The Essential Galileo (5 page)

meridian.
A circle on the surface of a sphere passing through both poles and cutting the equator at right angles.

Milky Way.
Wide band of faint light in the night sky stretching all the way around the celestial sphere. Corresponds to the galaxy of stars in which our solar system is located.

moment.
Aside from the obvious connotation of an instant of time, this word is used by Galileo with several other meanings. One is an approximation to the
momentum
of modern physics. Another is synonymous with the terms
magnitude
or
intensity
or
degree
, as in the phrase “the moment of the speed a body possesses.”

momentum.
In classical (Newtonian) physics, momentum is defined as the product of a body's mass and its velocity, taking velocity as a vector quantity (which has both a numerical magnitude and a spatial direction). The law of conservation of momentum states that in a closed system the total amount of momentum neither increases nor decreases but remains constant. Galileo's counterpart of this law is his principle of conservation of motion; but the correspondence is inexact because he had no conception that momentum is a vector and because he did not clearly distinguish between momentum and kinetic energy.

natural motion.
In Aristotelian natural philosophy, natural motion is the motion which a body has by nature; that is, motion which the body has because of its nature; namely, motion caused by the moving body's inherent nature; or again, motion caused by a force internal or inherent to the moving body. Thus, the natural motion of a terrestrial body is the motion it spontaneously tends to undergo in order to reach its natural place of rest, if it is not already there; for example, the natural motion of the elements earth and water is straight toward the center of the universe, and the natural motion of the elements air and fire is straight away from the center of the universe. Natural motion is contrasted with
violent motion
. Galileo partly accepted and partly modified this notion. He continued to speak of spontaneous (or internally caused) motion as one kind of natural motion, but dissociated it from the doctrine of natural places; so, for him the oscillation of a pendulum on the earth or the free fall of a rock on the moon would be as natural as the free fall of a rock on the earth. He sometimes added another meaning to the concept of natural motion, namely, motion that can last forever. And he contrasted natural motion with violent motion but also spoke of a third kind which is neither natural nor violent and which he labeled
indifferent
or
neutral
motion; an example of the latter would be horizontal motion on a frictionless surface.

opposition.
A configuration in the apparent position of two heavenly bodies when they appear to be on opposite sides from the observer on earth, namely, 180 degrees apart on the celestial sphere.

For example, a full moon occurs when it and the sun are in opposition.

orb.
A term that is partly synonymous with the term
orbit
, namely, the path followed by one heavenly body around another. The term also refers to the region of the heavens where a given orbital path is located. For example, the
lunar orb
could refer either to the path of the moon around the earth, or to the region of space surrounding the earth at a distance equal to that between the earth and the moon.

orbit.
The path followed by a heavenly body as it moves among the other bodies, usually around some particular body or point that is regarded as the center or focus of the orbit.

palm.
A term used in this book to translate literally Galileo's word
palmo
. This was an ancient inexact unit of length corresponding to either the width of the palm of a hand, the length of a hand, or the distance from the tip of the thumb to the tip of the little finger when extended.

parabola.
In mathematics, a plane curve defined as the set of all points equidistant from a fixed straight line (called the
directrix
) and a fixed point (called the
focus
). The shape generated is that of the intersection of a cone and a plane parallel to its side.

parallel.
This term has two relevant meanings, as an adjective and as a noun. Two lines lying in the same plane are said to be parallel to each other when they never meet regardless of how far they are extended; similarly, two planes are parallel to each other when they never meet regardless of how far they are extended. Used as a noun, a parallel is a circle on the surface of a sphere (such as the earth or the celestial sphere) that is parallel to the equator; these parallels become smaller and smaller as one moves on the sphere's surface from the equator to the poles.

perigee.
In the orbit of a heavenly body, this is the point closest to the earth.

Peripatetic.
A Greek word meaning literally a person who walks around. A nickname given to Aristotelians in Galileo's time. Peripatetics acquired this nickname because in the school founded by Aristotle the teachers had the habit of walking around while lecturing.

perturbed equidistance of ratios.
Given two sets of three magnitudes
a, b, c
, and
A, B, C
, such that
a:b = B:C
and
b:c = A:B
, it follows by
perturbed equidistance of ratios
that
a:c = A:C.

phases.
The phases of a nearby heavenly body (such as the moon and Venus) are the periodic changes in its apparent shape from round disk to semicircle to crescent and back to semicircle and round disk. They are caused by changes in the relative position between the sun, the earth, and the other body: a crescent is seen when the body is in the region between the earth and the sun; a semicircle is seen when the line connecting the three bodies forms an angle close to a right angle; and a full disk is seen when the body's entire surface illuminated by the sun can be seen from the earth, either because the earth is between the sun and the body (as in the case of the moon) or because the sun is between the earth and the body (as in the case of Venus).

Philolaus of Croton
(c. 470–c. 385 B.C.). Greek philosopher from southern Italy who accepted some of Pythagoras' ideas. He elaborated the view that the earth, together with the other planets (including the sun), moves around the center of the universe, where a central fire is located.

planet.
A term originating from Greek and meaning a wandering star. In the geostatic worldview, a planet is a heavenly body that appears to move both around the earth and in relation to other heavenly bodies; that is, a heavenly body that simultaneously performs two motions around the earth, the diurnal motion from east to west every day and another revolution from west to east in a definite period of time varying from one planet to another. There were seven planets, and their arrangement in the order of increasing orbit and period was as follows: moon, one month; Mercury, Venus, and sun, one year; Mars, two years; Jupiter, twelve years; and Saturn, twenty-nine years. In the Copernican view, a planet is a heavenly body that revolves around the sun, again in a definite period of time that varies from one planet to another: Mercury, three months; Venus, seven and a half months; earth, one year; Mars, two years; Jupiter, twelve years; and Saturn, twenty-nine years. In Galileo's time, the planets Uranus, Neptune, and Pluto had not yet been discovered.

