The EJS Galileo Sunspots Model illustrates the motion and changes in appearance of sunspots as they move across the disk of the Sun. Galileo's observation of the changing appearance of sunspots around 1611 provided strong evidence that the spots were actually blemishes on the surface of the Sun, an idea that conflicted with the Aristotelean notion of the perfections of the heavens.  Through these observations Galileo was able to determine the obliquity of the Sun (the angle between the Sun's equator and the Ecliptic plane) as well as the rotational period of the Sun.

The simulation shows four randomly generated sunspots, and one sunspot that has a user-defined latitude (to allow the user to examine the behavior of sunspots at various latitudes).  All sunspots are oval in shape.  As the spots move across the Sun their appearance changes due to foreshortening and a slight tilt as the spots approaches the limb (or edge) of the solar disk.  These effects come about because the Sun's surface is spherical, not flat.  So the sunspot changes it orientation relative to the viewer as it moves around on the Sun's surface.

In addition to controls that allow the user to define the latitude of one of the sunspots, this model allows the user to set the obliquity of a line representing the Sun's equator.  The user can adjust the obliquity of this line until the sunspots all move parallel to the line.  In this case the obliquity of the line is the actual obliquity of the Sun.  In addition, the model displays the elapsed time (in days) so that the user can determine the rotational period of the Sun by measuring the time required for a sunspot to complete its motion around the Sun.  (Note that the period of a sunspots motion is latitude-dependent, as it is in the real Sun.)

Please note that this resource requires at least version 1.5 of Java (JRE).