| A | B |
|---|
| elliptical galaxy | a cluster of billions of starts that take the shape of an extended oval (ellipse) |
| spiral galaxy | a cluster of billions of starts that take the shape of one or more spiraling streams issuing from a center (pinwheel shape) |
| irregular galaxy | a cluster of billions of starts that have no general shape |
| Milky Way galaxy | a spiral galaxy that the earth and solar system are part of |
| revolution (of a planet) | the time it takes for a planet to go once around the sun - we call this time period a year. The farther from the sun a planet is, the longer its period of revolution. |
| rotation (of a planet) | the time it takes for a planet to spin once on its axis - we call this time period a day. the change from day to night is caused by the Earth's rotation on its axis. |
| axis | an imaginary line running from the northern most point of a planet to the southern most point. the fact that the axis of the earth is tilted causes the seasons to change on Earth. |
| tile (of the Earth) | referring to how much the axis caries from a straight north-south alingment |
| gravity | the force of attraction between all matter |
| inner planets | the rocky planets closest to the sun; Mercury, Venus, Earth, and Mars |
| outer planets | the gas planets farther from the sun; Jupiter, Saturn, Uranus and Neptune |
| galaxy | a group of stars, dust and gases in the same general area of space. There are believed to be billions of galaxies in the universe. |
| universe | pertains to all existing matter, energy and space. It is believed to be expanding. |
| day (as it relates to rotation) | refers to the time it takes for a planet to rotate (spin) once on its axis. |
| year (as it relates to revolution) | refers to the time it takes for a planet to revolve one time around the sun. |
| hemisphere | one half of a sphere - half of the earth's surface |
