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
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
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
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
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
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
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!