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 pointsEccentricity: The “roundness” an ellipse

Distance between foci=dLength of major axis = LEccentricity= d/LFormula on front page of ESRT

NO UNITS (they cancel out)

Eccentricity of a circle = 0Eccentricity 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

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 larger2) Planet A’s revolution period is longer3) Planet A’s speed of rotation is greater4) Planet A’s day is longer