| A | B |
|---|
| mercury, venus, earth, mars | inner planets |
| solar system | sun, planets, and all objects that revolve around the sun |
| age of solar system | 5 billion years |
| venus | has the same size, mass, and density as Earth |
| Kepler | said planets moved in elliptical orbits around the sun |
| rotation | spin on axis |
| revolution | move around the sun |
| nebular theory | solar system began as a vast cloud of dust and gas |
| protoplanets | clumps of matter that became planets |
| orbit | path an object takes when moving around another object in space |
| closest planet to sun | mercury |
| pluto | smallest planet |
| planets revolve in this direction | counterclockwise |
| inertia | an object's motion will not change unless acted upon by another force |
| moons | satellites |
| 23.5 degrees | Earth's tilt |
| equator | imaginary line that encircles Earth halfway between north and south pole |
| summer in northern hemisphere | north pole tilted toward the sun |
| winter in northern hemisphere | north pole tiltied away from the sun |
| summer solstice | longest day |
| the length of day and night changes due to ______ | the tilt of the earth |
| seasons are caused by | the tilt of the earth |
| 24 hours | Earth's period of rotation |
| 365 days | Earth's period of revolution |
| equal day and night | equinox |
| winter solstice | shortest day |
| perigee | moon closest to the earth |
| navigation satellites | send continuous information to ships and planes |
| polaris | earth's north pole points toward this star |
| tides | caused by the gravitational attraction between the Earth and the moon |
