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\centerline{\bf Astronomy 101U Skylab \#3 -- ``Hey! Don't Touch That!''}
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\underline{Introduction}:  The gnomon, a fancy name for a vertical stick stuck in the ground, is the oldest of all 
astronomical instruments.  It can be used to analyze the Sun's motion across the sky.

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\underline{Equipment}:  A smooth, sunny location, a stick (at least one foot long), a compass (to find North), a 
protractor, a watch, a ruler and a large piece of paper (optional).

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\underline{Time Required}:  One full day (for a few minutes each hour).

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\underline{What to Do}:  Construct a gnomon in a location which is exposed to the Sun all day.  A flat, smooth (grass 
can make things uneven) surface is best; if the gnomon cannot be easily stuck in the ground, as on 
asphalt, then a small stand will help.  A "plumber's helper" turns out to be ideal.  If you want to 
record your measurements on paper, spread out a large sheet of paper (must be big to record the 
location of the tip of the shadow over the course of a day) around or under the gnomon.

		Use a compass to find true north (note:  compass needles point to magnetic north, which is 
approximately 8 degrees east of geographic north (or true north) in the Seattle area) and draw an 
East-West line on the ground through the gnomon.  A possible setup (bird's eye view) is:

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		From early morning to sunset, accurately measure the length of the shadow and the angle it 
makes relative to the East-West line, called the azimuth.  Record your results along with the height of 
the gnomon above the ground.  Make a scale drawing showing how the tip of the shadow moves in 
azimuth through the day.  If you know a little trigonometry, you can use the ratio of the length of the 
stick to the length of the shadow to calculate the elevation of the sun at each time of observation (its 
altitude).

		You will notice that the shadow is not shortest exactly at clock noon.  Think about the reason for 
this!  You will need to make many observations around TRUE noon -- at least every ten minutes from 
30 minutes before to 30 minutes after.  Don't forget about Daylight Savings Time.

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\underline{Questions}:
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(1)  Notice that the shadow of the Sun is always North of the East-West line (azimuth).  Will this ever 
change based on the time of year (if you do the experiment in Seattle)?  Why or why not?
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(2)  "True" noon occurs when the shadow's length is shortest (sun is closest to being directly overhead).  
At what latitude would you have to make your observation in order for there to be no shadow at true 
noon on June 21 (the summer solstice)?
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(3)  Besides daylight savings time, why doesn't true noon occur at noon right on the hour?  Hint:  Think 
about the way time zones are set up.  Where would you have to observe from in order for true noon 
to fall at exactly 1 pm Pacific Daylight Time?
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(4)  If you did this experiment again on the day after you did it the first time, would the shadow length 
be longer or shorter?  Why?
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(5)  List at least four sources of error in this experiment and for each list a way, if one exists, for that 
error to be reduced or eliminated.  Also, estimate the size of each error (in degrees for the angle or 
centimeters/inches/miles for the shadow length or seconds/minutes for time).

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