Monday Mar. 6, 2006

The Expt. 2 reports have been graded and were returned in class today.  Revised reports are due on Wed. Mar. 22.  Please return the original report with your revised report.

Optional Assignment #3 was returned in class.  Answers are available online.

Answers to the surprise optional assignment will become available sometime Tuesday.

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On the equinoxes, the days are 12 hours long everywhere on the globe (with the exception of the poles), the sun rises in the east and sets in the west, and the sun passes overhead at noon at the equator. 



There is a lot of information on this figure.  You need work through this figure numbered point by numbered point.  You'll find written explanations of each point on p. 77b in the photocopied notes which is reproduced below. 

The cloud shown next to Point 5 above refers to a band of clouds that circles the globe at the latitude where the sun passes overhead at noon.  You can usually make out this band of clouds on a global satellite picture.  At the present time, just a few weeks after the fall equinox the band of clouds is near the equator.  It will move south of the equator as we get closer to the winter solstice.  Then it will move back to the equator by next March and move into the northern hemisphere next summer.


We start out with the orienttation of the earth on the equinoxes and then "rotate the earth" until we end up with the bottom picture below.

Probably the easiest point to understand now is that the sun will be overhead at noon not at the equator but at 23.5 S latitude.  That is called the Tropic of Capricorn.  The sun's rays strike the earth perpendicularly at 23.5 S latitude.

Days in the northern hemisphere are 12 hours long or shorter.  They grow shorter the further north you go.  The sun rises in the south eastern part of the sky and sets in the southwest.




See p. 78b in the photocopied notes for a point by point discussion of this figure.


Now we start with the equinox orientation and twist the earth in the other direction.


The lower figure above shows the orientation of the earth on the northern hemisphere summer solstce.  The sun will pass overhead at 23.5 degrees N latitude, the Tropic of Cancer.  The sunrises in the northeast and sets in the northwest.  Days in the northern hemisphere are 12 hours long (near the equator) or longer (at higher latitudes). 


Refer to p. 80 in the photocopied notes.