CHAPTER 3
Gravity and the Rise of Modern Astronomy
CHAPTER OUTLINE
3-1 Galileo Galilei and the Telescope
1. Galileo was born in 1564 and was a contemporary of Kepler. He built his first
telescope in 1609, shortly after hearing about telescopes being constructed in the
Netherlands.
2. Galileo was the first person to use a telescope to study the sky. He made 5 important
observations that affected the comparison between the geocentric and heliocentric
theories.
(a) Mountains and valleys on the Moon
(b) Sunspots
(c) More stars than can be observed with the naked eye
(d) Four moons of Jupiter
(e) Complete cycle of phases of Venus
Observing the Moon, the Sun, and the Stars
1. Though Galileo’s first three observations do not disprove the geocentric model, they
cast doubt on its basic assumption of perfection in the heavens.
2. The existence of stars too dim to be seen with the naked eye also cast doubt on the
literal interpretation of some Biblical passages.
Jupiter’s Moons
1. In 1610 Galileo discovered that Jupiter had four satellites of its own, now known as the
Galilean moons of Jupiter.
2. The motion of Jupiter and its orbiting moons contradicted the Ptolemaic notions that
the Earth is the center of all things and that if the Earth moved through space it would
leave behind the Moon.
The Phases of Venus
1. Galileo observed that Venus goes through a full set of phases: full, gibbous, quarter,
crescent.
2. Venus’s full set of phases cannot be explained by the Ptolemaic model but can be
explained by the heliocentric model.
3. The Ptolemaic model predicts that Venus will always appear in a crescent phase, which
is not borne out by the observations.
4. Also, the heliocentric model explains the correlation between Venus’ phases and its
corresponding observed sizes.
5. Galileo is credited with setting the standard for studying nature through reliance on
observation and experimentation to test hypotheses.
3-2 Isaac Newton’s Grand Synthesis
Newton’s First Two Laws of Motion
1. The year Galileo died—1642—is the year Isaac Newton was born. Newton took the
work of Galileo and Kepler and created a new theory of motion.
2. Newton’s First Law (Law of Inertia): Unless a net, outside force, acts upon an object,
the object will maintain a constant speed in a straight line (if initially moving), or remain
at rest (if initially at rest).
3. Inertia is the tendency of an object to resist a change in its motion.
4. The first law indicates that a net force is necessary for an object to change its speed
and/or its direction of motion (i.e., to accelerate).
5. Newton’s second law quantifies and extends the first law. It tells us how much force is
necessary to produce a certain acceleration of an object.
An Important Digression — Mass and Weight
1. Mass is the quantifiable property of an object that is a measure of its inertia. It is an
intrinsic property of an object and independent of location.
2. Mass is NOT volume or weight. (The weight of an object on Earth is simply the
downward force experienced by the object due to its gravitational interaction with the
Earth.)
3. The international (SI) unit of mass is the kilogram. A kilogram weighs about 2.2
pounds on Earth.
Back to Newton’s Second Law
1. Newton’s Second Law: A net external force applied to an object causes it to accelerate
at a rate that is inversely proportional to its mass: Acceleration = net force / mass, or F =
m a.
2. When the net force is zero, there is no acceleration.
Newton’s Third Law
1. Newton’s Third Law: When object X exerts a force on object Y, object Y exerts an
equal and opposite force back on X.
2. The Third Law is sometimes stated as “For every action there is an equal and opposite
reaction,” but the first statement is more precise in terms of physical forces.
3-3 Motion in a Circle
1. Motion of an object in a circle at constant speed (uniform circular motion) is an
example of acceleration causing a change in direction.
2. Centripetal (“center-seeking”) force is the force directed toward the center of the
curve along which the object is moving. Centripetal force is simply a label we apply to a
net force that causes an object to move in a curve.
3-4 The Law of Universal Gravitation
1. The law of universal gravitation states that between every two objects there is an
attractive force, the magnitude of which is directly proportional to the mass of each
object and inversely proportional to the square of the distance between the centers of the
objects.
2. In equation form: F = Gm1m2 / d2, where G is a constant, m1 and m2 are the masses, and
d is the distance between their centers.
3. Weight is the gravitational force between an object and the planetary/stellar body
where the object is located.
4. According to Newton, gravity not only makes objects fall to Earth but keeps the Moon
in orbit around the Earth and keeps the planets in orbit around the Sun. His laws could
explain the planets’ motions and why Kepler’s laws worked.
Arriving at the Law of Universal Gravitation
1. Whether or not force is proportional to mass can be tested by showing that weight is
proportional to mass here on Earth.
2. To test the dependence of force on distance, Newton compared accelerations of objects
near the Earth’s surface to the Moon’s acceleration in orbit around the Earth.
3. Because the distance from the center of the Earth to the Moon is about 60 times the
distance from the center of the Earth to its surface, the centripetal acceleration of the
Moon should be (1/60)2 or 1/3600 of the acceleration of gravity on Earth. Newton’s
calculations showed this to be the case and confirmed the validity of his theory of
gravitation.
3-5 Newton’s Laws and Kepler’s Laws
1. Kepler’s first law (that the planets move in elliptical orbits) can be derived from
Newton’s laws but requires calculus.
