AY 102 Lecture Notes - Lecture 4: Copernican Revolution, Orbital Period, Semi-Major And Semi-Minor Axes

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24 Jan 2018
AY Lecture 4 1-24-18
Historical Underpinnings of Astronomy
The view of the location of Earth in the universe changed from:
- Geocentric (Erath-centered) to
- Heliocentric (Sun-centered
Six early scientists contributed to these development in astronomy:
- Ptolemy
Famous geocentric model of planetary motion
Plaets oe i irles o irles aroud the Earth
- Copernicus
Proposed heliocentric model
- Brahe
Precise observations of planetary positions
- Kepler
Using these observations, derived three empirical laws of planetary motion
Planets move along elliptical paths around the sun
- Galileo
Using a telescope to discover mountains on the moon, the moons of Jupiter,
sunspots, and the phases of Venus
- Newton
Using math to derive physical laws of motion and developed the universal law of
graitatio that eplaied Kepler’s las
What are planets?
- aderig stars
- ostellatios defie fied stars
Seeking evidence that Earth is also a planet and that it moves
- Two observations favored a heliocentric view
Retrograde motion of Mars, Jupiter, and Saturn
Venus and Mercury can never be seen at midnight
Normally planets move west to east in the sky, but every 780 days, Mars temporarily reverses
its oeet goes retrograde
- What causes this retrograde motion?
It is aused  a real loop i Mars’ orit
Heliocentric view
- Retrograde motion is an illusion caused by the relative movement of the Earth and a
planet moving on a larger, slower orbit around the sun
Retrograde motion occurs when an outer planet is at opposition. At this time, an outer planet:
- Is opposite the sun in the sky
- Is closest to Earth
- Rises at sunset and set at sunrise
- Crosses the meridian at midnight
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