| A | B |
|---|
| maria | dark, smooth plains on the Moon |
| planetesimals | hundreds of kilometers in diameter |
| retrograde motion | movement in the opposite direction |
| ellipse | an oval shape that is centered on two points instead of a single point, as in a circle |
| astronomical unit | unit of measurement used to find distances between the Sun and a planet in its orbit |
| eccentricity | the ratio of the distance between the foci to the length of the major axis in a planet's elliptical orbit |
| focus | one of the fixed points that with the corresponding directrix defines a conic section |
| inferometry | process that links separate telescopes so they act as one telescope |
| terrestrial planets | the four inner planets most similar in terrain to Earth |
| scarp | planet wide system of cliffs |
| similar | having characteristics in common |
| synchronous rotation | the state at which the Moon's orbital and rotational periods are equal |
| gas giant planet | a large, gaseous planet that does not have solid surfaces |
| liquid metallic hydrogen | a form of hydrogen with the properties of both a liquid and a metal which can exist only under conditions of very high pressure |
| belt | low, warm, dark-colored clouds that sink |
| zone | high, cool, light-colored clouds that rise |
| hypothesize | a tentative assumption made in order to draw out and test its logical or empirical consequences |
| dwarf planet | and object that, due to its own gravity, is spherical in shape and orbits the sun, but is not a satellite |
| meteoroid | interplanetary material that falls toward Earth and enters Earth's atmosphere |
| meteor | streak of light produced as a meteoroid burns up in Earth's atmosphere |
| meteorite | meteoroid that strikes Earth's surface |
| Kuiper belt | rocky asteroid belt that lies outside the orbit of Neptune |
| meteor shower | occurs when comet particles burn up enetering Earth's upper atmosphere |
| eventual | happening in the course of time or events |
