The Solar System and Planetary Motion

Aristotle Earth Centered The Geocentric Model

The planets periodically seemed to make a loop against the background stars Lets watch one more time

Are the planets really making this strange looping motion? What is going on here?

This looping motion is called retrograde motion Mars looping near the “teapot” a few years ago This looping motion is called retrograde motion

Ptolemy’s Model

Ptolemy’s Model

Ptolemy had all the known planets of the day on Epicycle orbits around the earth. The model lasted almost 1500 Years and was adopted by the catholic church

The Copernican model was much simpler than Ptolemy’s and it placed the sun rather than the earth in the center of the solar system

Copernican model Heliocentric Model This is where I belong

To an observer on the train going faster it appears as if the other train is going backwards 70mph 100mph See Ya

Kepler’s 1st Law of Planet Motion 1. The planets go around the sun in an ellipse with the sun at one focus of the ellipse

. Calculating Eccentricity Eccentricity = focal distance major axis Eccentricity ranges between 0 and 1 . focus 1 focus 2 asteroid focal distance major axis Eccentricity = focal distance major axis Notice the units cancel out

E = 0 (circle) E = 0.1 E = 0.4 E = 0.9 E = 0.7 The higher the eccentricity the more elongated the orbit

Earth Orbit = 0.017 (very slightly elliptical) Like the earth’s shape, the human eye probably can not distinguish earth’s orbit from a circle

Energy of an asteroid in orbit downhill Low kinetic energy Low potential energy perihelion aphelion High kinetic energy high potential energy uphill KE + PE = constant

Watch the comet change speed as it gets further and closer to the sun Low Velocity High Velocity

As this planet orbits the sun, where will it be going fastest? Slowest?

Gravity The force of attraction between any two objects in the universe The MORE MASS the object has, the stronger its force gravity The CLOSER the objects the stronger the attraction between them

Gravity Stronger gravitational attraction Weaker gravitational attraction

Gravity Stronger gravitational attraction Weaker gravitational attraction

Would you weigh the most North Pole Where on earth here equator Would you weigh the most South pole

If the sun has so much mass (& strong gravity), why don’t the planets fall into the sun?

Because of Inertia An object at rest will tend to remain at rest An object in motion will maintain its direction and speed unless a an opposing force affects is (like gravity)

Newton first explained gravity using the moon’s orbit around earth Solid arrow = Earth’s gravity Dashed arrow = moon’s inertia

Inertia + gravity give all objects in the solar system “orbits” around the sun

Apparent Diameter The size an objects appears to be depends on how far away it is. Objects far away appear small Objects nearby appear big

How large something appears in our What does this suggest about the sun’s distance? How large something appears in our Sky is called the apparent or angular diameter Yes, the sun is almost exactly 400 times further away than the moon The sun is actually 400 times larger than the moon SUN By shear coincidence the sun and moon both appear to have the same apparent diameter as viewed from Earth MOON

What could cause the moon to appear slightly larger than the sun? When the moon does appear larger than the Sun, a total eclipse can occur

As a result of the earth’s slightly elliptical orbit, the sun will appear slightly larger in January compared with July Which Sun appears slightly larger? Photo taken in January Photo taken in July In what season would the sun look larger?

Which photo was taken when Venus was closest to Earth? crescent quarter phase nearly full phase