| A | B |
|---|
| APHELION | the point in a planet's orbit when it is furthest from the sun (Earth- July 4th) |
| ELLIPTICAL ORBITS | ellipse-shaped orbits w/ two foci |
| EQUAL AREAS LAW | law that states that as a planet moves around the sun, an imaginary line joining the 2 will sweep over equal areas of space in equal periods of time |
| ESCAPE VELOCITY | the minimum velocity needed for an object to escape from another from which it is being held by gravity |
| GEOCENTRIC MODEL | model that has Earth at its center |
| HELIOCENTRIC MODEL | model that has Sun as its center |
| LAWS OF PLANETARY MOTION | 3 laws by Kepler that described the shape of planetary orbits & the speed in which they travel |
| ORBIT | the path of a revolving body; ex. Earth around Sun |
| PERIHELION | point in planet's orbit when nearest sun (Earth- Jan 2) |
| RETROGRADE MOTION | appearance of an apparent periodic backward, or westward, loop made by a planet |
| SOLAR FLARES | sudden outburst of energy that rises up in areas of sunspot activity |
| SOLAR SYSTEM | the Sun & its family of orbiting planets, asteroids, meteoroids & comets |
| SOLAR WIND | stream of elecrically charged particles that are blown out from the sun in all directions |
| UNIVERSAL LAW OF GRAVITY | Newton's law that states that the gravitational force is greater between objects of greater mass & less between objects of lesser mass |
| ECCENTRICITY | measure how unlike an ellipse is to a circle; formula is on ESRT |
