Podcast
Questions and Answers
What does retrograde mean?
What does retrograde mean?
backwards
When does the first period of apparent retrograde motion occur?
When does the first period of apparent retrograde motion occur?
when Mars appears to be in the constellation Taurus
When is the Sun visible in the top window of the video?
When is the Sun visible in the top window of the video?
when Earth and Mars are on opposite side of the Sun
At most times, how does Mars appear to move?
At most times, how does Mars appear to move?
What happens during periods of apparent retrograde motion for Mars?
What happens during periods of apparent retrograde motion for Mars?
To document the apparent retrograde motion of Mars, what would you need to measure and record?
To document the apparent retrograde motion of Mars, what would you need to measure and record?
What happens when we see Mars going through apparent retrograde motion?
What happens when we see Mars going through apparent retrograde motion?
What does Mars appear to do during its periods of apparent retrograde motion?
What does Mars appear to do during its periods of apparent retrograde motion?
Should these facts alone have been enough to convince the ancient Greeks that Earth really does go around the Sun?
Should these facts alone have been enough to convince the ancient Greeks that Earth really does go around the Sun?
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Study Notes
Apparent Retrograde Motion of Mars
- "Retrograde" refers to motion that appears to move backwards.
- The initial period of retrograde motion for Mars occurs while it is in the constellation Taurus.
- The Sun is visible in the top window of the video when Earth and Mars are positioned on opposite sides of the Sun.
Mars's Motion Relative to Stars
- Typically, Mars moves eastward relative to the stars in the night sky.
- During periods of retrograde motion, Mars appears to move westward relative to the stars.
Documenting Retrograde Motion
- To accurately document Mars's retrograde motion, one must measure its position among the constellations over several months.
Orbital Dynamics
- Apparent retrograde motion of Mars occurs when Earth overtakes Mars in their orbits around the Sun, resembling Mars being "lapped".
- When Mars is in the midst of retrograde motion, it shines brightly in the night sky, crosses the meridian around midnight, and is closest to Earth in its orbit, appearing full and opposite the Sun.
Historical Perspective
- Ancient Greeks were not convinced Earth orbited the Sun, as they adhered to an Earth-centered model that explained apparent retrograde motion through complex circular orbits of planets around Earth.
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