Physical science is the systematic study of the inorganic world, as distinct fr
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om the study of the organic world, which is the province of biological science. Physic
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al science is ordinarily thought of as consisting of four broad areas: astronomy, phys
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ics, chemistry, and the Earth sciences. Each of these is in turn divided into fields and
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subfields. b
• Physics, in its modern sense, was founded in the mid- b b b b b b b b b
19th century as a synthesis of several older sciences—
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namely, mechanics, optics, acoustics, electricity, magnetism, heat, and
b b b b b b b b
the physical properties of matter. The synthesis was based in large part
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on the recognition that the different forces of nature are related and are
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, in fact, inter-convertible because they are forms of energy.
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The boundary between physics and chemistry is somewhat arbitrary. As it dev
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eloped in the 20th century, physics is concerned with the structure and behavior of i
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ndividual atoms and their components, while chemistry deals with the properties an
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d reactions of molecules. These latter depend on energy, especially heat, as well as
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on atoms; hence, there is a strong link between physics and chemistry. Chemists ten
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d to be more interested in the specific properties of different elements and compoun
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ds, whereas physicists are concerned with general properties shared by all matter.
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ASTRONOMY
• Astronomy is the science of the entire universe beyond Earth; it includ b b b b b b b b b b b
es Earth’s gross physical properties, such as its mass and rotation, insof
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ar as they interact with other bodies in the solar system. Until the 18th c
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entury, astronomers were concerned primarily with the Sun, Moon, plan
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ets, and comets. During the following centuries, however, the study of s
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tars, galaxies, nebulas, and the interstellar medium became increasingl
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y important. b b
Celestial mechanics, the science of the motion of planets and other solid ob
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jects within the solar system, was the first testing ground for Newton’s laws of motio
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n and thereby helped to establish the fundamental principles of classical (that is, pre
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-20th-
century) physics. Astrophysics, the study of the physical properties of celestial bodie
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,he chemical composition of those bodies. In the 20th century, physics and astronom
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y became more intimately linked through cosmological theories, especially those ba
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sed on the theory of relativity.
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Heritage of antiquity and the Middle Ages
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• The physical sciences ultimately derive from the rationalistic materialis
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m that emerged in classical Greece, itself an outgrowth of magical and
b b b b b b b b b b b b
mythical views of the world. The Greek philosophers of the 6th and 5th c
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enturies BCE abandoned the animism of the poets and explained the wo
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rld in terms of ordinarily observable natural processes. These early philo
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sophers posed the broad questions that still underlie science: How did t
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he world order emerge from chaos? What is the origin of multitude and
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variety in the world? How can motion and change be accounted for? Wh
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at is the underlying relation between form and matter? Greek philosoph
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y answered these questions in terms that provided the framework for sci
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ence for approximately 2,000 years. b b b b
Ancient Middle Eastern and Greek astronomy
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• Western astronomy had its origins in Egypt and Mesopotamia. Egyptian
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astronomy, which was neither a very well- b b b b b b
developed nor an influential study, was largely concerned with time reck b b b b b b b b b b
oning. Its main lasting contribution was the civil calendar of 365 days, c
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onsisting of 12 months of 30 days each and five additional festival days
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at the end of each year. This calendar played an important role in the his
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tory of astronomy, allowing astronomers to calculate the number of day
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s between any two sets of observations.
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Babylonian astronomy, dating back to about 1800 BCE, constitutes one ofb b b b b b b b b b b
the earliest systematic, scientific treatments of the physical world. In contrast to the
b b b b b b b b b b b b b
Egyptians, the Babylonians were interested in the accurate prediction of astronomic
b b b b b b b b b b
al phenomena, especially the first appearance of the new moon. Using the zodiac as
b b b b b b b b b b b b b b
a reference, by the 4th century BCE, they developed a complex system of arithmetic
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progressions and methods of approximation by which they were able to predict first
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appearances. The mass of observations they collected and their mathematical meth
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ods were important contributions to the later flowering of astronomy among the Gre
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eks.
