History & Physical Concepts
LACC §1.1, 1.4, 2.1, 2.3
• Some basic Astronomy terminology
• Astronomy’s Giants: Copernicus’s heliocentric
model (1543), Kepler’s laws of planetary motion
(1605), Galileo’s telescopic observations of
Jupiter’s moon’s and the phases of Venus (1610)
• Some Physical Concepts: Kepler’s three laws of
planetary motion, Newton’s three laws of motion,
Newton’s universal law of gravity
Astronomy is the branch of science concerned with
the nature of space, e.g. stars, planets, the universe.

Welcome to
Astronomy 1
Discussion: What questions does Astronomy
seek to answer?

Astronomy Basics
• star • solar system
• planet • galaxy
• moon (or satellite) • universe
• asteroid • rotation
• comet • orbit

The Earth is (mostly) Round
Who first discovered that the Earth is round?

The Earth is Round
On the Heavens
By Aristotle
Written 350 B.C.E
Translated by J. L. Stocks
...the interposition of the earth that makes the eclipse, the form
of this line will be caused by the form of the earth's surface,
which is therefore spherical. Again, our observations of the stars
make it evident, not only that the earth is circular, but also that it
is a circle of no great size. For quite a small change of position to
south or north causes a manifest alteration of the horizon. There
is much change, I mean, in the stars which are overhead, and the
stars seen are different, as one moves northward or southward.
http://classics.mit.edu/Aristotle/heavens.2.ii.html

Size of the Sun
Aristarchus
of Samos
(310-230 BC)
also reasoned
that since the
Sun and the
Moon have
the same
angular size,
but the Sun is
19 times
further (it
is actually 390
times), then
the Sun must
be 19 times
bigger than
the Moon.
http://www.astro.cornell.edu/academics/courses/astro201/aristarchus.htm

The Earth is Round
about 200 BC
At Syene, the zenith distance was 0 degrees; at
Alexandria it was about 7 degrees.
http://www.astro.cornell.edu/academics/courses/astro201/eratosthenes.htm

Geocentric Model
Claudius Ptolemaeus’s
Almagest in c. 140 A.D.
Because the planets seem to
move backward some of the time,
however, their observed motion
cannot be explained by single
circles. Ptolemy adopted a
solution to this problem that he
attributes to Apollonius (although
earlier Greek writers, such as
Hipparchus, also used this
concept): Each planet moves on a
small circle, called an epicycle....
Although these complex motions
seem strange to those familiar
with modern astronomy, they
succeed in accounting for
observed motions.
http://www.answers.com/topic/almagest?cat=technology

Heliocentric Model
Nicolás Copérnic’s De
Revolutionibus Orbium Coelestium
in 1543.
De Revolutionibus famously proposed
the heliocentric theory: the (now taken
for granted) proposition that the Earth
rotates around the Sun rather than vice
versa. During Copernicus’ lifetime,
orthodox opinion asserted the contrary
view – that the Earth was fixed,
unmoving at the centre of the
Universe. This “geo-centric” myth was
not easy to de-bunk: it was popularly
held to be true by common sense
perception supported by two millennia
of philosophical tradition....
http://special.lib.gla.ac.uk/exhibns/month/apr2008.html

Retrograde Motion
According to Geocentric Model
http://astro.unl.edu/classaction/loader.html?filename=animations/renaissance/
marsorbit.swf&movieid=marsorbit&width=825&height=550&version=6.0.0
According to Heliocentric Model
http://astro.unl.edu/classaction/loader.html?filename=animations/renaissance/
retrograde.swf&movieid=retrograde&width=700&height=600&version=6.0.0

Galileo Discovers Four
Moons Around Jupiter
Galileo first observed the moons of Jupiter on
January 7, 1610 through a homemade telescope. He
originally thought he saw three stars near Jupiter,
strung out in a line through the planet. The next
evening, these stars seemed to have moved the
wrong way, which caught his attention. Galileo
continued to observe the stars and Jupiter for the
next week. On January 11, a fourth star (which
would later turn out to be Ganymede) appeared.
After a week, Galileo had observed that the four
stars never left the vicinity of Jupiter and appeared
to be carried along with the planet, and that they
changed their position with respect to each other
and Jupiter. Finally, Galileo determined that what he
was observing were not stars, but planetary bodies
that were in orbit around Jupiter. This discovery
provided evidence in support of the Copernican
system and showed that everything did not revolve
around the Earth.
http://www.telescope1609.com/
http://www.solarviews.com/eng/galdisc.htm
Galileo.htm