Prime Mobile.
A term meant to convey the idea of the “first body in motion.” In Aristotelian natural philosophy, the Prime Mobile was a sphere lying outside the celestial sphere and acted upon by the First Unmoved Mover; by rotating daily, the Prime Mobile carried along all the other heavenly bodies (excluding the earth). The Prime Mobile was needed by those Aristotelians for whom the celestial sphere could not be a source of the diurnal motion; in fact, there was evidence that it had another slower movement in the opposite direction (the precession of the equinoxes), and the idea was to have a distinct sphere for each distinct movement.

Pseudo-Aristotle.
The unknown author of a book entitled
Questions of Mechanics
, wrongly attributed to Aristotle according to a tradition that lasted past Galileo's time. The book was probably written a generation after Aristotle's death, by one of his followers.

retrograde motion.
Westward motion against the background of the fixed stars which a planet appears to have periodically for a brief period, thus reversing its usually eastward motion (called
direct motion
). In the geostatic system, it was explained by means of epicycles. In the Copernican system, it is explained in terms of the relative motion between the earth and the planet in question.

Sagredo
, Giovanfrancesco (1571–1620). Venetian aristocrat and diplomat who became Galileo's best friend when Galileo taught at the University of Padua. Sagredo has been immortalized as one of the three speakers in the
Dialogue
and the
Two New Sciences
.

Salviati
, Filippo (1582–1614). Wealthy Florentine nobleman whose interest in science and philosophy earned him membership in the Lincean Academy in 1612. One of Galileo's closest friends in Florence, Salviati has been immortalized as one of the three characters in the
Dialogue
and the
Two New Sciences
.

save the appearances
(or the phenomena). To explain observed natural phenomena by means of assumptions that are taken not to describe real physical processes, but rather to be merely convenient instruments for making calculations and predictions.

Seleucus.
A Babylonian who lived around 150 B.C. and who was a follower of Aristarchus' geokinetic theory.

Simplicio.
One of the three characters in the
Dialogue
and the
Two New Sciences
. In Italian,
Simplicio
denotes both a simpleton and the philosopher Simplicius.

Simplicius.
Greek philosopher who lived in the sixth century A.D., famous as one of the greatest commentators of Aristotle.

specific gravity.
A term used by Galileo to mean weight per unit volume (the weighing being done presumably in air). This is analogous to the modern concept of density (which means
mass
per unit volume), except that Galileo did not have a clear concept of mass as distinct from weight. His point is that he wants a concept enabling him to compare the weights of equal volumes of different substances, so as to say, for example, that wax is “specifically” heavier than cork but lighter than lead.

spyglass.
Term used by Galileo to refer to the telescope during the first two years that he used the instrument (
perspicillum
in Latin,
occhiale
in Italian). In 1611 the term
telescopium
was coined, and he adopted the new term.

telescope.
Optical instrument consisting of an arrangement of lenses, mirrors, or both, that magnifies the image of distant objects so that they appear larger or nearer, thus rendering our vision more powerful and enabling us to see things that cannot be seen by the naked eye. Invented in Holland in 1608, it was significantly improved by Galileo the following year and turned into an effective scientific instrument for acquiring new knowledge about the world. The word (
telescopium
in Latin) was not coined until 1611, and before 1611 Galileo called the instrument a
spyglass
.

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

Tycho Brahe
(1546–1601). Danish astronomer, best known as an excellent observer and collector of data and as the promoter of the so-called Tychonic system. In the Tychonic system, the earth is motionless at the center of the universe; the planets revolve around the sun; but the sun (together with all the planets) moves around the earth, daily in a westward direction and annually in an eastward direction. Kepler worked with him and inherited his data.

Venus.
A planet that revolves in its orbit in such a way that it always appears close to the sun. In the Copernican system, Venus is the second planet from the sun and completes its orbit in seven and a half months. In the geostatic system, opinions differed about whether it was the second, third, or fourth planet from the earth; but it was most commonly regarded as the third (between Mercury and the sun). With the telescope, Galileo discovered the phases of Venus, which are changes in its apparent shape similar to those which the moon exhibits each month; this proved conclusively that Venus revolves around the sun. However, this confirmed the system of Tycho Brahe as well as that of Copernicus, and so the choice between these two systems required other evidence for a conclusive demonstration.

violent motion.
In Aristotelian natural philosophy, violent motion is motion that occurs because of the influence of some external force; examples are the motion of a cart pulled by a horse, the motion of a rowboat pushed by rowing, and the lifting of a weight with a pulley. Violent motion was contrasted with
natural motion
. Galileo partly accepted and partly modified this doctrine of violent motion.

wandering star.
Heavenly body that appears not only to revolve daily around the earth, but also to change its position relative to other heavenly bodies; that is, in contrast to a
fixed star
, each wandering star moves around in the heavens according to a period that characterizes it. A wandering star is hence equivalent to a
planet
, a Greek word whose literal meaning is
wandering star.

zodiac.
Narrow belt on the celestial sphere along which the planets, sun, and moon appear to move. The zodiac is subdivided into twelve equal parts of thirty degrees, each part being the location of a group of stars arranged into a distinct pattern. These twelve patterns are the constellations of the zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. The sun, moon, and planets are always seen somewhere in one of these constellations, moving from one constellation to the next in the order listed. This order corresponds to an eastward direction (from the viewpoint of terrestrial observation), and so the order of the signs of the zodiac is a direction of motion opposite to that of apparent diurnal rotation.