, • The Pythagoreans (5th century BCE) were responsible for one of the f
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irst Greek astronomical theories. Believing that the order of the cosmos i
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s fundamentally mathematical, they held that it is possible to discover th
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e harmonies of the universe by contemplating the regular motions of th
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e heavens. Postulating a central fire about which all the heavenly bodies
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including Earth and the Sun revolve, they constructed the first physical
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model of the solar system. Subsequent Greek astronomy derived its cha
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racter from a comment ascribed to Plato, in the 4th century BCE, who is
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reported to have instructed the astronomers to “save the phenomena” i
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n terms of uniform circular motion. That is to say, he urged them to deve
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lop predictively accurate theories using only combinations of uniform cir
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cular motion. As a result, Greek astronomers never regarded their geom
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etric models as true or as being physical descriptions of the machinery o
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f the heavens. They regarded them simply as tools for predicting planet
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ary positions. b
Learn about solar-
b b
system theories by Aristotle, Ptolemy, Nicolaus Copernicus, and Johanne
b b b b b b b b
s Kepler
b
Discussion of four attempts to explain the structure of the solar system, from
b b b b b b b b b b b b b
Aristotle to Johannes Kepler. (Encyclopædia Britannica, Inc.)
b b b b b b
b
• Eudoxus of Cnidus (4th century BCE) was the first of the Greek astrono
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mers to rise to Plato’s challenge. He developed a theory of homocentric
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spheres, a model that represented the universe by sets of nesting conce
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ntric spheres the motions of which combined to produce the planetary a
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nd other celestial motions. Using only uniform circular motions, Eudoxu
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s was able to “save” the rather complex planetary motions with some su
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ccess. His theory required four homocentric spheres for each planet and
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three each for the Sun and Moon. The system was modified by Callippus
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, a student of Eudoxus, who added spheres to improve the theory, espec
b b b b b b b b b b b b
ially for Mercury and Venus. Aristotle, in formulating his cosmology, ado
b b b b b b b b b b
pted Eudoxus’s homocentric spheres as the actual machinery of the hea
b b b b b b b b b b
vens. The Aristotelian cosmos was like an onion consisting of a series of
b b b b b b b b b b b b b
some 55 spheres nested about Earth, which was fixed at the center. To
b b b b b b b b b b b b b
unify the system, Aristotle added spheres to “unroll” the motions of a giv
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en planet so that they would not be transmitted to the next inner planet.
b b b b b b b b b b b b b
The theory of homocentric spheres failed to account for two sets of obse
b b b b b b b b b b b b
, ❖ brightness changes suggesting that planets are not always the same distance
b b b b b b b b b b b
from Earthb
❖ and bounded elongations (i.e., Venus is never observed to be more than abou
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t 48° and Mercury never more than about 24° from the Sun).
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• Heracleides of Pontus (4th century BCE) attempted to solve these pr b b b b b b b b b b
oblems by having Venus and Mercury revolve about the Sun, rather tha
b b b b b b b b b b b
n Earth, and having the Sun and other planets revolve in turn about Eart
b b b b b b b b b b b b b
h, which he placed at the center. In addition, to account for the daily mo
b b b b b b b b b b b b b b
tions of the heavens, he held that Earth rotates on its axis. Heracleides’ t
b b b b b b b b b b b b b
heory had little impact in antiquity except perhaps on Aristarchus of Sa
b b b b b b b b b b b
mos (3rd century BCE), who put forth a heliocentric hypothesis similar t
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o the one Copernicus was to propound in the 16th century.
b b b b b b b b b b
• Hipparchus (flourished 130 BCE) made extensive contributions to bot b b b b b b b b
h theoretical and observational astronomy. Basing his theories on an im
b b b b b b b b b b
pressive mass of observations, he was able to work out theories of the S
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un and Moon that were more successful than those of any of his predece
b b b b b b b b b b b b b
ssors. His primary conceptual tool was the eccentric circle, a circle in whi
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ch Earth is at some point eccentric to the geometric center. He used this
b b b b b b b b b b b b b b
device to account for various irregularities and inequalities observed in t
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he motions of the Sun and Moon. He also proved that the eccentric circle
b b b b b b b b b b b b b
is mathematically equivalent to a geometric figure called an epicycle-
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deferent system, a proof probably first made by Apollonius of Perga a ce
b b b b b b b b b b b b
ntury earlier. b
• Among Hipparchus’s observations, one of the most significant was that
b b b b b b b b b b
of the precession of the equinoxes—
b b b b b
i.e., a gradual apparent increase in longitude between any fixed star an
b b b b b b b b b b b
d the equinoctial point (either of two points on the celestial sphere wher
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e the celestial equator crosses the ecliptic). Thus, the north celestial pol
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e, the point on the celestial sphere defined as the apparent center of rot
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ation of the stars, moves relative to the stars in its vicinity. In the helioce
b b b b b b b b b b b b b b
ntric theory, this effect is ascribed to a change in Earth’s rotational axis,
b b b b b b b b b b b b b
which traces out a conical path around the axis of the orbital plane.
