Download 3. 1 Astronomy before Copernicus 3. 2 Nicolaus Copernicus 3. 3

CHAPTER 3
THE ORIGIN OF MODERN ASTRONOMY
Topic Summaries
3. 1 Astronomy before Copernicus
How did people in ancient civilizations describe Earth's place in the universe?
•
Ancient philosophers accepted as a first principle that Earth was the unmoving
center of the universe. Another first principle w as that the heavens were perfect;
and, because the circle was the only perfect geometrical form, objects in the
heavens must move in uniform (constant speed) circular motion.
• The geocentric (Earth-centered) universe became part of the teachings of the
great philosopher Aristotle, who argued that the sun, moon, and stars were carried around Earth on rotating crystalline spheres.
About AD 140, Ptolemy gave mathematical form to Aristotl e's model. Ptolemy
kept the geocentric (Earth-centered) and uniform circular motion principles, but
he added off-center circles and variable speeds to better predict the motions of
the planets.
3. 2 Nicolaus Copernicus
How did Copernicus revise that ancient model?
Copernicus devised a heliocentric (sun-centered) model. He preserved the principle of uniform circular motion, but he argued that Eart rotates Ojl itS::Pxis and
circles the sun once a.year. His t heory w controve ial be ause i( contradic e
Church teaching.
Copernicus publi bed his theory in his book De Revolutionibus in 1543, the year
he died.
•
Why was the Copernican model gradually acce teCI 7
Because Copernicus kept uniform circular motion, his model did not pre ict th
motions of the planets well, but it did offer a simpl e explanation of retrograde
motion of planets without using large epicycles (circles on circles). He did have
to include small epicycles to account for some observed planetary motions.
The Copernican model was also more eloquent and straightforward. Venus and
Mercury were treated the same as all the other planets, and the velocity of each
planet was related to its distance from the sun.
first principle Something that
seems obviously true and needs no
further examination.
geocentric universe A model
universe with Earth at the center, such
as the Ptolemaic universe.
heliocentric universe A model
of the universe with the sun at the
center, such as the Copernican
universe.
uniform circular motion The
classical belief that the perfect
heavens could move only by the
combination of uniform motion along
circular orbits.
parallax The apparent change in
position of an object due to a change
in the location of the observer.
Astronomical parallax is measured in
secon(js of arc.
retrograde motion The apparent
~backward (westward) motion
of planets as seen against the
background of stars.
epicycle The small circle followed by
a planet in the Ptolemaic theory. The
center of the epicycle follows a larger
circle (the deferent) around Earth.
paradigm A commonly
accepted set of scientific ideas and
3. 3 rycho Brahe, Johannes Kepler, and
Planetary Motion
assumptions.
ellipse A closed curve around t wo
How did Tycho Bra he and Johannes Kepler contribute to the Copernican
Revolution?
points, called the foci, such that the
• Tycho's great contribution was to compile the most preci se and detailed nakedeye observations of the planets and stars ever made, observations that were
later analyzed by Kepler.
curve and back to the other focus
Kepler inherited Tycho's books of observations in 1601 and used them to uncover three laws of planetary motion. He found that the planets follow ellipses
with th e sun at one focus, that they move faster w hen near the sun, and that a
planet's orbital period squared is proportional to its orbital radius cubed.
total distance from one focus to the
remain s constant.
semi-major axis (a) Half of the
longest diameter of an ellipse.
eccentricity (e) A number between
1 and 0 that describes the shape of an
ellipse; the distance from one focus to
the center of the ellipse divided by the
semi major axis.
Key Terrns
empirical Description of a
phenomenon without explaining why
it occurs.
Topic Surnrnaries
3. 4 Galileo Galilei
Why was Galileo condemned by the Inquisition?
Galileo used the newly invented telescope to observe the heavens, and
he recognized the significance of what he saw there. His discoveries of the
phases of Venus, the satellites of Jupiter, the mountains of the moon, and
other phenomena helped undermine the Ptolemaic model.
hypothesis A conjecture, subject
to further tests, that accounts for a set
of facts.
theory A system of assumptions
Galileo based his analysis on observational evidence. In 1633, he was
condemned before the Inquisition for refusing to obey an order to halt his
defense of Copernicus's model.
and principles applicable to a wide
range of phenomena that has been
repeatedly verified.
natural law A theory that has been
3.5 Isaac Newton, Gravity, and Orbital Motion
so well confirmed that it is almost
How did Isaac Newton change humanity's view of nature?
universally accepted as correct.
Newton used the work of Kepler and Galileo to discover three laws of motion
and the law of gravity. These laws made it possible to understand such
phenomena as orbital motion and the tides.
mass A measure of the amount of
matter making up an object.
weight The force that gravity exerts
on an object.
inverse square relation A rule
that the strength of an effect (such as
•
Newton's laws gave scientists a unified way to think about nature. Every
effect has a cause, and science is the search for those causes.
The 144 years from Copernicus's book De Revolutionibus to Newton's book
Principia marked the beginning of modern science. From that time on, science depended on evidence to test theories and relied on the methods of
gravity) decreases in proportion as the
distance squared increases.
spring tide Ocean tide of jarge
range that occurs at full and new
moon.
neap tide Ocean tide of small range
occurring at first- and third-quarter
moon.
circular velocity The velocity an
object needs to stay in orbit around
another object.
geosynchronous satellite A
satellite that orbits eastward around
Earth with a period of 24 hours and
remains above the same spot on
Earth's surface.
center of mass The balance point
of a body or system of masses. The
point about which a body or system
of masses rotates in the absence of
external forces.
closed orbit An orbit that
repeatedly returns to the same
starting point.
escape velocity The initial velocity
an object needs to escape from the
surface of a celestial body.
open orbit An orbit that carries
an object away, never to return to its
starting point.
heavens were made up of
perfect crystalline spheres
moving at constant speeds?
2. How did the Ptolemaic model
explain retrograde motion of the
planets?
3. In what ways were the models of
Ptolemy and Copernicus similar?
4. Why did the Copernican
hypothesis gradually win
acceptance?
5. Why is it difficult for scientists to
replace an old paradigm with a
new paradigm?
6. Why did Tycho Bra he expect
the new star of 1572 to show
parallax? Why was the lack of
parallax evidence against the
Ptolemaic model?
7. Explain how Kepler's laws
contradict uniform circular
motion.
etween a hypothesis, a theory,
and a law?
9. Review Galilee's telescope
discoveries and explain why
they supported the Copernican
model and contradicted the
Ptolemaic model.
10. If you lived on Mars, which
planets would describe
retrograde loops? Which would
be always seen near the sun?
Which wou ld never be visible as
crescent phases?
11. Galileo was condemned by the
Inquisition, but Kepler, also a
supporter of Copernicus, was
not. Why not?
12. Why did Newton conclude that
gravitation had to be universal?
13. Explain w hy you might describe
the orbital motion of the moon
with the statement, "The moon
is falling."