The Essential Galileo (4 page)

Glossary of Terms and Names

This glossary includes terms and names which appear in more than one section, and whose meaning is relatively peculiar, unusual, or obscure; such words are not explained in the notes. When such a word occurs in only one section, it is usually explained in a note and not here. Words whose meanings are generally known or commonly included in small desk-top dictionaries are not included here or in the notes.

Academician.
A term referring to Galileo, used by him in some of his books written in dialogue form. It is meant to remind readers that he was a member of the Lincean Academy.

ad hominem.
In the 17th century, this expression referred to an argument designed to examine the correctness of a controversial view by showing that it implies consequences not acceptable to those who hold that view. This Galilean meaning should not be confused with the most common modern meaning, referring to the fallacy of criticizing a controversial view by questioning the motives, character, or circumstances of those who hold that view, instead of criticizing the reasons and evidence they offer.

annual motion.
In the geostatic worldview, this was the orbital revolution of the sun around the central motionless earth, in an eastward direction relative to the fixed stars and taking one year to complete. In the Copernican system, the annual motion is simply the earth's orbital revolution around the sun, also in an eastward direction and lasting one year.

apogee.
In the orbit of a heavenly body, this is the point farthest from the earth.

Aristarchus of Samos
(c. 310–250 B.C.). Greek astronomer who elaborated the theory that the earth moves around the sun.

Bellarmine
, Robert (1542–1621). Jesuit theologian, perhaps the most influential Catholic churchman of his time, and now a saint. Besides being a cardinal, he also served as a professor at the Roman College (the Jesuit university in Rome), an archbishop, the pope's theologian, a consultant to the Inquisition, and a member of both the Congregation of the Inquisition and the Congregation of the Index.

Brahe
, Tycho. See
Tycho Brahe
.

Caccini
, Tommaso (1574–1648). A Dominican friar from Florence who held various administrative positions in his order and earned various academic degrees and positions in theology. He accused Galileo of heresy in a sermon in 1614 and testified against him with the Inquisition in 1615.

Castelli
, Benedetto (1578–1643). Benedictine monk, student of Galileo at the University of Padua, his successor at the University of Pisa, and friend and collaborator; also an important figure in his own right, mainly for his contributions to the science of hydraulics, and as the teacher of many outstanding Italian scientists of the period.

comet.
A large heavenly body appearing as a luminous mass to which is attached a long tail, and visible for only brief periods ranging from a few days to several months. Though comets had been observed since antiquity, in Galileo's time their nature and origin remained controversial; the main issue was whether they were heavenly bodies or atmospheric phenomena. Nowadays, comets are known to be bodies of great volume but very small mass, to consist mostly of ice, and to follow definite (elliptical or parabolic) orbits around the sun; furthermore, the periodic recurrence of some of them can be predicted with great accuracy; but many more details remain controversial or unknown.

conjunction.
A configuration in the apparent position of two heavenly bodies when they appear to be on the same side of the earth, namely, close to each other or separated by only a few degrees on the celestial sphere. For example, a new or thinly crescent moon occurs when it and the sun are in conjunction.

cubit.
An ancient unit of distance corresponding to the length of a forearm, and thus approximately one and one-half to two feet. This is the term used to translate Galileo's term
braccio
.

declination.
The angular distance of a star from the celestial equator as seen from the earth. The analogue for the celestial sphere of what latitude is for the earth's surface.

direct motion.
Apparent motion which planets exhibit most of the time in their journeys against the background of the fixed stars; its direction is eastward, namely it follows the order of the constellations of the zodiac. Used primarily in contexts where one wants to contrast direct motion to retrograde motion, whose direction is opposite (namely, westward).

diurnal motion.
At the level of observation, diurnal motion is the apparent motion of all heavenly bodies around the earth, occurring every day in a
westward
direction. In the geostatic worldview, such apparent motion corresponds to reality. In the Copernican system, the diurnal motion is simply the earth's daily rotation around its own axis, in an
eastward
direction.

eccentric.
An eccentric is a circular orbit of one heavenly body around another such that the second body is not located at the geometrical center of the orbit but off that center. This device enables the distance between the two bodies to vary.

eclipse.
An eclipse occurs when the sun or moon becomes partially or completely invisible due to their relative position vis-à-vis the earth. In a lunar eclipse, the moon is eclipsed; that is, the earth is directly between the sun and the moon, and the moon is in the earth's cone-shaped shadow. In a solar eclipse, the sun is eclipsed; that is, the moon is directly between the sun and the earth, and the earth is in the moon's cone-shaped shadow.

ecliptic.
A term used to denote the annual orbit of the sun around the earth (in the geostatic system) or of the earth around the sun (in the Copernican system). The term also denotes both the plane on which the annual orbit lies and the circle resulting from projecting the annual orbit onto the celestial sphere.

element.
One of the four basic substances out of which all terrestrial bodies were thought to be composed: earth, water, air, and fire.

elemental.
Pertaining to the four terrestrial elements (earth, water, air, and fire).

epicycle.
A circle whose center lies on and moves along the circumference of a larger circle, called
deferent
. The postulation of epicycles enabled astronomers to analyze the motion of heavenly bodies as a combination of circular motions, so that there would be variations in the distance from the heavenly body to the center of the deferent, as well as in the body's direction of motion as seen from that center.

equidistance of ratios.
If
a, b, c, d
, … ,
l
is one set of magnitudes, and
A, B, C, D
, … ,
L
is another, such that
a:b = A:B, b:c = B:C
, …, and
k:l = K:L
, then it follows by
equidistance of ratios
that
a:l = A:L.

