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written by Todd Timberlake
The Galileo's Moon Mountain Model illustrates the method used by Galileo to measure the height of a mountain on the Moon.  Using his improved telescope design, Galileo was able to see spots of light in the otherwise dark potion of the Moon.  He interpreted these spots as mountain peaks which caught the rays of the sun even though the sun did not illuminate the Moon's surface at the base of the mountain.  He measured the distance of the bright spot from the terminator (the line separating the lit and unlit portions of the Moon) as a fraction of the Moon's radius.  Then he was able to use a geometrical argument to determine the height of the mountain as a fraction of the Moon's radius.  Galileo knew that the Moon's radius was approximately 1600 km (he didn't use those units, of course), which allowed him to determine the absolute height of the mountain.  (Note that the modern value for the Moon's radius is about 1740 km.)

One window shows the view from above the North pole of the Moon.  The mountain appears near the bottom of this window.  A ray of sunlight which just grazes the Moon's surface at the terminator is shown. Controls allow the user to adjust the angle of sunlight (thus altering the Moon's phase) and the height of the mountain.

The other window shows the view from Earth.  When sunlight strikes the top of the mountain a bright spot becomes visible in the dark area of the Moon. Likewise, when the mountain is in the bright region it casts a shadow.  The distance across the Moon's face from terminator to mountain in shown.

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
- Solar System
= The Moon
- Lower Undergraduate
- 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, Moon, mountain, terminator
Record Creator:
Metadata instance created May 12, 2011 by Todd Timberlake
Record Updated:
June 6, 2014 by Andreu Glasmann
Last Update
when Cataloged:
May 15, 2011
Other Collections:

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Record Link
AIP Format
T. Timberlake, Computer Program GALILEO'S MOON MOUNTAIN, Version 1.0 (2011), WWW Document, (
T. Timberlake, Computer Program GALILEO'S MOON MOUNTAIN, Version 1.0 (2011), <>.
APA Format
Timberlake, T. (2011). Galileo's Moon Mountain (Version 1.0) [Computer software]. Retrieved July 29, 2014, from
Chicago Format
Timberlake, Todd. "Galileo's Moon Mountain." Version 1.0. (accessed 29 July 2014).
MLA Format
Timberlake, Todd. Galileo's Moon Mountain. Vers. 1.0. Computer software. 2011. Java (JRE) 1.5. 29 July 2014 <>.
BibTeX Export Format
@misc{ Author = "Todd Timberlake", Title = {Galileo's Moon Mountain}, Month = {May}, Year = {2011} }
Refer Export Format

%A Todd Timberlake
%T Galileo's Moon Mountain
%D May 15, 2011
%O 1.0
%O application/java

EndNote Export Format

%0 Computer Program
%A Timberlake, Todd
%D May 15, 2011
%T Galileo's Moon Mountain
%7 1.0
%8 May 15, 2011

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Galileo's Moon Mountain:

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's Moon Mountain.

relation by Wolfgang Christian

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