The Motions of the Planets

Planet means “Wanderer”

A planet moving against the stars

How can we classify How can we classify the planets to make the planets to make

sense of their motions?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 Motion of Mars

• “Retrograde” = opposite direction

• Always occurs when planet is high at midnight (i.e. opposite sun)

Retrograde Motion of Mars

mars_retrograde.MOV

Stellarium – planetary motionStellarium – planetary motion

END OF DATA AVAILABLE TO END OF DATA AVAILABLE TO ANCIENTSANCIENTS

DATA and MODELSDATA and MODELS

Where you get the data:Where you get the data:

Experimental Sciences: Physics, Experimental Sciences: Physics, Chemistry, BiologyChemistry, Biology

Observational Sciences: Observational Sciences: Astronomy, Geology, Astronomy, Geology, Atmospheric/Oceanic Sciences, Atmospheric/Oceanic Sciences, BiologyBiology

ModelsModels A good model:A good model:

-Is as simple as possible-Is as simple as possible

-Organizes data-Organizes data

-Explains observations-Explains observations

-Makes predictions-Makes predictions

A model can be a:A model can be a:

Hypothesis - educated guess Hypothesis - educated guess

Theory – Well-developed and Theory – Well-developed and accepted body of ideasaccepted body of ideas

Venus and Mercury: Ptolemy

Retrograde Motion: Ptolemy

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

sphere_around_earth.MOV

Copernican Model

Copernicus’ View of the Solar System

coperincus_terrestrials.MOV

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

copernicus_sun_motions.MOV

Seasons: Copernicus

Copernicus: What Causes the Seasons?

copernicus_seasons.MOV

Venus and Mercury: Copernicus

Why Venus and Mercury only appear near the sun

coperincus_terrestrials.MOV

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

On January 1st, Venus will _____ be in Virgo.

• A. always

• B. sometimes

• C. never

Can Venus ever set at midnight?

• A. Yes

• B. No

Retrograde Motion: Copernicus

Copernicus: Why Mars shows Retrograde Motion

copernicus_mars_retrograde.MOV

Retrograde Cars

How often does Neptune undergo retrograde loops?

A.About once per weekB.About once per monthC.About once per yearD.About once per decade

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