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written by Todd Timberlake
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).
View the source code document attached to this resource
Subjects Levels Resource Types
- Astronomy Education
= Curricula
- Historical Astronomy
= History of Astronomy
- The Sun
= Magnetic Activity
= Space Weather
- Lower Undergraduate
- Middle School
- High School
- Instructional Material
= Simulation
Intended Users Formats Ratings
- Learners
- Educators
- General Publics
- application/java
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Access Rights:
Free access
This material is released under a GNU General Public License Version 3 license.
Galileo, obliquity, solar rotation, sunspot
Record Creator:
Metadata instance created May 12, 2011 by Todd Timberlake
Record Updated:
June 6, 2014 by Andreu Glasmann
Last Update
when Cataloged:
May 12, 2011
Other Collections:

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Record Link
AIP Format
T. Timberlake, Computer Program GALILEO SUNSPOTS, Version 1.0 (2011), WWW Document, (
T. Timberlake, Computer Program GALILEO SUNSPOTS, Version 1.0 (2011), <>.
APA Format
Timberlake, T. (2011). Galileo Sunspots (Version 1.0) [Computer software]. Retrieved August 1, 2014, from
Chicago Format
Timberlake, Todd. "Galileo Sunspots." Version 1.0. (accessed 1 August 2014).
MLA Format
Timberlake, Todd. Galileo Sunspots. Vers. 1.0. Computer software. 2011. Java (JRE) 1.5. 1 Aug. 2014 <>.
BibTeX Export Format
@misc{ Author = "Todd Timberlake", Title = {Galileo Sunspots}, Month = {May}, Year = {2011} }
Refer Export Format

%A Todd Timberlake
%T Galileo Sunspots
%D May 12, 2011
%O 1.0
%O application/java

EndNote Export Format

%0 Computer Program
%A Timberlake, Todd
%D May 12, 2011
%T Galileo Sunspots
%7 1.0
%8 May 12, 2011

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Galileo Sunspots:

Is Based On Easy Java Simulations Modeling and Authoring Tool

The Easy Java Simulations Modeling and Authoring Tool is needed to explore the computational model used in the Galileo Sunspots.

relation by Wolfgang Christian

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