| A | B |
|---|
| astronomy | the study of all physical objects beyond Earth |
| calendar | a system for organizing time; most calendars organize time within a single unit called a year |
| 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 |
| heliocentric | the theory that the sun is the center of the universe |
| geocentric | the theory that the Earth is the center of the universe |
