Total orbital energy is negative. What does that mean?

• The object barely escapes
• The object leaves for good
• The orbit is bound and looping

Why does a planet sweep equal areas in equal times?

• Gravity is central, so angular momentum is conserved
• The Sun pushes the planet along its path
• All orbits are perfect circles

A planet moves fastest at aphelion, its farthest point from the Sun.

You know position and velocity at one point. How do you find the speed at a new radius?

• Assume the speed never changes
• Divide the old speed by the new radius
• Use energy conservation between the two radii

Total energy is exactly zero. Which path does the object follow?

• A closed ellipse
• A parabola that just barely escapes
• A plunge straight into the star

Why does the Sun also move?

• The solar wind pushes the Sun around
• Both bodies orbit their shared centre of mass
• Sunlight pressure rocks it back and forth

An object has kinetic energy 4 units and potential energy minus 10 units. What is its total orbital energy?

A starlight spectrum shifts red and blue in a steady rhythm. What is the likely cause?

• The star cooling down and shrinking
• Dust dimming the telescope at random
• An unseen planet tugging its star around