Celestial Ebooks Catalog
That I am mortal I know, and that my days are numbered, but when in my mind I follow the multiply entwined orbits of the stars, then my feet do no longer touch the Earth. At the table of Zeus himself do I eat Ambrosia, the food of the Gods . These words by Ptolemy from around 125 A.D. are handed down together with his famous book The Almagest, the bible of astronomy for some 1500 years. They capture mankind's deep fascination with the movements of the heavens, and the miracles of the physical world. After the Babylonians observed the motions of the Sun, Moon, and planets for millennia, the ancient Greeks were the first to speculate about the nature of these celestial bodies. Yet it is only as a consequence of developments in the last 150 years that a much clearer picture of the physical universe has begun to emerge. Among the most important discoveries have been the stellar parallax, confirming Copernicus's heliocentric system, the realization that galaxies are comprised of billions...
Although we cannot yet resolve stellar disks, except for those of a few very large supergiant stars, it is possible to detect the radial velocity variations of a star's orbit around the center of mass. This is achieved by carefully analyzing absorption lines in the stellar spectrum. Recent advances in the precision of these measurements now allow us to detect velocity variations of as little as 3 m s. Figure 4.2b shows the geometry of such an orbit. The star orbits (solid) around the center of mass in a plane inclined at an angle i against the plane of the sky, the imagined celestial sphere perpendicular to the line of sight from the Earth. The radial velocity vr (Fig. 4.2b) is measured by the Doppler shift of an absorption line of the stellar spectrum. Only the fraction vr v sin i of the true velocity v in the star's orbit is observed (see Fig. 4.2b).
In some species, the young accompany the female on the southward migration, but most young migrate on their own, relying on inbuilt genetic programming to fly hundreds and sometimes thousands of miles using the sun or stars or both as navigation aids. Many species memorize landmarks such as lakes and river courses, which supplement celestial navigation aids on future migrations. Studies have also suggested that at least some ducks are able to tap into the earth's magnetic field for directional aid on overcast days, when other cues are unavailable.
A number of geologists and paleontologists, using new dating techniques based on the rate of radioactive decay in rocks, are reassessing the ages traditionally assigned to fossils by less sophisticated techniques in the past. These studies should eventually reveal whether the mass extinction events of the remote past occurred in a very short or over a relatively longer period of time. Other geologists are searching for impact craters, to lend further credence to the theory that at least some and perhaps all mass extinctions resulted from periodic collisions between the earth and large celestial bodies.
Looking at the nature, origin, and evolution of life on Earth is one way of assessing whether extraterrestrial life exists on Earth-like planets elsewhere (see Chaps. 6 and 7). A more direct approach is to search for favorable conditions and traces of life on other celestial bodies, both in the solar system and beyond. Clearly, there is little chance of encountering nonhuman intelligent beings in the solar system. But there could well be primitive life on Mars, particularly as in the early history of the solar system the conditions on Mars were quite similar to those on Earth. In addition, surprisingly favorable conditions for life once existed on the moons of Jupiter. Yet even if extraterrestrial life is not encountered in forthcoming space missions, it would be of utmost importance to recover fossils of past organisms as such traces would greatly contribute to our basic understanding of the formation of life. In addition to the planned missions to Mars and Europa, there are...
BETA (the Billion-channel Extraterrestrial Assay), the follow-up program of META I (see SETI 2005), began observations in October 1995. It broke down in 1999, but is scheduled to resume operation in 2006. It employs a new strategy of rapid and automatic observation of candidate events, a better discrimination of terrestrial interference, and a much greater frequency coverage. With a 240-million-channel spectrometer, the output of which feeds an array of programmable feature recognizers , BETA searches the full water hole of 1.4-1.7 GHz. The antenna incorporates two (east west) feed horns and a third low-gain terrestrial one. When, as a consequence of the Earth's rotation, a suspicious celestial signal is first seen in the east, then in the west, but not in the terrestrial horn (detection there is attributed to terrestrial sources), it triggers the antenna to jump to the west, forcing the source to move through the detection sequence again. If the signal is confirmed, the antenna will...
Scientists studying the neurophysiology underlying animal behavior (neuroetholo-gists) have been amazed for many years at the ability of migrating animals unerringly to find their destinations in the apparent absence of obvious terrestrial or celestial orienting features. Factors such as the positions of stars, sun, and moon (when visible) and water chemical content (where relevant) have been implicated as navigational guides, and in some cases may be guides. However, behavioral evidence has accumulated showing that certain diverse animals such as yellowfin tuna, trout, sea turtles, pigeons, spiny lobsters, honeybees, and certain mollusks and bacteria use the Earth's magnetic field to guide their migratory and systematic movements, often in conjunction with certain other geophysical factors mentioned above.
How Arctic terns orient and navigate along their migratory routes is usually studied by means of laboratory-conducted behavioral experiments. Birds are exposed to various combinations of stimuli (magnetic fields, planetarium-like celestial fields, light levels), and their orientation, activity levels, and physiological states are measured. Experiments involving surgical or chemical manipulation of known sensory systems are sometimes conducted to compare behavioral reactions to experimental stimuli. In such experiments, the birds (or other test animals) are rarely harmed.
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