click for hint or
click for answer
A lesson developed with partial support of NSF Grant # DUE9554744.
Dr. Dan R. Overcash, South Carolina State University, Orangeburg, SC 29117
Prof. Cynthia W. Peterson, University of Connecticut, Storrs, CT 06269
When you see the following icons,
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Now we must examine the phases of the planets at the respective positions.
Two basic orreries are
Note that in the geocentric orrery, Venus and Mercury can never go behind the sun. Since these inferior planets always lie between the earth and sun, they can never show gibbous or full phases.
Now look at the recent satellite photo of Venus as seen from earth!
What conclusion about the geocentric model is the result of observing Venus at gibbous phase?
Does the heliocentric model allow Venus to be seen at gibbous phase?
Thus this single observation, first made by Galileo's telescope in
1603, allowed astronomers to distinguish between these two proposed
models for the geometry of the solar system.
Now watch a short movie of the planets seen from the earth as they orbit
the sun in a heliocentric orrery (Helios Planetarium). Watch carefully and
observe Mercury's phases change through a complete cycle. It is suggested that
you step through the first orbit frame by frame to see the labelled positions clearly.
A double click will replay the movie and a single click during play will pause the
movie so labels can be read. Click right mouse button outside the movie frame to bring
'back' button into view, soas to return to the lesson.
Here is a short quiz to solidify the concepts.
1. In the following orrery for April 25, 1999 at 0:00 UT, Mars is at what phase?
2. In the above orrery, Venus has just passed what phase?
Click here for additional review
quiz questions about the planetary phases associated
with various orbital positions.
click here to see
movie of Mercury phases seen from earth in a heliocentric orrery.
orrery for question 1