Ancient Planetary Astronomy
Ptolemy
Ptolemy
Geocentric Solar System
Copernicus
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07System
Heliocentric Solar
Galileo
Announcements
• The first homework assignment is on the course website
– Due January 25
– Start early!
• Starry Night Backyard software program
– The CD is in the back of the book – it is a blue CD, right behind the yellow
one (there are 2 CDs that accompany the textbook!)
• Preceptors
– Please fill out a preceptor application.
– It would be very helpful if you would also give me your schedule on the back
of the application
• Mission updates
– Extra credit (tbd) for an informal and brief (~5 minute) presentation of a
current (or past) mission or other topic related to the class
• LPL Public Lecture Series
– There will be some extra credit (tbd) given to attend these
– This 206
is a great opportunity
to Planetary
learn about
cutting-edge research at the UofA!
PTYS/ASTR
Ancient
Astronomy
– See www.lpl.arizona.edu/COLPL
for details
1/16/07
Today’s Topics
• Preliminary info
– Powers-of-ten notation
– Units and unit conversion
– Speed, time, velocity
• Ancient Planetary Astronomy
–
–
–
–
Ancient Greeks
Retrograde motion
Ptolemaic model of the solar system
Parallax
• The Copernican revolution
– A long-standing Earth-centered view of the solar system is
turned on its head with the invention of the telescope
– Phases of Venus
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
Powers-of-ten notation is a useful shorthand
system for writing numbers
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
Common Prefixes
Factor
(billion)
109
Name
Giga-
Symbol
G
(million)
(thousand)
(hundredth)
(thousandth)
106
103
10-2
10-3
Megakilocentimilli-
M
k
c
m
(millionth)
(billionth)
10-6
10-9
micronano-

n
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
Powers of ten manipulation
• 128 Billion miles = 128x109 miles = 1.28x1011 miles
• 20 milliseconds = 20 x 10-3 seconds = 2x10-2 seconds
• 6000 x 20000 = (6x103) x (2x104) = 12x107 = 1.2x108
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
Some common units used by planetary
scientists
• Distance
– Between the planets → AU
• 1AU = mean distance between Earth and Sun
– Size of craters and other features on planets → km
• Mass → kg
– Note that mass is different from weight
– Weight is the force exerted by gravity
• Will depend on where you are (you would weigh less on the Moon
and Mars)
• Speed → km/s
• Temperature → K (Kelvin), C (Centigrade), or F
(Fahrenheit)
PTYS/ASTR 206
Ancient Planetary Astronomy
– Kelvin (K) is the most
common
1/16/07
Speed, Distance, and Time
• Distance traveled = speed x time
• Basic algebra gives two other formulas from this:
– Average Speed = total distance traveled / time
– Time it takes = total distance traveled / speed
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
An Example
• When Voyager 2 spacecraft sent back pictures of
Neptune (the most distant planet in our solar system) in
1989, the radio signals took 4 hours to reach Earth. How
far away was the spacecraft?
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
An Example
• When Voyager 2 spacecraft sent back pictures of
Neptune (the most distant planet in our solar system) in
1989, the radio signals took 4 hours to reach Earth. How
far away was the spacecraft?
• The correct formula to use is:
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
An Example
• When Voyager 2 spacecraft sent back pictures of
Neptune (the most distant planet in our solar system) in
1989, the radio signals took 4 hours to reach Earth. How
far away was the spacecraft?
• The correct formula to use is:
(3x108 m/s) x (4 hours)
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
An Example
• When Voyager 2 spacecraft sent back pictures of
Neptune (the most distant planet in our solar system) in
1989, the radio signals took 4 hours to reach Earth. How
far away was the spacecraft?
• The correct formula to use is:
(3x108 m/s) x (4 hours)
(3x108 m/s) x ( 0.001 km/m)
x (4 hours) x (3600 s/hour)
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
An Example
• When Voyager 2 spacecraft sent back pictures of
Neptune (the most distant planet in our solar system) in
1989, the radio signals took 4 hours to reach Earth. How
far away was the spacecraft?
