Admired And Honored

Beginning in 1962 Hess chaired the Space Science Advisory Board of the National Academy of Sciences. The board's responsibility was to advise NASA. In 1966 he was at Woods Hole, Massachusetts, chairing a meeting to discuss the scientific objectives of lunar exploration when he began having chest pains. He died of a heart attack on August 25, 1969, and was buried at the Arlington National Cemetery.

Hess was elected to membership of several academic societies including the National Academy of Sciences (1952), the American Philosophical Society (1960), and the American Academy of Arts and Sciences (1968). He served as president of the Geodesy Section (1951-53) and the Tectonophysics Section (1956-58) of the American Geophysical Union, the Mineralogical Society of America (1955), and the Geological Society of America (1963). Because he was well respected as a scientist, he also was appointed chairman of the Committee for Disposal of Radioactive Wastes, chairman of the Earth Sciences Division of the National Research Council, and chairman of the Space Science Advisory Board of the National Academy of Sciences. Along with oceanographer Walter Munk, he was a principal player in the Mohole Project, the goal of which was to drill beneath the oceanic crust into the mantle. The Geological Society of America awarded Hess the Penrose Medal for distinguished achievement in the geological sciences in 1966, and NASA awarded Hess a Distinguished Public Service Award posthumously. Because of his outstanding achievements the American Geophysical union created the Harry H. Hess Medal for outstanding achievements in research in the constitution and evolution of the Earth and sister planets.

Hess's friends have described his personality as puckish and courageous. Part of Hess's greatness as a scientist was his willingness to entertain new ideas, even if they conflicted with his own previous conclusions. After all, sometimes wrong ideas ushered in new eras of scientific accomplishment. In suggesting that the ocean basins were continuously recycled, Hess explained why seafloor spreading did not cause Earth to grow, why the layer of sediment on the ocean floor was thinner than expected, and why oceanic rocks are younger than continental rocks. Hess's model of seafloor spreading has become part of the foundation knowledge of the geological sciences and has evolved into the theory of plate tectonics. Many questions regarding the forces that occur deep within the Earth are still being actively investigated today.

See also convergent plate margin processes; divergent plate margin processes; plate tectonics; Wegener, Alfred.

FURTHER READING

Carruthers, Margaret W., and Susan Clinton. Pioneers of Geology: Discovering Earth's Secrets. New York: Franklin Watts, 2001. Hess, Harry H. "Comments on the Pacific Basin." Geological Survey of Canada. Special Paper (1966): 311-316.

-. "The Oceanic Crust." Journal of Marine Research

-. "Geological Hypotheses and the Earth's Crust

Under the Oceans." Proceedings of the Royal Society of London, Series A: Mathematical and Physical Sciences 222 (1954): 341-348.

Hipparchus (160-120 b.c.e.) Greek Astronomer, Geographer, Mathematician Hipparchus was one of the greatest astronomical observers of ancient Greece. He is best known for his accurate quantitative models for the motion of the Sun and Moon. He used a trigonometric table and techniques from the Chaldeans (a Semitic people from Mesopotamia) to derive the sizes of Sun and Moon, to determine the latitude and longitude of places on the Earth, and to predict solar eclipses. He is credited also with compilation of the first star catalog and invention of the astrolabe.

Hipparchus was born in Nicaea in the ancient Greek district of Bithynia (now Iznik, Turkey). Historical accounts of his life from Ptolemy and Pliny the Elder suggest that he was born in 190 b.c.e., and it is known from his writings that he visited Alexandria, Egypt, and Babylon, but the dates of these trips are not known. Hipparchus is thought to have spent the later years of his life, and to have died in about 120 b.c.e., on the Greek island of Rhodes in the Aegean Sea.

Much of Hipparchus's scientific work has been lost to antiquity, and most is known from writings by scientists from more recent times, especially Ptolemy. From these writings it is known that Hipparcus wrote at least 14 books and published a star catalog later incorporated into Ptolemy's star catalog.

The world's first known trigonometric table is that of Hipparchus, who used the tables to calculate the eccentricity and orbits of the Sun and Moon. He was also concerned with calculating the distances to the Sun and Moon, and the sizes of these objects. He wrote about his trigonometric methods in the book Toon en kuklooi eutheioon (Of lines inside a circle), which has been lost to civilization. Hippar-chus worked on stereographic projections (a mapping function that projects a sphere onto a plane), showing that the projections can be made to preserve angles so that they are the same on the projection as in the physical world, and that circles on the sphere that do not pass through the center of the sphere project as circles on the projection (i.e., they are not great circles). Hipparchus used these principles to develop the astrolabe, a historical astronomical instrument used to locate the positions of the Sun, Moon, stars, and planets. Astrolabes also proved useful in calculating location and time for ships at sea, and for navigation.

one of the topics of great interest to Hippar-chus was the motion of the Moon. He calculated accurately the Moon's period and predicted eclipses with great accuracy. He was also concerned with the length of the year and the apparent motion of the Sun, and observed the summer solstice in 135 b.c.e., as well as many solar equinoxes. He used these measurements to calculate the length of the year. Toward the end of his career Hipparchus wrote a book on his solar observations and calculations, Peri eniausiou megethous (On the length of the year). In this book he concluded that the year lasted 365 1/4-365 1/300 days.

Hipparchus worked to determine the size of the Sun and the Moon, and the distance to these objects, publishing his work in two books, Peri megethoon kai 'apostemdtoon (On sizes and distances). Although his books do not survive, later accounts suggest that he calculated the Sun to be 2,550 Earth radii and the mean distance to the Moon to be 60.5 Earth radii.

Hipparchus may be most famous for his discovery of the precession of the equinoxes through his observations of the Sun and Moon. He published books on precession titled On the Displacement of the Solsticial and Equinoctial Points and On the Length of the Year, and Ptolemy and others later used these works in their celestial observations and catalogs.

See also astronomy; solar system; Sun.

historical geology Historical geology is the science that uses the principles of geology to reconstruct and interpret the history of the Earth. It includes study of the changes in the Earth's surface, the record of life, stratigraphy, dating of different geologic units, the history of the motions of plates and past positions of continents, the formation of mountain belts, basins, and past climates. The Earth can be like a jigsaw puzzle, and historical geology attempts to understand the causes and sequence of events that led to the observable features in the geological record, including the origin and destiny of living things and establishing the chronology of events in Earth history by examining the rock record. Geologists who have studied the history of Earth have gradually come to realize that the planet is very old and has had a complex history. The rock record shows that the life-forms on the planet have evolved from preexisting species by means of slow, gradual changes over long periods of time, and this evolution has been punctuated by several major episodes of mass extinctions, where large numbers of species and individual organisms within species have suddenly died off, to be replaced by totally new species in the next layer of younger strata.

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