| A | B |
|---|
| astronomy | The scientific study of the universe |
| galaxy | a collection of stars, dust & gas bound together by gravity |
| astronomical unit | the average distance between the Earth & the sun; approximately 150 million kilometers |
| electromagnetic spectrum | all of the frequencies or wavelengths of electromagnetic radiation |
| telescope | an instrument that collects electromagnetic radiation from the sky & concentrates it for better observation |
| refracting telescope | a telescope that uses a set of lenses to gather & focus light from distant objects |
| reflecting telescope | a telescope that uses a curved mirror to gather & focus light from distant objects |
| rotation | the spin of a body on its axis |
| revolution | the motion of a body that travels around another body in space; one complete trip along an orbit |
| perihelion | the point in the orbit of a planet at which the planet is closest to the sun |
| aphelion | the point in the orbit of a planet at which the planet is farthest from the sun |
| equinox | the moment when the sun appears to cross the celestial equator |
| solstice | the point at which the sun is as far north or as far south of the equator as possible |