• The correct formula to use is:
(3x108 m/s) x (4 hours)
(3x108 m/s) x ( 0.001 km/m)
x (4 hours) x (3600 s/hour)
4.32 x 109 km
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
Revolutions in Planetary Science
•
Greek Philosophy (500
BC-200 AD)
– Greece, Turkey,
Egypt, Syria
– Systematic
philosophy
– Knew that Earth is a
sphere206
PTYS/ASTR
•
Copernican Revolution
(1500-1700 AD)
– Europe
– Modern telescopes
Ancient Planetary Astronomy
1/16/07
•
Age of spacecraft
(1960-present)
Ancient Greek Astronomy
• The word “planet” is derived from a
Greek word meaning wanderer
– They recognized that the planets did not
stay stationary relative to the background
of stars
• Only 5 were visible to the naked eye
– Mercury, Venus, Mars, Jupiter, and Saturn
• They were able to measure the size of
the Earth and the relative distances
between the Earth, Moon, and Sun
• They knew the Earth to be a sphere
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
The Ancient Greeks were able to measure
the size of the Earth
LIGHT RAYS FROM SUN
• Erotosthenes (276197 BC) – Accurately
measured the size of
the Earth using simple
geometry and the
assumption of a
spherical Earth
PTYS/ASTR 206
Deep Well
at Syrene
post at Alexandria
(which casts a shadow)
Erotosthenes method:
Ancient Planetary Astronomy
1/16/07
• Ancient astronomers believed the
Earth was the center of the
Universe.
– Plato’s views of celestial bodies
moving in perfect circles guided
much of this thinking
– Some did consider the helio- (or
Sun-) centered system.
• Aristarchus advocated the suncentered system because the
Sun was so big, it had to be at
the center! His views were not
accepted, however.
Ancient Greek
Astronomy
"At the centre, they [the
Pythagoreans] say, is fire, and the
earth is one of the stars, creating
night and day by its circular motion
about the centre." -- in Aristotle’s “Of
the
Heavens”
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206
Ancient Planetary Astronomy
1/16/07
Aristotle dismissed the heliocentric system he saw
no parallax
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Ancient Planetary Astronomy
1/16/07
Parallax Shift
Only small apparent
shifts can be seen
using the Earth’s
rotation to change the
observing point.
Larger shifts can be
seen using Earth’s orbit
around the Sun (but still
to small to be noticed
without a telescope)
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
Retrograde motion of planets
• When observed from one night to the next, a planet appears to move from
West to East against the background stars most of the time.
• Sometimes it will appear to reverse direction. For a short time, it moves
from East to West against the background constellations.
• This reversal is known as retrograde motion. All planets exhibit this
behavior as seen from Earth.
It is due to the relative motion of Earth and
PTYS/ASTR 206
Ancient Planetary Astronomy
the planet.
1/16/07
Ptolemy
• Egypt (~127-145 AD)
– Not much known about
his life.
– Used the concept of
Epicycles to explain
the motion of the Sun
and planets
crater Ptolemeaus
Consolidated Lunar Atlas
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
Ptolemy’s Universe
• The epicycle picture
explains the retrograde
motion of the planets
• This picture lasted 1000
years!
– Newton’s physics has
only been around for
about 400 years and
Einstein has already
corrected it !
• But, as we now know, it
is flawed.
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
The inner planets
• Venus is only seen easily (when it is
dark outside) in the morning or evening
– The ancients first thought that the
morning/evening stars were different objects
– Pythagoras was the first to note that they
were the same object -- Venus
• Their orbits must be between that of the
Earth and Sun
The Same is True for Mercury
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
The Copernican Revolution
A short list of key people
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Ancient Planetary Astronomy
1/16/07
Nicholas Copernicus
• 1473-1543, Polish
• Re-proposed heliocentric
theory
• Put the Sun at the center, but
still believed the orbits of the
planets were circles +
epicycles
• He felt that this was a more
natural explanation of the
solar system
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
Ptolemy vs. Copernicus Solar Systems
Path of Mars in the sky
(Ptolemy’s epicyles)
Path of Mars in the sky
(Copernicus’s system)
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07
Today’s in-class activity
• You may work together in small (2-3 people)
groups
– You must hand in your own work
• Note the scoring system
• After the activity is turned in, we will toss a coin
to decide if it will be graded
– Heads – it is graded
– Tails – it will not be graded
PTYS/ASTR 206
Ancient Planetary Astronomy
1/16/07