b b b b b b b b b b b b
Precession of the equinoxes
b b b
Effects of precession on Earth's axis of rotation. (Encyclopædia Britannica, Inc.)
b b b b b b b b b b
b b b b b b b b b b b b
om the study of the organic world, which is the province of biological science. Physic
b b b b b b b b b b b b b b
al science is ordinarily thought of as consisting of four broad areas: astronomy, phys
b b b b b b b b b b b b b
ics, chemistry, and the Earth sciences. Each of these is in turn divided into fields and
b b b b b b b b b b b b b b b b
subfields. b
• Physics, in its modern sense, was founded in the mid- b b b b b b b b b
19th century as a synthesis of several older sciences—
b b b b b b b b
namely, mechanics, optics, acoustics, electricity, magnetism, heat, and
b b b b b b b b
the physical properties of matter. The synthesis was based in large part
b b b b b b b b b b b b
on the recognition that the different forces of nature are related and are
b b b b b b b b b b b b
, in fact, inter-convertible because they are forms of energy.
b b b b b b b b b
The boundary between physics and chemistry is somewhat arbitrary. As it dev
b b b b b b b b b b b
eloped in the 20th century, physics is concerned with the structure and behavior of i
b b b b b b b b b b b b b b
ndividual atoms and their components, while chemistry deals with the properties an
b b b b b b b b b b b
d reactions of molecules. These latter depend on energy, especially heat, as well as
b b b b b b b b b b b b b b
on atoms; hence, there is a strong link between physics and chemistry. Chemists ten
b b b b b b b b b b b b b
d to be more interested in the specific properties of different elements and compoun
b b b b b b b b b b b b b
ds, whereas physicists are concerned with general properties shared by all matter.
b b b b b b b b b b b b
ASTRONOMY
• Astronomy is the science of the entire universe beyond Earth; it includ b b b b b b b b b b b
es Earth’s gross physical properties, such as its mass and rotation, insof
b b b b b b b b b b b
ar as they interact with other bodies in the solar system. Until the 18th c
b b b b b b b b b b b b b b
entury, astronomers were concerned primarily with the Sun, Moon, plan
b b b b b b b b b
ets, and comets. During the following centuries, however, the study of s
b b b b b b b b b b b
tars, galaxies, nebulas, and the interstellar medium became increasingl
b b b b b b b b
y important. b b
Celestial mechanics, the science of the motion of planets and other solid ob
b b b b b b b b b b b b
jects within the solar system, was the first testing ground for Newton’s laws of motio
b b b b b b b b b b b b b b
n and thereby helped to establish the fundamental principles of classical (that is, pre
b b b b b b b b b b b b b
-20th-
century) physics. Astrophysics, the study of the physical properties of celestial bodie
b b b b b b b b b b b
,he chemical composition of those bodies. In the 20th century, physics and astronom
b b b b b b b b b b b b
y became more intimately linked through cosmological theories, especially those ba
b b b b b b b b b b
sed on the theory of relativity.
