| A | B |
|---|
| axis | an imaginary line the Earth rotates on; the North and South Poles are at the ends of the axis |
| rotation | the spinning motion of the Earth on its axis; it takes just about 24 hours for the Earth to make one rotation |
| revolution | Earth's path around the Sun; one revolution takes 365 1/4 days (one year) |
| summer solstice | (June 21 or 22) the day when the North Pole leans toward the Sun |
| winter solstice | (December 21 or 22) the day when the North Pole leans away from the Sun |
| equinox | when Earth's axis is neither away nor toward the Sun; days and nights are nearly equal worldwide |
| spring (vernal) equinox | (March) the Sun's vertical rays strike directly on the Equator; first day of Spring |
| fall (autumnal) equinox | (September) the Sun's vertical rays strike directly on the Equator; first day of Fall |
| solar angle | the angle at which light strikes a surface |
| beam spreading | the way a light beam covers a larger area when it hits a surface |
