Solstices and Equinoxes

The first motion is rotation. Earth rotates on its axis once every 24 hours. One-half of the Earth’s surface is therefore facing the Sun at all times. Rotation about Earth’s axis takes place in an eastward direction. Thus, the Sun appears to rise in the east and set in the west. (See fig. 1-3.) The second motion of Earth is its revolution around the Sun. The revolution around the Sun and the tilt of Earth on its axis are responsible for our seasons. Earth makes one complete revolution around the Sun in approximately 365 1/4 days. Earth’s axis is at an angle of 23 1/2° to its plane of rotation and points in a nearly fixed direction in space toward the North Star (Polaris). Solstices and Equinoxes When Earth is in its summer solstice, as shown for June in figure 1-4, the Northern Hemisphere is inclined 23 1/2° toward the Sun. This inclination results in more of the Sun’s rays reaching the Northern Hemisphere than the Southern Hemisphere. On or about June 21, direct sunlight covers the area from the North Pole down to latitude 66 1/2°N (the Arctic Circle). The area between the Arctic Circle and the North Pole is receiving the Sun’s rays for 24 hours each day. During this time the most perpendicular rays of the Sun are received at 23 l/2°N latitude (the Tropic Of Cancer). Because the Southern Hemisphere is tilted away from the Sun at this time, the indirect rays of the Sun reach only to 66 1/2°S latitude (the Antarctic Circle). Therefore, the area between the Antarctic Circle and the South Pole is in complete darkness. Note carefully the shaded and the not shaded area of Earth in figure 1-4 for all four positions. At the time of the equinox in March and again in September, the tilt of Earth’s axis is neither toward nor away from the Sun. For these reasons Earth receives an equal amount of the Sun’s energy in both the Northern Hemisphere and the Southern Hemisphere. During this time the Sun’s rays shine most perpendicularly at the equator. 1-6 Agf0103 MIDNIGHT SUNSET SUNRISE NOON Figure 1-3.—Rotation of the Earth about its axis (during equinoxes).