GEOL-1041-ONLINE-Lab Exercise 4 IEL -The Analemma and EM Radiation-Summer2016

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Lab Exercise #4The Analemma & EM RadiationThe Analemma
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Chapter 6 / Exercise 6
An Introduction to Physical Science
Shipman/Wilson
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How to find Seasonal Variation in the Sun’s Declination and Altitude
Seasonal variations are a response to changes in the Sun’saltitude, the angle between the horizon and thenoon Sun. The Sun’sdeclination, the latitude of the subsolar point, migrates annually through 47º oflatitude-between theTropic of Cancerat 23º30` N and theTropic of Capricornat 23º30’S.Theanalemmais a convenient device to track the passage of the Sun’s path and declination throughout theyear (Figure 5-4). The horizontal lines are latitudes from 25º N to 25º S. The “figure 8” is a calendar witheach day of the year represented by a black or whitesegment.Find August 20thon the analemma; note that it layson the 12º N line-the subsolar point on that date.While you should remember the subsolar points forthe solstices and equinoxes, an analemma will helpyou determine the declination for the other days ofthe year.The analemma also is useful for ascertaining thepositive and negative equations of time and periodsof “fast-Sun times” (greatest in October andNovember) and “slow-Sun times” (greatest inFebruary and March). A 24-hour (86,400 seconds) average day determinesmean solar time. However, anapparent solar dayis based on observed successive passages of the Sun over a given meridian. Anydifference between observed solar time and mean solar time is called theequation of time. On successivedays, if the Sun arrives overhead at a meridianafter12:00 noon local standard time (taking longer than the24-hours of a mean solar day)-like an airliner arriving later than scheduled-the equation of time isnegative, and the Sun is described as “slow”. If, on the other hand, the Sun arrives overhead before 12:00noon local time on successive days (taking less time than 24-hours)-like an airliner arriving ahead ofschedule- the equation of time ispositive, and the Sun then is described as “fast”. The equation of time canrange up to 16 minutes slow and 14 minutes fast at differing times during the year. The equation of time isacross the top of the analemma.Why a Figure Eight?An analemma takes on a figure-eight shape because of two main factors, thedeclination of the sun which governs the north and south movement of the sun's imageand the equation of time which accounts for the easterly and westerly changes.The sun's declination is its north and south angle as compared to the earth's celestialequator. It determines the distance that the sun's image will fall to the north of a pointdirectly below our gnomon. Toward the winter solstice, the sun's angle is moresoutherly, the sun appears lower in the sky and the declination is less. The image willfall a longer distance north of the gnomon, nearly to the north wall. Toward thesummer solstice, the sun is higher in the sky and its image will be so close to the pointdirectly below the gnomon that it will fall on the window sill. See an enlarged view ofanalemma path (left)

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Term
Spring
Professor
AmandaKGiesler
Tags
Geology, Light, Electromagnetic spectrum, EM Spectrum
We have textbook solutions for you!
The document you are viewing contains questions related to this textbook.
An Introduction to Physical Science
The document you are viewing contains questions related to this textbook.
Chapter 6 / Exercise 6
An Introduction to Physical Science
Shipman/Wilson
Expert Verified

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