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This is a question for a physical sciences or astronomy buff.

An elementary school teacher recently posed an embarrassing question regarding observations of the sun's seasonal movement in the sky made by her her fifth grade class.  I could not explain the observations, nor confirm if they were accurate, but suspecte
d they were real and related to minor deviations in the Earth's rotational precession.  After consulting my authoritative source, the Farmers' Almanac..who knows where my college astronomy text is, I'm not so sure any more.  I also realize now that the ti
me frame for the change must be too short relative to possible wobbles in the Earth's motion.

The experiment:  Basically a sundial..the location of the shadow cast by a pointer was measured at the same time each day (+/- minutes error) and plotted over the time span of several months.

The observation:  Around the Winter solstice, the shadow position did not change in a regular manner but in fact on several occasions moved back and forth.  

The known and obvious (to me):  As the Earth precesses about its axis of revolution, the normal presented to the sun varies causing the seasonal movement of the Sun in the sky.

The less obvious (to me):  The sun reaches its maximum Northerly position on Dec 21 but does not cross the meridian at noon (civil time) until Dec 25.  This leading and lagging of the sun's position relative to the "civil clock" ..by up to 16 minutes..is 
due to the Earth's elliptical orbit.

The Questions: 
	#1  Is the Earth's elliptical orbit the explaination for the "back and forth" motion of the sundial pointer shadow?  What influence does the inclination of the Earth's orbital plane to its rotational plane have on the motion of the sundial shadow? Does t
he Earth precess about its axis of rotation as well as its axis revolution and does this "planetary" motion have an affect on the shadow position?  What is the period of rotational precession?

	#2 What causes the asymmetry of time change for sunrise and sunset about the Summer and Winter solstices?  i.e. the time of sunrise changes more rapidly than that of sunset before Dec21 but more slowly afterwards.

The answer:  I would appreciate receiving your comments to the above or a reference to a clear lay description of the fine structure in the motion of a sundial shadow.



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hbstrock@world.std.com			phone/fax 508/829-9610
Harold B. Strock, PhD
Strock Technology Associates, Inc.	Independent Licensing Executive
40 Driftwood DR				Consultant in advanced materials