Ancient Planetary Astronomy - Lunar and Planetary …jweirich/PTYS_206_2/Lectures/Lec2-011607.pdf · PTYS/ASTR 206 Ancient Planetary Astronomy 1/16/07 Announcements • The first

PTYS/ASTR 206 Ancient Planetary Astronomy1/16/07

Ptolemy

Copernicus

Geocentric Solar System

Heliocentric Solar System

Galileo

Ancient Planetary AstronomyPtolemy


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 is a great opportunity to learn about cutting-edge research at the UofA!– See www.lpl.arizona.edu/COLPL for details


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


Powers-of-ten notation is a useful shorthand system for writing numbers


Common Prefixes

Factor Name Symbol(billion) 109 Giga- G(million) 106 Mega- M(thousand) 103 kilo- k(hundredth) 10-2 centi- c(thousandth) 10-3 milli- m(millionth) 10-6 micro- μ(billionth) 10-9 nano- n


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


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) – Kelvin (K) is the most common


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


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 (4 hours)(3x108 m/s) x ( 0.001 km/m)

x (4 hours) x (3600 s/hour)





(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


Revolutions in Planetary Science

• Greek Philosophy (500 BC-200 AD)– Greece, Turkey,

Egypt, Syria– Systematic

philosophy– Knew that Earth is a

sphere

• Copernican Revolution (1500-1700 AD)– Europe– Modern telescopes

• 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


The Ancient Greeks were able to measure the size of the Earth

Deep Wellat Syrene

post at Alexandria(which casts a shadow)

LIGHT RAYS FROM SUN

• Erotosthenes (276-197 BC) – Accurately measured the size of the Earth using simple geometry and the assumption of a spherical Earth

Erotosthenes method:


Ancient Greek Astronomy

• 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 sun-centered system because the Sun was so big, it had to be at the center! His views were not accepted, however.

"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”


Aristotle dismissed the heliocentric system he saw no parallax


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)


• 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 the planet.

Retrograde motion of planets


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 PtolemeausConsolidated Lunar Atlas



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.


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



The Copernican RevolutionA short list of key people


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


Ptolemy vs. Copernicus Solar Systems

Path of Mars in the sky(Ptolemy’s epicyles)

Path of Mars in the sky(Copernicus’s system)


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

Documents

Ancient Planetary Astronomy - Lunar and Planetary …jweirich/PTYS_206_2/Lectures/Lec2-011607.pdf · PTYS/ASTR 206 Ancient Planetary Astronomy 1/16/07 Announcements • The first