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| Astronomy | The study of the moon, stars, and other objects in space. |
| Axis | An imaginary line that passes through Earth's center and the North and South poles, about which Earth rotates. |
| Rotation | The spinning motion of a planet on its axis. |
| Revolution | The movement of an object around another object. |
| Orbit | The path of an object as it revolves around another object in space. |
| Solstice | The two days of the year on which the sun reaches its greatest distance north or south of the equator. |
| Equinox | The two days of the year on which neither hemisphere is tilted toward or away from the sun. |
| Gravity | The force pulls objects toward each other. |
| Inertia | The tendency of an object to resist any change in its motion. |
| Phase | One of the different apparent shapes of the moon as seen from Earth. |
| Eclipse | The partial or total blocking of one object in space by another. |
| Solar Eclipse | The blocking of sunlight to Earth that occurs when the moon is directly between the sun and Earth. |
| Lunar Eclipse | The blocking of sunlight to the moon that occurs when Earth is directly between the sun and the moon. |
| Geocentric | A model of the universe in which Earth is at the center of the revolving planets and stars. |
| Heliocentric | A model of the solar system in which Earth and the other planets revolve around the sun. |
| Ellipse | An oval shape, which may be elongated or nearly circular; the shape of the planets' orbits. |
| Astronomical Unit | A unit of measurement equal to Earth's average distance from the sun, about 150 million kilometers. |
| Nuclear Fusion | The process by which hydrogen atoms join together in the sun's core to form helium. |
| Core | The central region of the sun , where nuclear fusion takes place. |
| Terrestrial Planets | The name often given to the four inner planets: Mercury, Venus, Earth, and Mars. |
| Gas Giants | The name often given to the first four outer planets: Jupiter, Saturn, Uranus, and Neptune. |
| Comet | A loose collection of ice, dust, and small rocky particles, typically with a long, narrow orbit. |
| Coma | The fuzzy outer layer of a comet. |
| Nucleus | The solid inner core of a comet. The central core of an atom. |
| Asteroids | Rocky objects revolving around the sun that are too small and numerous to be considered planets. |
| Asteroid Belt | The region of the solar system, between the orbits of Mars and Jupiter, where many asteroids are found. |
| Meteoroid | A chunk of rock or dust in space. |
| Meteor | A streak of light in the sky produced by the burning of meteoroid in Earth's atmosphere. |
| Meteorite | A meteoroid that passes through the atmosphere and hits Earth's surface. |
| Electromagnetic Radiation | Energy that can travel through space in the form of waves. |
| Visible Light | Electromagnetic radiation that can be seen with the unaided eye. |
| Wavelength | The distance between the crest of one wave and the crest of the next wave. |
| Spectrum | The range of wavelengths of electromagnetic waves. |
| Constellation | An imaginary pattern of stars in the sky. |
| Apparent Brightness | The brightness of a star as seen from Earth. |
| Absolute Brightness | The brightness of a star would have if it were at a standard distance from Earth. |
| Light Year | The distance that light travels in one year, about 9.5 million million kilometers. |
| Hertzsprung-Russell Diagram | A graph relating the surface temperatures and absolute brightnesses of stars. |
| Main Sequence | A diagonal area on an H-R diagram that includes more than 90 percent of all stars. |
| Nebula | A large cloud of gas and dust in space, spread out in an immense volume. |
| Protostar | A contracting cloud of gas and dust with enough mass to form a star. |
| Planetary Nebula | A huge cloud of gas that is created when the outer layers of a red giant star drift out into space. |
| White Dwarf | The blue-white hot core of a star that is left behind after its outer layers have expanded and drifted out into space. |
| Supernova | The brilliant explosion of a dying supergiant star. |
| Neutron Star | The small, dense remains of a high-mass star after a supernova. |
| Pulsar | A rapidly spinning neutron star that produces radio waves. |
| Black Hole | An object whose gravity is so strong that nothing, not even light, can escape. |
| Galaxy | A huge group of single stars, star systems, star clusters, dust, and gas bound together by gravity. |
| Spiral Galaxy | A galaxy with a bulge in the middle and arms that spiral outward in a pinwheel pattern. |
| Elliptical Galaxy | A galaxy shaped like a round or flattened ball, generally containing only old stars. |
| Irregular Galaxy | A galaxy that does not have a regular shape. |
| Universe | All of space and everything in it. |
