena, it sought to give a genotypic understanding of hery which they felt was lacking in previous studies of hery which focused on phenotypic approaches.

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