Shade Planning for Schools

37 Shade Planning for America’s Schools Chapter 5 The Earth’s axis of rotation is not perpendicular to its orbit around the sun, but is tilted at an angle of approximately 23.5°. Solar Declination is the angle that a given hemisphere is tilted toward the sun on any given day. It is marked by the latitude on the Earth where the location of the sun is directly overhead at solar noon. Because of the 23.5° tilt, this location is always somewhere between 23.5° north and 23.5° south, depending on the time of the year. Solar declination is 0° when the sun lines up with the equator on the equinoxes. Equinox is one of the two periods when the declination of the sun is 0°, or the sun is lined up exactly with the equator. The autumnal equinox occurs on or about September 21st, and the vernal equinox occurs on or about March 22nd. Only on these 2 days are the hours of the day and night equal, and only on these 2 days does the sun rise due east and set due west. Solstice is either the longest or the shortest day of the year. In the northern hemisphere, summer solstice is the longest day of the year and occurs on or about June 21st. Winter solstice occurs on or about December 22nd. True north , also known as geographic north, is the northernmost point on the Earth as determined by the Earth’s rotation. This usually differs from what a compass indicates as north. When a compass points to north, it is pointing toward magnetic north, which in some locations in the United States, may be as far as 20° from true north. K e y T e r m s a n d C o n c e p t s The Earth’s Rotation and Revolution Every 24 hours, the Earth makes one rotation on its axis; every 365.24 days, the Earth makes one revolution around the sun. If one were to view the Earth spinning on its axis, one would note that the axis is not perpendicular to the Earth’s orbit around the sun, but is tilted at an angle of 23.5°, with the North Pole always pointing directly at the North Star. Furthermore, the Earth’s orbit around the sun is not circular, but elliptical, causing the Earth’s distance from the sun to vary by as much as 3 million miles throughout the year. The annual variation in the Earth’s distance from the sun affects the amount of solar radiation intercepted by the Earth by as much as 7%. The changing distance from the sun, however, is not responsible for the changes in seasons. The changes in season are caused instead by the constant 23.5° tilt of the Earth and the Earth’s rotation around the sun. Image Source: National Geophysical Data Center Summer solstice, which is on or about June 21st, marks the beginning of summer in the northern hemisphere. The Earth is positioned so that the North Pole is leaning toward the sun at 23.5°. On summer solstice, the length of the day, from sunrise until sunset is greater than 12 hours for all latitudes north of the equator, and less than 12 hours for all latitudes south of the equator. On summer solstice, the center of the sun lines up with the latitude known as the Tropic of Cancer, which is at 23.5° north. At the winter solstice, on or about December 22nd, the Earth is positioned so that the North Pole is leaning away from the sun, and all latitudes south