b b b b b b
Heritage of antiquity and the Middle Ages
b b b b b b
• The physical sciences ultimately derive from the rationalistic materialis
b b b b b b b b
m that emerged in classical Greece, itself an outgrowth of magical and
b b b b b b b b b b b b
mythical views of the world. The Greek philosophers of the 6th and 5th c
b b b b b b b b b b b b b
enturies BCE abandoned the animism of the poets and explained the wo
b b b b b b b b b b b
rld in terms of ordinarily observable natural processes. These early philo
b b b b b b b b b b
sophers posed the broad questions that still underlie science: How did t
b b b b b b b b b b b
he world order emerge from chaos? What is the origin of multitude and
b b b b b b b b b b b b b
variety in the world? How can motion and change be accounted for? Wh
b b b b b b b b b b b b
at is the underlying relation between form and matter? Greek philosoph
b b b b b b b b b b
y answered these questions in terms that provided the framework for sci
b b b b b b b b b b b
ence for approximately 2,000 years. b b b b
Ancient Middle Eastern and Greek astronomy
b b b b b
• Western astronomy had its origins in Egypt and Mesopotamia. Egyptian
b b b b b b b b b b
astronomy, which was neither a very well- b b b b b b
developed nor an influential study, was largely concerned with time reck b b b b b b b b b b
oning. Its main lasting contribution was the civil calendar of 365 days, c
b b b b b b b b b b b b
onsisting of 12 months of 30 days each and five additional festival days
b b b b b b b b b b b b b
at the end of each year. This calendar played an important role in the his
b b b b b b b b b b b b b b
tory of astronomy, allowing astronomers to calculate the number of day
b b b b b b b b b b
s between any two sets of observations.
b b b b b b
Babylonian astronomy, dating back to about 1800 BCE, constitutes one ofb b b b b b b b b b b
the earliest systematic, scientific treatments of the physical world. In contrast to the
b b b b b b b b b b b b b
Egyptians, the Babylonians were interested in the accurate prediction of astronomic
b b b b b b b b b b
al phenomena, especially the first appearance of the new moon. Using the zodiac as
b b b b b b b b b b b b b b
a reference, by the 4th century BCE, they developed a complex system of arithmetic
b b b b b b b b b b b b b b
progressions and methods of approximation by which they were able to predict first
b b b b b b b b b b b b b
appearances. The mass of observations they collected and their mathematical meth
b b b b b b b b b b
ods were important contributions to the later flowering of astronomy among the Gre
b b b b b b b b b b b b
eks.
, • The Pythagoreans (5th century BCE) were responsible for one of the f
b b b b b b b b b b b
irst Greek astronomical theories. Believing that the order of the cosmos i
b b b b b b b b b b b
s fundamentally mathematical, they held that it is possible to discover th
b b b b b b b b b b b
e harmonies of the universe by contemplating the regular motions of th
b b b b b b b b b b b
e heavens. Postulating a central fire about which all the heavenly bodies
b b b b b b b b b b b
including Earth and the Sun revolve, they constructed the first physical
b b b b b b b b b b b b
model of the solar system. Subsequent Greek astronomy derived its cha
b b b b b b b b b b
racter from a comment ascribed to Plato, in the 4th century BCE, who is
b b b b b b b b b b b b b b
reported to have instructed the astronomers to “save the phenomena” i
b b b b b b b b b b
n terms of uniform circular motion. That is to say, he urged them to deve
b b b b b b b b b b b b b b
lop predictively accurate theories using only combinations of uniform cir
b b b b b b b b b
cular motion. As a result, Greek astronomers never regarded their geom
b b b b b b b b b b
etric models as true or as being physical descriptions of the machinery o
b b b b b b b b b b b b
f the heavens. They regarded them simply as tools for predicting planet
b b b b b b b b b b b
ary positions. b
Learn about solar-
b b
system theories by Aristotle, Ptolemy, Nicolaus Copernicus, and Johanne
b b b b b b b b
s Kepler
b
Discussion of four attempts to explain the structure of the solar system, from
b b b b b b b b b b b b b
Aristotle to Johannes Kepler. (Encyclopædia Britannica, Inc.)
