A1 01 History and Concepts

Early Astronomers
§2-1, 3-6, 4-1, 4-2, 4-3, 4-5

• Earth: Shape and Size?
• Solar System: Geocentric or Heliocentric?
• Galileo Galilei
Astronomy is the branch of science concerned with the
nature of space, e.g. stars, planets, the universe.

In fact, th e Ea rt h is
a n Obl ate Sph eroid--
it b ul ge s sl igh tl y at
Q: Who ﬁrst discovered that the Earth is round? When? th e eq uato r.
1
Monday, August 16, 2010

The Earth is Round
ways
a re t wo t h i s
On the Heavens
H e re e e ar
to sh o w t h n yo u By Aristotle
a
ro u n d . C o re? Written 350 B.C.E.
m
t hi n k o f
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
2

Earth is 25,000 Miles Around
...Eratosthenes. About 240 B.C., as librarian of Alexandria's already unsurpassed library of
scrolls, he learned that Syene...stands almost exactly on the Tropic of Cancer. At noon the
reﬂection of the midsummer sun was there visible in the water of a deep well. This showed that
the sun was directly overhead and that its beams therefore pointed in a straight line toward the
middle of the earth. On the same day, measurement of the noon shadow cast by a pillar at
Alexandria shows that the sunbeam strikes the earth at an angle of 7.2°off the vertical. Sunbeams
travel in parallel, so we may account for the difference only by the curve of the
earth…. Eratosthenes thus knew that the angle between Alexandria … and Syene must be
7.2°, one ﬁftieth of the 360 degrees circle…. Syene lies nearly due south of Alexandria, and the
road between them therefor lies almost exactly on a great circle passing through the North
and the South
poles. Since it is
almost exactly
480 miles long,
the great circle
is 50 times 480
miles in length:
that is, the
circumference
of the earth is
about 24,000
miles.
http://library.thinkquest.org/25672/earth.htm http://www.juliantrubin.com/aboutfairs.html
Actual value: 24,900 miles at the equator. 3

“Wandering Stars”

West
East
West
East

http://mars.jpl.nasa.gov/allaboutmars/nightsky/nightsky04/

Ancient astronomers noticed that ﬁve “stars” seemed to wander through the ecliptic--
they didn’t stay ﬁxed in a constellation like most stars. (The greeks called them “asterai
planetai”.) In general, these “wandering stars” move west to east through the
constellations of the zodiac. Occasionally however, they reverse direction, and move
east to west; this is called retrograde motion. How to explain this behavior?
4

Geocentric Model
Claudius Ptolemaeus’s Almagest
in c. 140 A.D.
Because the planets seem to move
backward some of the time...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

http://astro.unl.edu/classaction/loader.html?ﬁlename=animations/renaissance/
marsorbit.swf&movieid=marsorbit&width=825&height=550&version=6.0.0 0:30
5

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

http://mars.jpl.nasa.gov/allaboutmars/nightsky/nightsky04/
0:05
6

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
7

Galileo Discovers Four
Moons Around Jupiter
Galileo Galilei’s Sidereus Nuncius, March 1610.

http://www.solarviews.com/eng/galdisc.htm
8

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://astronomy.fm/skylogs/skysafari/520/
http://www.telescope1609.com/Galileo.htm
Galileo---Father-of-Modern-Astronomy.html
9

Phases of Venus

According to Geocentric Model
ptolemaic.swf&movieid=ptolemaic&width=900&height=660&version=6.0.0

According to Heliocentric Model
venusphases.swf&movieid=venusphases&width=870&height=600&version=6.0.0

Since we do sometimes see Venus in a gibbous phase,
which model can be ruled out?

10

Elongation
the angle between the
planet and the sun

When are Mars, Jupiter,
and Saturn brightest?
What phase would they
be in?

When are Mercury and
Venus brightest?
(

be in?

http://www.eso.org/public/outreach/eduoff/vt-2004/Background/Infol2/EIS-D3.html
11

Elongation
the angle between the
The maximum elongation of Venus is
about 47°. Venus is a remarkable planet at then sun
object in the night sky at its
brightest, 35 days before or after
inferior conjunction, when one third
of the visible surface is illuminated.
Under favourable conditions it is When are Mars, Jupiter,
even possible to see the crescent
shape of Venus with binoculars. and Saturn brightest?
be in?

When are Mercury and
Venus brightest?
(

be in?

http://www.eso.org/public/outreach/eduoff/vt-2004/Background/Infol2/EIS-D3.html
12

Early Astronomers
§2-1, 3-6, 4-1, 4-2, 4-3, 4-5
• Earth: Shape and Size?

• Round--Aristotle, 350 BC

• About 12756.32 kilometers or 7926.41 miles--
Eratosthenes, about 240 BC

• Solar System and the Motion of the Planets, esp. Retrograde:

• Geocentric--Ptolemy, 140 AD

• Heliocentric--Copernicus, 1543 AD

• Galileo Galilei: Moons of Jupiter, Phases of Venus

13

AstroTeam Classwork

• Class Action Questions

• Give four ways to demonstrate that the Earth is
round.

All classwork due presently.

14

Freedman, Kaufmann, Robert Geller;
Universe, 9th ed.

• Ch 4: Online Quiz accessible from:
http://bcs.whfreeman.com/universe9e
must know: 1, 2, 3, 8, 9, 11, 12, 13, 14, 15, 16, 19, 20
can’t hurt: 4, 6, 7, 10, 17, 18
wish we could skip: 5

• Ch. 4, p. 92: 2, 3, 20 (name at least two observations).
Due at the beginning of the next class.

15

Physical Concepts
§1.2, 1.4, 4.4, 4.5, 4.6, 4.7

• Some basic Astronomy terminology
• Kepler’s laws of planetary motion (1609)
• Newton’s three laws of motion, Newton’s universal
law of gravity (1687)
Astronomy is the branch of science concerned with
the nature of space, e.g. stars, planets, the universe.

16

Astronomy Basics

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

17

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.
18

Kepler’s Laws of
Planetary Motion

http://rst.gsfc.nasa.gov/Sect19/Sect19_2.html

http://astro.unl.edu/classaction/
animations/renaissance/kepler.html 3
19

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.

http://www.nasa.gov/audience/foreducators/
diypodcast/nl-video-index.html: 11,14;19,21;25 3
20

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

21

Newton's law of

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/circles/u6l3c.html
22

Newton's law of

23

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.

24

Physical Concepts
§1.2, 1.4, 4.4, 4.5, 4.6, 4.7

• Some basic Astronomy terminology
• rotation vs revolution
• solar system vs galaxy vs universe
• Kepler’s three laws of planetary motion (1609)
• Newton’s three laws of motion, Newton’s universal
law of gravity (1687)

25

AstroTeam Classwork

• Class Action Questions

• 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?

All classwork due presently.

26

Freedman, Kaufmann, Robert Geller;
Universe, 9th ed.

• Ch 2: Online Quiz accessible from:
http://bcs.whfreeman.com/universe9e
must know: 3, 5, 6, 7, 8, 9, 10, 11, 12, 16, 17, 18, 20
can’t hurt: 1, 4, 13, 14, 15, 19
wish we could skip: 2

• Ch. 4, p. 92: 16 (yes, one of these questions is a trick question),
26 (no need to explain), 27 (No more than three sentences).

Due at the beginning of the next class.

27

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