Poseidonius (born c. 135 bce—died c. 51 bce) was a Greek philosopher, considered the most-learned man of his time and, possibly, of the entire Stoic school.
Poseidonius, nicknamed “the Athlete,” was a native of Apamea in Syria and a pupil of the Greek Stoic philosopher Panaetius. He spent many years in travel and scientific research in Spain, Africa, Italy, Gaul (modern France), Liguria, and Sicily. When he settled as a teacher at Rhodes, his adopted Greek city, his fame attracted numerous scholars. By his writings and his personal relations, he did more to spread Stoicism in the Roman world than anyone else except Panaetius. He was known to many leading men of his time, including the Roman statesman Cicero, who studied under him in 78–77 and whom he mentioned as a friend. Such other Roman writers as Strabo and Seneca provide the major source of knowledge about his life; until the 20th century, scholars accorded him only a minor place in the development of Stoicism.
The titles and subjects of more than 20 of his works, now lost, are known. Like other Stoics of the middle period in the school’s history, Poseidonius was an eclectic who combined the views of older Stoics and of Plato and Aristotle. His well-known ethical doctrine diverged from contemporary Stoicism, however, in asserting that human passions are inherent qualities, not mere faulty judgments. Also interested in natural science, geography, astronomy, and mathematics, Poseidonius tried to calculate the diameter of the Earth, the influence of the Moon on tides, and the distance and magnitude of the Sun. His history of the period 146–88 bce filled 52 volumes and was undoubtedly a storehouse of knowledge for early writers. A gifted dialectician, Poseidonius was notable for his powers of observation, his travel reports, his ironic humour, and his practice of Stoic doctrine.
Examine the observable universe's place within the whole universeLearn about defining and measuring the observable universe within the “whole” universe.
cosmology, field of study that brings together the natural sciences, particularly astronomy and physics, in a joint effort to understand the physical universe as a unified whole. The “observable universe” is the region of space that humans can actually or theoretically observe with the aid of technology. It can be thought of as a bubble with Earth at its centre. It is differentiated from the entirety of the universe, which is the whole cosmic system of matter and energy, including the human race. Unlike the observable universe, the universe is possibly infinite and without spatial edges.
If one looks up on a clear night, one will see that the sky is full of stars. During the summer months in the Northern Hemisphere, a faint band of light stretches from horizon to horizon, a swath of pale white cutting across a background of deepest black. For the early Egyptians, this was the heavenly Nile, flowing through the land of the dead ruled by Osiris. The ancient Greeks likened it to a river of milk. Astronomers now know that the band is actually composed of countless stars in a flattened disk seen edge on. The stars are so close to one another along the line of sight that the unaided eye has difficulty discerning the individual members. Through a large telescope, astronomers find myriads of like systems sprinkled throughout the depths of space. They call such vast collections of stars galaxies, after the Greek word for milk, and call the local galaxy to which the Sun belongs the Milky Way Galaxy or simply the Galaxy.
The Sun is a star around which Earth and the other planets revolve, and by extension every visible star in the sky is a sun in its own right. Some stars are intrinsically brighter than the Sun; others, fainter. Much less light is received from the stars than from the Sun because the stars are all much farther away. Indeed, they appear densely packed in the Milky Way only because there are so many of them. The actual separations of the stars are enormous, so large that it is conventional to measure their distances in units of how far light can travel in a given amount of time. The speed of light (in a vacuum) equals 3 × 1010 cm/sec (centimetres per second); at such a speed, it is possible to circle the Earth seven times in a single second. Thus in terrestrial terms the Sun, which lies 500 light-seconds from the Earth, is very far away; however, even the next closest star, Proxima Centauri, at a distance of 4.3 light-years (4.1 × 1018 cm), is 270,000 times farther yet. The stars that lie on the opposite side of the Milky Way from the Sun have distances that are on the order of 100,000 light-years, which is the typical diameter of a large spiral galaxy.
Andromeda GalaxyThe Andromeda Galaxy, also known as the Andromeda Nebula or M31. It is the closest spiral galaxy to Earth, at a distance of 2.48 million light-years.
If the kingdom of the stars seems vast, the realm of the galaxies is larger still. The nearest galaxies to the Milky Way system are the Large and Small Magellanic Clouds, two irregular satellites of the Galaxy visible to the naked eye in the Southern Hemisphere. The Magellanic Clouds are relatively small (containing roughly 109 stars) compared to the Galaxy (with some 1011 stars), and they lie at a distance of about 200,000 light-years. The nearest large galaxy comparable to the Galaxy is the Andromeda Galaxy (also called M31 because it was the 31st entry in a catalog of astronomical objects compiled by the French astronomer Charles Messier in 1781), and it lies at a distance of about 2,000,000 light-years. The Magellanic Clouds, the Andromeda Galaxy, and the Milky Way system all are part of an aggregation of two dozen or so neighbouring galaxies known as the Local Group. The Galaxy and M31 are the largest members of this group.
The Galaxy and M31 are both spiral galaxies, and they are among the brighter and more massive of all spiral galaxies. The most luminous and brightest galaxies, however, are not spirals but rather supergiant ellipticals (also called cD galaxies by astronomers for historical reasons that are not particularly illuminating). Elliptical galaxies have roundish shapes rather than the flattened distributions that characterize spiral galaxies, and they tend to occur in rich clusters (those containing thousands of members) rather than in the loose groups favoured by spirals. The brightest member galaxies of rich clusters have been detected at distances exceeding several thousand million light-years from the Earth. The branch of learning that deals with phenomena at the scale of many millions of light-years is called cosmology—a term derived from combining two Greek words, kosmos, meaning “order,” “harmony,” and “the world,” and logos, signifying “word” or “discourse.” Cosmology is, in effect, the study of the universe at large.
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