| Astronomical Unit (AU) | An astronomical unit (AU) is the average distance between Earth and the Sun, about 150 million kilometers. |
| Atmosphere | An atmosphere is a layer of gases that surround a planet or moon. |
| Coronagraphy | Coronagraphy is a technique used to block out the bright light of a star so that dimmer objects nearby can be observed. |
| Cosmic Clock | A cosmic clock is a natural phenomenon that can be used to measure time. |
| Distance | Distance is the amount of space between two objects or points. In the solar system, distance is often measured in terms of astronomical units (AU) or light-years (ly). |
| Doppler Effect | The Doppler effect is the change in frequency or wavelength of a wave for an observer who is moving relative to the wave source. |
| Eclipse | An eclipse is an event in which one object in space blocks another from view. |
| Exoplanet | A planet that orbits a star outside the solar system. Also called extrasolar planet. |
| Fluctuation | A fluctuation is a change in something that is not stable, especially in an irregular way. |
| Gas | In context of planets, gas refers mostly to hydrogen and helium. |
| Gravitational Pull | Gravitational pull is the force of gravity acting between two objects. |
| Light-Year (ly) | A light-year is the distance light travels in one year, about 9.5 trillion kilometers. |
| Mass | Mass is the amount of matter in an object. In the solar system, mass is often measured in terms of Earth's, Jupiter's, or the Sun's mass. |
| Orbit | An orbit is the path one object in space takes around another. |
| Planet | A planet is a large object that orbits a star and does not produce light of its own. |
| Pulsar | A pulsar is a spinning neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing toward Earth. |
| Radiation | Radiation is energy that travels in the form of waves or particles. |
| Radius | Radius is the distance from the center of a sphere or circle to its edge. |
| Solar System | A solar system is a star and all the objects that travel in orbit around it. |
| Star | A star is a large, glowing ball of gas that generates heat and light through nuclear fusion. |
| Starshade | A starshade is a large, specially shaped screen that can be used to block out the light of a star so that dimmer objects nearby can be observed. |
| Terrestrial | Something that is terrestrial is related to Earth or land. |