2. Kepler’s second law (that an imaginary line connecting a planet and the Sun sweeps
out equal areas in equal times) can also be derived from Newton’s laws. As planets orbit
the Sun they show a change in both speed and direction.
3. Newton showed mathematically that Kepler’s third law—the period-distance
relationship—derives from the inverse square law for gravitation.
4. Newton’s modified version of Kepler’s third law, a(AU)3/P(yrs)2 = (m1 + m2)/mSun, is
valid for any two objects orbiting each other as a result of their mutual gravitational
attraction (a binary system). For the case where the two objects are the Sun and a planet,
Newton’s modified version reduces to Kepler’s third law.
3-6 Orbits and the Center of Mass
1. Center of mass is the average location of the various masses in a system, weighted
according to how far each is from that point. The center of mass of a system is not always
the same as the center of gravity.
2. The center of mass for the Earth-Moon system is inside the Earth, about 4,800 km
from its center.
3. Historically, the location of the center of mass of the Earth-Moon system was
determined by observing parallax of nearby planets due to the Earth’s motion around the
center of mass.
4. When we consider the motion of the Earth around the Sun, it is the center of mass of
the Earth-Moon system that follows an elliptical path.
3-7 Beyond Newton
1. Newtonian ideas succeed in describing a wide range of observations based on a few
basic ideas: for example, Kepler’s laws can be derived from them, they explain tides and
precession, and their use predicted the existence of the planet Neptune.
2. Newton’s laws provide a way to measure things quantitatively and predict the motion
of things. They were the first “laws” ever that could be shown to hold for both the
heavens and the Earth. They offer a unifying view of the universe.
3. Newton’s work confirmed the belief of the ancient Greeks that nature is explainable.
3-8 General Theory of Relativity
1. We defined mass as the measure of the inertia of an object. However, in Newton’s law
of gravity, mass determines the strength of gravitational attraction. The same quantity
measures two seemingly different physical properties.
2. Experiments show that the measures of inertia and gravitational attraction are identical
to one part in a trillion. This is not a coincidence.
3. Einstein, in his General Theory of Relativity, showed mathematically that the two
types of masses are indeed equivalent.
4. According to the principle of equivalence, effects of acceleration are
indistinguishable from gravitational effects.
5. Space warp, or the curvature of space, is used by Einstein to explain the similarity of
gravity and acceleration as well as the bending of light near a massive object.
6. For Einstein, as objects move, they follow the curvature of space created by the
presence of mass. (“Matter tells space how to curve, and space curvature tells matter how
to move.”) This view differs from Newton’s attractive gravitational force between
objects.
3-9 Gravitation and Einstein
1. The many tests of Einstein’s general theory of relativity show that it provides a better
description of gravity than Newton’s theory.
Test 1: The Gravitational Bending of Light
1. The general theory predicts that light will curve in the presence of a massive object.
This prediction, made in 1915, was first confirmed during a solar eclipse in 1919.
2. The bending of starlight by a massive object (the Sun, for example) is fully explained
by the general theory of relativity but is not by Newtonian mechanics.
Test 2: The Orbit of Mercury
1. 19th century measurements showed that Mercury’s elliptical orbit slowly precesses
about the Sun at the rate of 574 arcseconds per century. (Precession of an orbit
corresponds to the change in orientation of the major axis of the elliptical path of an
object.)
2. Calculations based on Newtonian mechanics can account for 531 arcseconds of this
precession of Mercury’s orbit. However, 43 arcseconds are unaccounted for by classical
theory.
3. Einstein’s general theory of relativity was able to explain the “mysterious” 43
arcseconds of precession by considering the properties of curved space. The Sun’s mass
curves space in its vicinity and Mercury’s orbit is affected by this space curving.
Additional Tests
1. According to the general theory of relativity, massive objects warp not only space
around them but also time: time slows down in the presence of gravity.
2. This effect was tested in 1960 and the results (discussed in Chapter 15) were in
complete agreement with the theory.
3. Also, the theory predicts the existence of gravitational waves, ripples in the curvature
of space produced by changes in the distribution of matter.
4. Strong indirect evidence for gravitational waves comes from observations of a binary
system of stars (discussed in Chapter 15).
5. Newton’s theory of gravity does not include any effect that gravity might have on time
and does not predict the existence of gravitational waves.
The Correspondence Principle
1. The correspondence principle states that the predictions of a new theory must agree
with the theory it replaces in cases where the previous theory has been found to be
correct.
2. The general theory of relativity is in accord with the correspondence principle. It
agrees with Newtonian mechanics where the older theory provided correct results.
3. All tests of Einstein’s relativity theory have confirmed it. We still use Newtonian
mechanics, however, when it fits, because it is easier to understand it.
Advancing the Model: The Special Theory of Relativity
1. The special theory of relativity is based on two principles.
(a) All laws of physics are the same for all non-accelerating observers, independent of
their speeds.
(b) The speed of light is the same for all non-accelerating observers, no matter what their
motion relative to the source of light.
2. The special theory of relativity predicts that:
(a) the observed length along the line of motion of a moving object becomes less than its
length when measured at rest;
(b) the observed passage of time becomes slower for the moving object;
(c) the observed inertia of an object becomes greater than its inertia when at rest; and
(d) mass can be transformed to energy and vice versa.
3. Experimental results confirm the predictions made by the special theory of relativity.