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Sun Lab

  The purpose of the sun lab was to determine the sun's rotational period, and also to determine its diameter and angular size.  We also calculated the size of a sunspot, and learned about what a sunspot is.

  First, we set up a telescope so that the sun was reflecting through the telescope and into a box with a white piece of paper on the background.  We used a digital camera to take a picture of the projection of the sun.  We then measured the picture to determine how far a sunspot had moved around the face of the sun.


Picture of the sun projection
Click here for an enlarged view


  The next step was to calculate how much the sun had rotated.  Here are the diagrams that I used to calculate this:


Diagram of how far the sun rotated through the duration of the experiment


There were 70 hours and 40 minutes between the two sun
readings (Friday, 10:50, and Monday, 10:30), so:


  So, according to these calculations, the sun makes one complete rotation in 25.8 days.  The next step is to determine the diameter and angular size of the sun.


 

  So the sun's diameter is 1.36*109 meters, and it has an angular size of 0.521o.  The last remaining calculation is that for the sunspot size:

  So the sunspot that was used as the test point for this lab was 6.87*107 meters across.  Normally, it would be necesary to find the length of the arc on the sun that corresponds to the sunspot, but because the sunspot is centered on the sun, and because the sunspot is so small relative to the sun that it is virtually flat, the difference between this length and the length of the appropriate arc is insignificant.



  Now, for a description of what, exactly, sunspots are. Sunspots are regions on the surface of the sun that are significantly cooler than other regions, and so give off less energy, in the form of light.  The reason for this cooling is that strong electromagnetic forces are preventing hot gasses from reaching the surface of the sun in that region.  This prevents heat transfer through convection, and since in the environment of the sun convection is a major method of heat transfer, much less heat reaches the surface.  The source of the magnetic force that causes this effect is not well understood, but it is thought to be related to the sun's magnetic field. This field, a field much like that which we have here on Earth, although notably more volatile, is thought to be made by moving fluids inside the sun.

  (I obtained information for the last paragraph from our textbook, Astronomy: The Revolving Universe, and from the Exploratorium's website on sunspots, http://www.exploratorium.edu/sunspots/)