 Period of Rotation: The amount of time it takes for a planet to make one spin around its imaginary axis  Period of rotation determines the length of a planet’s day Rotation

 Revolution: a planet’s movement around the sun in an orbit.  Determines the length of a year  Planets revolve in a counterclockwise motion Revolution

 Inertia (a good unit test bonus question…hint, hint!): The concept that an object in motion will maintain the direction and speed of that motion unless an opposing force affects it

 Ellipse: a closed curve around two foci in which the sum of the distances between any point of the curve and the foci is a constant  Foci: Two fixed points  Eccentricity: The “roundness” an ellipse

 Distance between foci=d  Length of major axis = L  Eccentricity= d/L  Formula on front page of ESRT  NO UNITS (they cancel out)

 Eccentricity of a circle = 0  Eccentricity of a straight line = 1  Your answer MUST be between 0 and 1  The smaller the number of the answer, the closer orbit is to being a perfect circle

 Major Axis: the longest straight-line distance across an ellipse and it cuts through both foci  F1 is Focus 1, F2 is Focus 2

 The distance between two foci is 4 centimeters and the major axis is 5 centimeters. What is the eccentricity of the ellipse?

 Gravitation: the attractive force that exists between any two objects in the universe; depends on the mass of the objects and the distance between their centers

 Planets and all other bodies that orbit around the sun operate under the balance between inertia and gravitation  The greater the mass of one or both objects, the more gravitational attraction there is between them  The farther apart two objects are, the lower the gravitational attraction

 When a planet is closer to the sun, its orbital speed is faster  When a planet is farther from the sun, its orbital speed is slower  Comet Revolving Around the Sun Comet Revolving Around the Sun

 The closer a planet is to the sun, the smaller its orbit, the shorter its period of revolution, and the faster the speed of revolution (see page 15 of the ESRT)

 Planet A has a greater mean distance from the sun than planet B. On the basis of this fact, which further comparison can be correctly made between the two planets?  1) Planet A is larger  2) Planet A’s revolution period is longer  3) Planet A’s speed of rotation is greater  4) Planet A’s day is longer