Ch: 2.4 - The Ancient Mystery of the Planets

Planetary Motion

·         3 laws devised by Johannes Kepler to define the mechanics of planetary motion:

o    1st Law - states that planets move in an elliptical orbit, with the Sun being one focus of the ellipse. This law identifies that the distance between the Sun and Earth is constantly changing as the Earth goes around its orbit.

o    2ng Law - states that the radius of the vector joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse. As such, the planet moves quickest when the vector radius is shortest (closest to the Sun), and moves more slowly when the radius vector is long (furthest from the Sun).

o    3rd Law - states that the ratio of the squares of the orbital period for two planets is equal to the ratio of the cubes of their mean orbit radius. This indicates that the length of time for a planet to orbit the Sun increases rapidly with the increase of the radius of the planet's orbit.

• Ancient ppl believed in Earth-centered universe
• retrograde was difficult to explain.....what makes planets go around / backward if things move in circular motion around Earth?

Apparent Retrograde Motion

·         the motion of a planetary or other body in a direction opposite to that of other bodies within its system as observed from a particular vantage point

·         This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such as precession or nutation of the axis.

·         Reverse course (backwards)

Greeks rejected real explanation for planetary motion...

·         because their inability to detect stellar parallax..... Greeks were well aware that stellar parallax would occur if Earth orbits Sun (they couldn’t detect it)

Stellar Parallax

·         is created by the relative motion between the Earth and a star, can be seen, in the Copernican model, as arising from the orbit of the Earth around the Sun: the star only appears to move relative to more distant objects in the sky

·         the difference in direction of a celestial object as seen by an observer from two widely separated points