fixed star.
A heavenly body that is visible normally only at night and appears to revolve daily around the earth without changing its position relative to other stars; thus all fixed stars appear to move in unison as if they were fixed on a celestial sphere, whose daily rotation carries them all along. A fixed star corresponds to what is nowadays called simply
star
, but in Galileo's time
star
meant simply
heavenly body
and stars were divided into fixed and wandering.

force.
In modern physics, a force is defined by means of Newton's second law of motion, and so it is a cause of
changes
of speed or direction of motion. In Aristotelian physics, a force was a cause of motion and could be internal or external; internal forces caused natural motions, external forces caused violent motions. In Galileo's work, force had a less clear and less precise meaning that overlaps with both the Aristotelian and the Newtonian concepts, as well as with the concept of energy; although he was groping toward the Newtonian concept, he did not really possess it; Galileo's notion was also interwoven in confusing ways with his talk of
power
and
moment.

Foscarini
, Paolo Antonio (1580–1616). Head of the order of Carmelites in the province of Calabria and professor of theology at the University of Messina. He published in early 1615 a book that attempted to show the compatibility between the Bible and the earth's motion. This book was condemned and totally banned by the Index's Decree of 1616.

fourth proportional.
Given three quantities
A, B, C
, the fourth proportional to them is a quantity
X
such that
A:B = C:X
.

geokinetic.
Pertaining to the earth's motion or claiming that the earth moves. The geokinetic worldview claims that the earth rotates daily on its axis from west to east and revolves yearly around the sun in the same direction. This term is contrasted with
geostatic
and may be taken to correspond to
Copernican
.

gravity.
A term used interchangeably with
weight
and
heaviness
. In the Aristotelian worldview, gravity is the property of the elements earth and water whereby they tend to move toward the center of the universe; it manifests itself either as weight or free fall; and it is contrasted with a property called
levity
(or
lightness
), which is attributed to the elements air and fire, which consists of the tendency to move away from the center of the universe, and which manifests itself as buoyancy or spontaneous upward motion; bodies with gravity are called
heavy bodies
, and those with levity are called
light bodies;
it follows that light bodies go up because of their intrinsic property of levity, and not because they weigh less than the surrounding medium; in short, bodies with levity are thought to have no weight. Galileo abandoned the dichotomy between gravity and levity and held that all bodies have weight, thus explaining buoyancy and spontaneous upward motion in terms of the relative weight or specific gravity of the bodies involved; for him gravity was a property belonging to all bodies in the universe (heavenly as well as terrestrial), but consisted of the tendency to go toward the center of the whole of which one was a part, so that a rock on the moon would tend to move toward the center of the moon. Thus for both Aristotle and Galileo gravity could be labeled a universal property, but in different senses; for Aristotle it was universal in the sense that it was defined in terms of the center of the universe, a unique point yielding an absolute frame of reference; for Galileo it was universal in the sense that it characterized all material bodies in the universe; but even Galileo did not conceive of gravity as universal in the sense of Newton's
gravitation
, namely, in terms of mutual attraction among all bodies in the universe, and thus as acting between the earth and the moon.

great circle.
On a spherical surface, a circle whose center coincides with the center of the sphere. For example, on the earth the equator and the meridians are great circles, but the parallels are not.

Heraclides of Pontus.
Ancient Greek who lived in the fourth century B.C.

Hicetas of Syracuse.
Ancient Greek who lived about 400 B.C.

impetus.
In late medieval physics, the impetus of a projectile was the power to move that had been transferred to it by the projector and that would be gradually lost. Galileo uses the term to refer to the power that a body has due to either the quantity of motion it embodies or the tendency it has to move in a particular way. Thus, the Galilean meaning is inexact and corresponds partly to the late medieval meaning and partly to what modern physics would call either momentum, kinetic energy, or even potential energy.

Index.
Short for the Congregation of the Index, the department of the Catholic Church in charge of book censorship. Officially created in 1572 by Pope Gregory XIII, it was meant to formalize the periodic publication of the
Index of Prohibited Books
, whose first edition had already appeared in 1564 as a result of the Council of Trent. The Congregation was formally abolished by Pope Benedict XV in 1917, when the task of book censorship was taken over by the Inquisition.

Then in 1966 the Inquisition relinquished this task by decreeing that within the Church book regulation is a moral and not a legal issue.

Inquisition.
The common name for the Congregation of the Holy Office, the department of the Catholic Church whose purpose was to defend and uphold faith and morals. It was officially instituted in 1542 by Pope Paul III and was meant to take over the suppression of heresies and heretics begun by the medieval Inquisition. By the time of Galileo, the notion of heresy had been given a legal definition, and inquisitorial procedures had been codified. Nowadays this department is called the Congregation for the Doctrine of the Faith.

Jupiter.
A planet whose orbit is bigger than the annual orbit and whose period of revolution is about twelve years. In the geostatic system, it is the sixth planet from the earth; in the Copernican system, it is the fifth planet from the sun.

Lincean Academy.
Also called Academy of the Linceans (in Italian
Accademia dei Lincei
, which means literally “academy of those who have lynx eyes”), this was the first modern international scientific academy, founded in 1603 by Prince Federico Cesi (1585–1630), although it fell apart soon after his death. Galileo was made a member in 1611, became a friend of Cesi, and received support from the Academy for the publication of many of his works.

mean proportional.
Given two quantities,
A
and
B
, their mean proportional is a quantity
X
such that
A:X = X:B.

Medicean Planets or Stars.
A term used by Galileo to refer to Jupiter's satellites, which he discovered. He named the new bodies in honor of Cosimo II de' Medici, who ruled Florence and the Grand Duchy of Tuscany.