Part 2: Motions of the Planets 1

Planets  While stars move through the sky they stay in the same place in relation to each other.  Ancient observers of the sky saw that a few “stars” seem to wander among the others.  The word planet comes from the Greek name for those odd stars, astéres planéta, which literally mean wandering star. 2

A Geocentric Universe? Early Greek scientist/philosophers Aristotle (4 th century BC) and Ptolemy (2 nd century BC) theorized that the Earth was at the center of the universe and everything else revolved around it. The sky according to Ptolemy Source: Pearson Scott Foresman, donated to the Wikimedia Foundation, public domain 3

A Geocentric Universe? Even in Ptolemy’s time there were those who disagreed with the geocentric universe idea, but as with Empedocles and his theory of 4 elements, the incorrect theory remained the prevalent world view into the 17 th century and beyond. The sky according to Ptolemy Source: Pearson Scott Foresman, donated to the Wikimedia Foundation, public domain 4

A Heliocentric Universe In the early 1500s, Nicolaus Copernicus developed a theory that Earth and the other planets revolve around the sun, all in the same direction, at different rates, and in perfect circles. 5

Elliptical Orbits  Johannes Kepler (16 th century mathematician and astronomer) calculated the shape of the orbits of planets to be elliptical, or oval, rather than spherical.  The planets move counterclockwise.  The closer it is to the sun, the faster a planet moves. 6

7 The small circles are necessary to account for the backward motion of the planets in the night sky that periodically occurs. The backward (retrograde) motion of the planets in the night sky occurs because the planets move at different speeds so inner planets periodically pass outer planets.

Orbital Motion: Inertia and Gravity  According to Isaac Newton, a planet’s motion around the sun is the results of two factors: inertia and gravity.  Inertia causes the planet to move in a straight line.  Gravity pulls the planet toward the sun.  Both working together causes a planet to move in an elliptical orbit. 8

Planetary Revolution vs. Rotation Revolution: Movement around the sun (orbit) Rotation: Turning on axis 9

Period of Revolution  The amount of time it takes a planet to orbit (revolve) once around the sun.  One revolution equals one year.  Each planets’ year is a different length.  What pattern do you see? Period of Revolution (Earth Days) Period of Revolution (Earth Years) Mercury880.2 Venus Earth Mars Jupiter4, Saturn10, Uranus30, Neptune60,

Period of Rotation  Axis: an imaginary line drawn through the center of a planet upon which it turns.  Period of Rotation: The time it takes for a planet to make one complete turn around its axis. Period of Rotation (Earth Days) Mercury58.6 Venus243 Earth0.99 Mars1.03 Jupiter0.41 Saturn0.45 Uranus0.72 Neptune