Overview
This lecture covers Earth's movements, the lunar cycle, the structure of the solar system, gravitational forces, star life cycles, galaxy types, cosmological expansion, and evidence for the Big Bang Theory.
Earth's Rotation and Orbit
- Earth orbits the Sun in a nearly circular path, completing one orbit in about 365 days (one year).
- Earth's axis is tilted at 23.4° from the vertical, causing seasonal changes.
- One full Earth rotation (24 hours) causes day and night.
- The Sun appears to rise in the east and set in the west due to Earth's rotation.
Seasons and Equinoxes/Solstices
- Seasons result from Earth's axial tilt and orbit around the Sun.
- During equinoxes, day and night are equal in length; the Sun rises due east and sets due west.
- In the northern hemisphere, summer solstice (June 21): longest day; winter solstice (December 21): shortest day.
The Moon and Its Phases
- The Moon orbits Earth every 27.3 days, always showing the same side to Earth.
- Moon phases result from its position relative to Earth and Sun; new moon (invisible), full moon (fully illuminated).
- The Moon reflects sunlight; it does not emit its own light.
Orbital Speed and Calculations
- Average orbital speed: ( V = 2\pi R / T ) (R = orbit radius, T = orbital period).
- Radius of orbit is measured from the center of the object being orbited.
The Solar System Structure
- The Sun (a star) is at the center, containing over 99% of solar system mass.
- Eight planets: Mercury, Venus, Earth, Mars (rocky); Jupiter, Saturn, Uranus, Neptune (gas giants).
- Venus is hotter than Mercury due to a thick greenhouse atmosphere.
- Includes moons (natural satellites), dwarf planets, asteroids, comets, and meteoroids.
Gravity and Orbits
- Gravitational force: directly proportional to product of masses, inversely proportional to square of distance.
- Newton’s third law: forces between two objects are equal and opposite.
- Orbital motion is caused by gravitational force providing centripetal force.
- Further distance from Sun = weaker gravity, slower orbital speed, and longer period.
Formation of Solar System and Stars
- Solar system formed by accretion from interstellar dust and gas (nebula).
- Closer to Sun: rocky planets formed; farther: gas giants.
- Stars form from collapsing nebula, initiating nuclear fusion at their cores.
Stars, Galaxies, and Universe
- The Sun is a medium-sized star, mainly hydrogen and helium.
- Stars are classified by color (surface temperature): red (coolest) to blue (hottest).
- Star life cycle: nebula → main sequence star → red giant/supergiant → planetary nebula/supernova → white dwarf/neutron star/black hole.
- Galaxies: spiral, elliptical, irregular; Milky Way is spiral.
Astronomical Distances
- 1 Astronomical Unit (AU): average Earth-Sun distance (~1.5 × 10^8 km).
- 1 Light Year: distance light travels in a year (~9.5 × 10^12 km).
Cosmology and Universal Expansion
- Doppler effect: red shift in galaxy light indicates they are moving away.
- Hubble's Law: galaxy recession velocity is proportional to distance (V = H₀D).
- The universe is expanding; estimated age ≈ 14 billion years.
- Cosmic Microwave Background (CMB): uniform radiation evidence for the Big Bang; hot/cold regions relate to galaxy distribution.
Key Terms & Definitions
- Equinox — Day and night are equal in length.
- Solstice — Point of longest/shortest day (summer/winter) in a hemisphere.
- Orbital Speed — Speed calculated by ( V = 2\pi R / T ).
- AU (Astronomical Unit) — Mean Earth-Sun distance.
- Light Year — Distance light travels in one year.
- Red Shift — Increase in observed wavelength due to an object’s motion away from observer.
- Hubble's Law — Recession velocity proportional to distance.
- CMB (Cosmic Microwave Background) — Remnant radiation from the Big Bang.
Action Items / Next Steps
- Review syllabus details in the accompanying figure.
- Practice calculations using the orbital speed formula.
- Study diagrams of the solar system and phases of the Moon.
- Read up on the Big Bang Theory and red shift evidence.