Overview
This lecture introduces key vocabulary and foundational concepts for Astronomy Unit 1, covering celestial models, planetary motion, and essential scientific principles.
Astronomical Measurement & Models
- The Astronomical Unit (AU) is the average distance from Earth to the Sun, about 150 million km.
- A constellation is a region of the sky; 88 are officially recognized.
- The geocentric model assumes Earth is at the center of the universe.
- The Ptolemaic model is an ancient geocentric system developed by Ptolemy.
- The heliocentric model proposes that planets, including Earth, revolve around the Sun.
- The Copernican Revolution marks the shift to the heliocentric model.
Observations & Planetary Motion
- Apparent retrograde motion describes when a planet appears to move westward relative to the stars as seen from Earth.
- Stellar parallax is the apparent shift in a nearby star’s position due to Earth's movement around the Sun.
Scientific Methods & Tools
- A scientific model is a testable explanation or representation of natural phenomena.
- Occam’s Razor states that simpler models are preferred when two explain observations equally well.
- Reflecting telescopes use mirrors, and refracting telescopes use lenses to focus light.
Orbits & Laws of Planetary Motion
- An ellipse is an oval shape describing planetary orbits.
- Eccentricity measures how much an ellipse deviates from being a circle.
- Each ellipse has two foci (singular: focus) along the major axis.
- Kepler's 1st Law: Planets orbit the Sun in ellipses, with the Sun at one focus.
- Kepler's 2nd Law: Planets move faster in their orbit closer to the Sun (at perihelion) and slower farther away (at aphelion).
- Kepler's 3rd Law: The square of a planet's orbital period is proportional to the cube of its average distance from the Sun.
Gravity & Orbital Points
- The Universal Law of Gravitation states that gravity increases with object mass and decreases with greater distance.
- Perihelion is when an object is closest to the Sun; aphelion is when it is farthest from the Sun.
Key Terms & Definitions
- Astronomical Unit (AU) — Average Earth-Sun distance (~150 million km).
- Constellation — Sky region with a recognized group of stars.
- Apparent retrograde motion — Westward, backward motion of a planet as observed from Earth.
- Stellar parallax — Apparent positional shift of a star due to Earth's orbit.
- Scientific model — Representation or explanation for natural phenomena.
- Geocentric model — Universe model with Earth at the center.
- Ptolemaic model — Ancient geocentric planetary model (Ptolemy).
- Copernican revolution — Shift to Sun-centered (heliocentric) model.
- Heliocentric model — Sun at the center of the solar system.
- Reflecting telescope — Uses mirror to focus light.
- Refracting telescope — Uses lens to focus light.
- Ellipse — Closed, oval, planet orbital path.
- Eccentricity — Measure of an ellipse's "stretch."
- Focus (ellipse) — Special points inside an ellipse.
- Kepler's Laws — Three laws describing planetary motion.
- Universal Law of Gravitation — Law describing gravitational force.
- Perihelion — Closest orbital point to the Sun.
- Aphelion — Farthest orbital point from the Sun.
- Occam's Razor — Principle preferring simpler explanations.
Action Items / Next Steps
- Study and memorize key terms and their definitions.
- Be able to apply Kepler’s Laws and the Universal Law of Gravitation to example problems.
- Review differences between geocentric and heliocentric models for class discussion.