(4) The Angle of the Sun's Rays

How a steeper arrival direction of sunlight
causes greater heating of the ground.

Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern

 This lesson plan supplements:       #4   "Angle of the Sun's Rays" at Sunangle.htm,                           on the web http://www.phy6.org/stargaze/Sunangle.htm "From Stargazers to Starships" home page and index: on disk Sintro.htm, on the web           http://www.phy6.org/stargaze/Sintro.htm

 Goals: The student will learn That the greatest elevation of the Sun above the horizon is at noon. At that time, in the middle, northern latitudes (e.g. in the continental US or Europe) the Sun is exactly south of the observer. That the closer to perpendicular is the direction at which the Sun's rays arrive, the greater is their heating power. Terms: Elevation (or height) of the Sun above the horizon. Stories and extras: The reason why lichens in a forest grow on the north side of tree trunks, why solar collectors face south and are inclined, and why French wine grape growers treasure vineyards on slopes facing south. The placement of windows--facing south in cold climates, facing north in hot climates, and the use of overhangs. Guiding questions and additional tidbits The questions below may be used in the presentation, the review afterwards or both, and suggested answers are provided. Brackets [ ] enclose comments by the teacher or optional material. -- At any given place, at what time of the day do the Sun's rays fall most steeply (towards the perpendicular) onto the ground? At what time of the day do they fall least steeply? The rays fall steepest at noon, when the Sun is highest above the horizon; The angle is least steep at sunrise and sunset. -- Does the Sun heat the ground more when its rays fall more steeply? Why? Yes, it does. When a cylinder of sunlight 1 meter wide (or 1 foot wide) hits the ground, the steeper it falls, the smaller the area (its "footprint") to which it gives its energy, and the more concentrated is its heating. -- In the summertime in northern Alaska, the Sun never sets. Still, it does not get very hot there. Why? The Sun is never far from the horizon, and its rays fall at a shallow angle -- In the US, how would you orient a solar collector (for heating water or generating electricity) to collect sunlight most efficiently? And in Argentina? In the US, have it face south, at an angle of 90 degrees minus the latitude where the collectors will be placed. In Argentina, have it face north at a similar angle. -- In the hot climate of Texas or New Mexico, Where should the main windows of the house face? Northward, to reduce the amount of heat generated inside the house by sunlight.

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Author and Curator:   Dr. David P. Stern
Mail to Dr.Stern:   audavstern("at" symbol)erols.com .

Last updated: 12.17.2001