Nicolaus Copernicus
P
olish astronomer Nicolaus Copernicus revolutionized
science by postulating that the earth and other planets
revolve about a stationary sun. Developed in the early
1500s but not published until years later, his
heliocentric (sun-centered) theory disputed the Ptolemaic
theory, popular at the time, which held that the sun and
the planets revolved about the fixed earth. Copernicus at
first hesitated in publishing his findings because he
feared criticism from the scientific and religious
communities. After suffering initial disbelief and
rejection, however, the Copernican system ranked as the
most accepted concept of the universe by the late 17th century.
Johannes Kepler
J
ohannes Kepler believed that the Copernican heliocentric
view of the solar system, which held that Earth rotates
around the stationary Sun, was correct. Kepler formulated
an accurate mathematical description of planetary orbits
that gave mathematical rigor to the heliocentric model.
His contributions dramatically increased scientists'
understanding of planetary motion; Isaac Newton drew upon
Kepler's work in formulating his theory of gravitation.
Isaac Newton
I
saac Newton's work represents one of the greatest
contributions to science ever made by an individual.
Most notably, Newton derived the law of universal
gravitation, invented the branch of mathematics called
calculus, and performed experiments investigating the
nature of light and color.
Al-Battani
A
l-Battani, (circa 858-929), leading Arab astronomer and
mathematician of his time. Known also as Albatenius, his
full name was Abu-Abdullah Muhammad ibn Jabir al-Battani.
His astronomical observations at Ar Raqqah, Syria,
extended for a period of more than 40 years. He also
made important contributions to pure mathematics. He
introduced the use of sines in mathematical calculations,
computed a table of cotangents, and formulated certain
propositions in spherical trigonometry. His astronomical
works, published as De Motu Stellarum (Concerning the
Motion of the Stars, 1537), corrected errors of the
Alexandrian astronomer Ptolemy in regard to the
inclination of the ecliptic and the length of the year.
Aristarchus of Samos
A
ristarchus of Samos (310?-250?BC), Greek astronomer who
was the first to assert that the earth revolves around
the sun. His belief that the earth revolves around the sun
is known only through the writings of Greek mathematician
and inventor Archimedes; none of the works written by
Aristarchus on the subject have survived. In his only
surviving work, On the Dimensions and Distances of the
Sun and Moon, Aristarchus described a method for
estimating the relative distances of the sun and moon from
the earth. Although his method was essentially correct,
his estimates were wrong because of his inadequate
knowledge of mathematics and his lack of accurate instruments.
Eratosthenes
E
ratosthenes (276?-196?BC), Greek mathematician,
astronomer, geographer, and poet, who measured the
circumference of the earth with extraordinary accuracy by
determining astronomically the difference in latitude
between the cities of Syene (now Aswân) and Alexandria,
Egypt. He was born in Cyrene (now Shaât, Libya). Among
his teachers was the Greek poet Callimachus. About 240BC,
Eratosthenes became the head of the library at Alexandria,
Egypt. His calculation of the earth's circumference was
only about 15 percent too large. Eratosthenes also
measured the obliquity of the ecliptic with an error of
only seven minutes of arc and created a catalog (now lost)
of 675 fixed stars. His most important work was a
systematic treatise on geography. After becoming blind,
he died in Alexandria of voluntary starvation.
Eudoxus
Eudoxus (408-355BC), Greek astronomer and mathematician, who made important contributions to the field of geometry and who proposed the first systematic explanation for the motions of the sun, moon, and planets. Eudoxus was born in Cnidus (in what is now Turkey). He became a pupil of the Greek philosopher Archytas and studied for a short time under the Greek philosopher Plato. Later Eudoxus founded a school at Cyzicus.
Eudoxus is often credited with the discovery that the solar year is about 6 hours longer than 365 days. Eudoxus also attempted to explain the motions of the sun, moon, and planets through a model of the solar system based on a complicated arrangement of rotating spheres. His model was moderately successful in predicting these motions. Eudoxus also made important discoveries in mathematics. It is believed that he discovered much of the geometry later included in Elements, the comprehensive treatise on mathematics written by the Greek mathematician Euclid
Halley, Edmond
H
alley, Edmond (1656-1742), British astronomer, who first calculated the orbit of a comet. He was born in London and educated at the University of Oxford. Halley was intrigued by the theories of the British physicist Sir Isaac Newton and encouraged him to write the Principia, which Halley then published in 1687 at his own expense. He was made Astronomer Royal in 1721 and began an 18-year study of the complete revolution of the moon through its ascending and descending nodes.
Halley's most important scientific treatise was Astronomiae Cometicae Synopsis (Synopsis on Cometary Astronomy), begun in 1682 and published in 1705. In this work, Halley applied Newton's laws of motion to all available data on comets and then mathematically demonstrated that comets move in elliptic orbits around the sun. His accurate prediction of the return in 1758 of a comet (now known as Halley's comet) validated his theory that comets are part of the solar system. See Comet.
Hipparchus
H
ipparchus (c. 190-120 BC), the most important Greek astronomer of his time. Hipparchus was born in Nicaea, Bithynia (now Ýznik, Turkey). He was extremely accurate in his research, a record of which was preserved in the Almagest, the scientific treatise by the Alexandrian astronomer Ptolemy, who was greatly influenced by Hipparchus. By comparing his own celestial studies with those of earlier astronomers, Hipparchus discovered the precession of the equinoxes (see Ecliptic). His calculation of the tropical year, the length of the year measured by the sun, was within 6.5 minutes of modern measurements. Hipparchus devised a method of locating geographic positions by means of latitudes and longitudes. He cataloged, charted, and calculated the brightness of perhaps as many as 1000 stars. Hipparchus also compiled a table of trigonometric chords that became the basis for modern trigonometry
Hubble, Edwin Powell
H
ubble, Edwin Powell (1889-1953), American astronomer who made important contributions to the study of galaxies, the expansion of the universe, and the size of the universe. Hubble was the first to discover that fuzzy patches of light in the sky called spiral nebula were actually galaxies like the earth's galaxy, the Milky Way. Hubble also found the first evidence for the expansion of the universe, and his work led to a much better understanding of the universe's size.
Brahe, Tycho
B
rahe, Tycho (1546-1601), Danish astronomer, who made precise, comprehensive astronomical measurements of the solar system and more than 700 stars. The data Brahe accumulated was superior to all other astronomical measurements made until the invention of the telescope in the early 17th century.
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