| A | B |
|---|
| astronomy | The study of all physical objects beyond Earth. |
| calendar | A system for organizing time. |
| year | The time required for the Earth to orbit the sun once. |
| month | Roughly the amount of time required for the moon to orbit the Earth once. |
| day | The time required for the Earth to rotate once on its axis. |
| leap year | A year in which an extra day is added to the calendar. |
| constellation | A section of the sky that contains a recognizable star pattern. |
| altitude | The angle between an object in the sky and the horizon. |
| right ascension | A measure of how far east an object is from the point at which the sun appears on the first day of spring. |
| declination | A measure of how far north or south an object is from the celestial equator. |
| celestial equator | Imaginary circle created by extending Earth's equator into space. |
| ecliptic | The apparent path the sun takes across the celestial sphere each year. |
| light-year | A unit of length equal to the distance that light travels through space in one year. |
| telescope | An instrument that collects electromagnetic radiation from the sky and concentrates it for better observation. |
| refracting telescope | A telescope that used a set of lenses to gather and focus light. |
| reflecting telescope | A telescope that uses curved mirrors to gather and focus light. |
| electromagnetic spectrum | All the wavelengths of electromagnetic radiation. |