Galileo Discovers Four
Moons Around Jupiter
Galileo Galilei’s Sidereus Nuncius, March 1610.
http://www.solarviews.com/eng/galdisc.htm

Galileo: Phases of Venus
Possibly the most compelling argument Galileo made in favor of the Heliocentric
Universe of Copernicus was based on the observations of Venus. Galileo observed the
phases of Venus throughout the year. At times Venus presented a small but circular disk
and at other times a large crescent. Based on these facts as illustrated in his drawings in
Sidereus Nuncius, Galileo reasoned that Venus must orbit the Sun; proof of the
Copernican Universe.
http://www.telescope1609.com/Galileo.htm

Phases of Venus
According to Geocentric Model
http://astro.unl.edu/classaction/loader.html?filename=animations/renaissance/
ptolemaic.swf&movieid=ptolemaic&width=900&height=660&version=6.0.0
According to Heliocentric Model
http://astro.unl.edu/classaction/loader.html?filename=animations/renaissance/
venusphases.swf&movieid=venusphases&width=870&height=600&version=6.0.0

Kepler’s Laws of
Planetary Motion
1. The orbit of a planet about the Sun is an
ellipse with the Sun at one focus.
2. A line joining a planet and the Sun sweeps out
equal areas in equal intervals of time.
3. The squares of the periods of the planets are
proportional to the cubes of their semi-major
axes (i.e. orbital radiuses):
paraphrased from Johannes Kepler's
Astronomia Nova and Harmonices Mundi
published in 1609 and 1619.

Kepler’s Laws of
Planetary Motion
Planet P (yr.) a (AU) T2 R3
Mercury 0.24 0.39 0.06 0.06
Venus 0.62 0.72 0.39 0.37
Earth 1 1 1 1
Mars 1.88 1.52 3.53 3.51
Jupiter 11.9 5.2 142 141
Saturn 29.5 9.54 870 868

Kepler’s Laws of
Planetary Motion
http://astro.unl.edu/classaction/loader.html?filename=animations/renaissance/
kepler.swf&movieid=kepler&width=900&height=710&version=6.0.0

Newton’s Laws of Motion
1. An object at rest remains at rest, and an
object in motion remains in motion, unless
acted upon by on outside force. (aka: The Law
of Inertia)
2. A force causes a mass to accelerate. (aka:
F = ma)
3. For every action, there is an equal and
opposite reaction.
paraphrased from Isaac Newton’s Philosophiæ
Naturalis Principia Mathematica published in
1687.

Newton's law of
universal gravitation
F = Force
G = a constant: 6.67 x 10-11 m3/(kg•s2)
M’s = two masses
R = distance between the two masses

Newton's law of
universal gravitation
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/circles/u6l3c.html

Newton's law of
universal gravitation
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/circles/u6l3c.html

Newton + Kepler
M
a
M = mass of central object
a = radius of orbit
p = period of orbit, i.e. how This equation is an
long it takes to orbit once approximation. It works
π = pi, 3.14 when the mass of the
orbiting object is much less
G = Gravitational constant, than the mass of the central
6.67 x 10-11 m3/(kg•s2) object.
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/circles/u6l3c.html

AstroTeam Classwork
• Give four ways to demonstrate that the Earth is
round. (Voyages, Ch. 1, pp. 40-41: 4)
• The International Space Station orbits about
500 km above the surface of the earth. The
Earth’s radius is 6378 km). How does the force
of gravity in the I.S.S. compare with that on the
ground? (Voyages, Ch. 2, pp. 58-59: 8)
Due presently.

LACC: Franknoi, Morrison, and Wolff,
Voyages Through the Universe, 3rd ed.
• Ch. 1, pp. 40-41: 1.
• Ch. 2, pp. 58-59: 11.
• Ch 3: Tutorial Quizzes accessible from:
www.brookscole.com/cgi-brookscole/course_products_bc.pl?
http://
fid=M20b&product_isbn_issn=9780495017899&discipline_number=19
Due at the beginning of the next class period.