# The Motions of the Planets. Planet means “Wanderer”

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The Motions of the Planets

Planet means “Wanderer”

A planet moving against the stars

How can we classify the planets to make sense of their motions?

When is Jupiter high in the Sky? 1=Morn.,2=Noon,3=Afternoon, 4=Eve,5=Midnight

When is Venus Visible? 1=Morn.,2=Noon,3=Afternoon, 4=Eve,5=Midnight

2 Kinds of Planetary Motion “Inferior Planets” - Stay close to sun on ecliptic, quickly moving from one side of the sun to the other (ME,V) “Superior Planets” - Can be anywhere along ecliptic, slowly move W to E, occasionally looping E to W (MA, J, S) (Retrograde loop)

Mercury and Venus Are always seen “close” to the Sun “Close” means angular separation Mercury always less than 28° from Sun Venus always less than 47° from Sun

“Retrograde” = opposite direction Always occurs when planet is high at midnight (i.e. opposite sun)

Stellarium – planetary motion

END OF DATA AVAILABLE TO ANCIENTS

When it is Noon in Wisconsin, what time is it in Japan? A. 8 AM B. 4 PM C. 8 PM D. 4 AM

When Gemini is high in Wisconsin, what zodiacal constellation is highest in Greece? A. Leo B. Cancer C. Pisces D. Libra

We want to know the reasons for: Rotation of the celestial sphere The sun’s motion against the stars Seasons Direct motion of planets Retrograde motion of superior planets Why inferior planets stay close to the Sun

DATA and MODELS

Where you get the data: Experimental Sciences: Physics, Chemistry, Biology Observational Sciences: Astronomy, Geology, Atmospheric/Oceanic Sciences, Biology

Models A good model: A good model: -Is as simple as possible -Organizes data -Explains observations -Explains observations -Makes predictions -Makes predictions

A model can be a: Hypothesis - educated guess Theory – Well-developed and accepted body of ideas

Claudius Ptolemy (c. 85-165)

The ancients knew the Earth was round The masts of sailing ships disappear when they sail away Shadows are shorter when you are nearer the equator The shadow of the earth is curved during a lunar eclipse

The Celestial Sphere

Ptolemy: Celestial Sphere Rotates around Stationary Earth

Sun Motion as Seen from Earth

Ptolemy’s Sun Sphere

The Sun on the Celestial Sphere

Ptolemy’s Sun Motion

Venus and Mercury: Ptolemy

Does position of Sun matter in this movie? A = Yes B = No

Ptolemy’s model explains... Night/Day = Rotation of Cel. Sphere Sun against Stars = Sun Sphere Motion Seasons = Sun Sphere Tilted Inf. Planets = Epicycles Linked to Sun Sup. Planet Motions = Planet Spheres Retrograde Motion (Sup.) = Epicycles

William of Occam (1288-1348)

Occam’s Razor: Simplest Model is Usually Best

Occam’s Razor non sunt multiplicanda entia praeter necessitatem (that entities are not to be multiplied beyond necessity) I.e. Keep it simple!

Nicolaus Copernicus (1473- 1543)

Rotation of Sky: Copernicus Celestial sphere (or distant stars, not necessarily all on a sphere) is fixed Earth rotates about its axis Same result, the sky seems to rotate Day when our part of the Earth facing Sun; night when our part is away from Sun

Copernicus: Earth rotates

Copernican Model

Copernicus’ View of the Solar System

Inferior and Superior Planets’ motion against sky all explained through circular orbits -- Simple Rule: Planets Nearer the Sun Move Faster

Sun’s Motion through the Stars

Copernicus: Why the Sun Seems to Move Through the Sky

Exercises: “The Earth in Orbit Around the Sun” “Why the Sun appears in different constellations throughout the year”

What constellation is the Sun in on August 1 st ? A. Libra B. Aquarius C. Aries D. Cancer

Now look at SFA starchart for August 1 st to see its precise location.

Exercise: Why the Constellations are up at different times of the year

On January 20, is Virgo visible at midnight? A. Yes B. No

What zodiacal constellation is highest in the sky on January 20? A. Taurus B. Gemini C. Cancer D. Virgo

Set up planisphere for midnight, January 20 and note which zodiacal constellations are visible

Seasons: Copernicus

Copernicus: What Causes the Seasons?

Venus and Mercury: Copernicus

Why Venus and Mercury only appear near the sun

Exercise: Why Inferior planets like Venus always appear somewhere near the Sun.

On January 1 st, Venus will _____ be in Virgo. A. always B. sometimes C. never

Can Venus ever set at midnight? A. Yes B. No

Copernicus: Why Mars shows Retrograde Motion

How often do retrograde motions occur? A=Less than 1/yr B= 1/yr C=More than 1/yr

Exercise: How the superior planets like Jupiter mover through the constellations.

On January 1 2007, Jupiter will be in the constellation: A. Ophiuchus B. Aquarius C. Taurus D. Leo

On January 1 2009, Jupiter would appear in the ____ sky. A. Daytime B. Nighttime

Show April 2006 Star Map

Exercise: Determining Planetary Rise and Set Times.

What time did Jupiter set on July 1, 2005? A. 9:30 PM B. 11:30 PM C. 1:30 AM D. 3:30 AM

“How the superior planets like Jupiter undergo retrograde loops”

In 2007, when does Jupiter begin to move retrograde? A. January B. April C. August D. December

Copernicus’s model explains… Star’s Rotation & night/day: Earth’s rotation The Sun’s path through the stars The seasons: Earth’s axis tilt Direct motion of the planets by their orbits Retrograde motion by Earth passing planet Mercury’s and Venus’s behavior by their actual proximity to the Sun

Stellar Parallax

Problems with Copernicus’s Model Parallax -- Copernicus: “Stars far away” If Earth is rotating, wouldn’t we fly off? –Copernicus’s answer: not rotating that fast –Physics not yet good enough to show that Copernicus was right Wouldn’t there be a wind? –Copernicus’s answer: there would be worse forces on the spheres of Ptolemy’s model –Modern answer: atmosphere (mostly) rotates with Earth

Which Model? Explains available data Predicts new phenomena –Both models predict planet position to about the same degree of accuracy –Actually neither model works with high degree of detail Fits in with other scientific models Aesthetically pleasing –Importance of spheres –Simplicity (Occam’s razor) Need more data!