b b b b b b
b
• Eudoxus of Cnidus (4th century BCE) was the first of the Greek astrono
b b b b b b b b b b b b
mers to rise to Plato’s challenge. He developed a theory of homocentric
b b b b b b b b b b b b
spheres, a model that represented the universe by sets of nesting conce
b b b b b b b b b b b
ntric spheres the motions of which combined to produce the planetary a
b b b b b b b b b b b
nd other celestial motions. Using only uniform circular motions, Eudoxu
b b b b b b b b b
s was able to “save” the rather complex planetary motions with some su
b b b b b b b b b b b b
ccess. His theory required four homocentric spheres for each planet and
b b b b b b b b b b
three each for the Sun and Moon. The system was modified by Callippus
b b b b b b b b b b b b b
, a student of Eudoxus, who added spheres to improve the theory, espec
b b b b b b b b b b b b
ially for Mercury and Venus. Aristotle, in formulating his cosmology, ado
b b b b b b b b b b
pted Eudoxus’s homocentric spheres as the actual machinery of the hea
b b b b b b b b b b
vens. The Aristotelian cosmos was like an onion consisting of a series of
b b b b b b b b b b b b b
some 55 spheres nested about Earth, which was fixed at the center. To
b b b b b b b b b b b b b
unify the system, Aristotle added spheres to “unroll” the motions of a giv
b b b b b b b b b b b b
en planet so that they would not be transmitted to the next inner planet.
b b b b b b b b b b b b b
The theory of homocentric spheres failed to account for two sets of obse
b b b b b b b b b b b b
, ❖ brightness changes suggesting that planets are not always the same distance
b b b b b b b b b b b
from Earthb
❖ and bounded elongations (i.e., Venus is never observed to be more than abou
b b b b b b b b b b b b
t 48° and Mercury never more than about 24° from the Sun).
b b b b b b b b b b b b
• Heracleides of Pontus (4th century BCE) attempted to solve these pr b b b b b b b b b b
oblems by having Venus and Mercury revolve about the Sun, rather tha
b b b b b b b b b b b
n Earth, and having the Sun and other planets revolve in turn about Eart
b b b b b b b b b b b b b
h, which he placed at the center. In addition, to account for the daily mo
b b b b b b b b b b b b b b
tions of the heavens, he held that Earth rotates on its axis. Heracleides’ t
b b b b b b b b b b b b b
heory had little impact in antiquity except perhaps on Aristarchus of Sa
b b b b b b b b b b b
mos (3rd century BCE), who put forth a heliocentric hypothesis similar t
b b b b b b b b b b b
o the one Copernicus was to propound in the 16th century.
b b b b b b b b b b
• Hipparchus (flourished 130 BCE) made extensive contributions to bot b b b b b b b b
h theoretical and observational astronomy. Basing his theories on an im
b b b b b b b b b b
pressive mass of observations, he was able to work out theories of the S
b b b b b b b b b b b b b
un and Moon that were more successful than those of any of his predece
b b b b b b b b b b b b b
ssors. His primary conceptual tool was the eccentric circle, a circle in whi
b b b b b b b b b b b b
ch Earth is at some point eccentric to the geometric center. He used this
b b b b b b b b b b b b b b
device to account for various irregularities and inequalities observed in t
b b b b b b b b b b
he motions of the Sun and Moon. He also proved that the eccentric circle
b b b b b b b b b b b b b
is mathematically equivalent to a geometric figure called an epicycle-
b b b b b b b b b b
deferent system, a proof probably first made by Apollonius of Perga a ce
b b b b b b b b b b b b
ntury earlier. b
• Among Hipparchus’s observations, one of the most significant was that
b b b b b b b b b b
of the precession of the equinoxes—
b b b b b
i.e., a gradual apparent increase in longitude between any fixed star an
b b b b b b b b b b b
d the equinoctial point (either of two points on the celestial sphere wher
b b b b b b b b b b b b
e the celestial equator crosses the ecliptic). Thus, the north celestial pol
b b b b b b b b b b b
e, the point on the celestial sphere defined as the apparent center of rot
b b b b b b b b b b b b b
ation of the stars, moves relative to the stars in its vicinity. In the helioce
b b b b b b b b b b b b b b
ntric theory, this effect is ascribed to a change in Earth’s rotational axis,
b b b b b b b b b b b b b
which traces out a conical path around the axis of the orbital plane.
b b b b b b b b b b b b
Precession of the equinoxes
b b b
Effects of precession on Earth's axis of rotation. (Encyclopædia Britannica, Inc.)
b b b b b b b b b b
