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| Astronomy | the science that deals with the material universe beyond the earth’s atmosphere |
| Galaxy | a large system of stars held together by mutual gravitation and isolated from similar systems by vast regions of space |
| Astronomical unit | a unit of length, equal to the mean distance of the earth from the sun: approx. 93 million miles |
| Electromagnetic spectrum | the entire spectrum, considered as a continuum, of all kinds of electric, magnetic, and visible radiation |
| Telescope | an optical instrument for making distant objects appear larger and therefore nearer |
| Refracting telescope | an objective lens set into one end of a tube and an adjustable eyepiece or combination of lenses set into the other end of a tube that slides into the first and through which the enlarged object is viewed directly |
| Reflecting telescope | a concave mirror that gathers light from the object and focuses it into an adjustable eyepiece or combination of lenses through which the reflection of the object is enlarged and viewed |
| Rotation | the movement or path of the earth or a heavenly body turning on its axis; one complete turn of such a body |
| Revolution | the orbiting of one heavenly body around another |
| Perihelion | the point in the orbit of a planet or comet at which it is nearest the sun |
| Aphelion | the point in the orbit of a planet or comet at which it is farthest from the sun |
| Equinox | the time when the sun crosses the plane of the earth’s equator, making night and day of approximately equal length all over the earth |
| Solstice | either of the two times a year when the sun is at its greatest distance from the celestial equator; about June 21, when the sun reaches its northernmost point on the celestial sphere, or about December 22, when it reaches its southernmost point |
| Solar wind | a stream of protons moving radially from the sun |
