Solar System Concepts

Geocentric Model:
- Proposed by the Greeks, with Earth at the center of the universe.
Aristotle's Model:
- Described Earth as a sphere surrounded by 55 crystalline spheres (one for each planet and star).
Ptolemy (140 A.D.):
- Enhanced the geocentric model using epicycles to explain retrograde motion (when planets appear to move backward).

Copernicus:
- Suggested a heliocentric model, where planets orbit the Sun (late 1500s).
Galileo (early 1600s):
- Provided evidence for heliocentrism.
- Observed that Venus has phases similar to the Moon.
- Discovered 4 moons orbiting Jupiter (now known as the Galilean moons).
Kepler (early 1600s):
- Found that planetary orbits are elliptical, not circular.
- Definitions:
- Perihelion: Closest point to the Sun.
- Aphelion: Furthest point from the Sun.
- Earth is 3% closer to the sun at perihelion in January than at aphelion.

Formed approximately 4.6 billion years ago from a solar nebula (cloud of gas and dust).
- Gravity caused the nebula to contract into a rotating disk, where:
- Most gas became the Sun.
- Denser elements formed the inner planets.
- Lighter gases formed around the larger outer planets.

Distance from Earth to the Sun: 150 million kilometers (93 million miles), known as 1 astronomical unit (AU).
All planets orbit the Sun counter-clockwise.

Size: 110 times larger than Earth; mass constitutes 99.8% of the solar system.
Rotation: Completes one rotation every 27 days.
Core Temperature: Approximately 15 million°C, where nuclear fusion occurs (hydrogen atoms combine to form helium).
Layers:
- Radiative Layer: Energy transferred through radiation.
- Convective Layer: Gases circulate due to convection currents.
Sun's Atmosphere:
- Photosphere: Visible surface; coolest part.
- Chromosphere: Layer of spiked gases.
- Corona: Thin gases visible during an eclipse, source of solar wind.

Sunspots: Dark spots indicating cooler regions due to slowed convection, cycle approximately every 11 years.
Prominences: Loops of gases along magnetic fields.
Solar Flares: Eruptions near sunspots, emitting radiation that disrupts Earth communications.
Coronal Mass Ejections (CMEs): Eruptions of energized gas that can take days to reach Earth.
Auroras: Lights created when charged particles interact with Earth's magnetic field (Northern Lights in the north).

Categories:
- Planets: Eight total, defined by their ability to orbit the Sun, be spheroid, and clear their orbit of debris.
- Dwarf Planets: Five known, only meet the first two requirements.
- Small Solar System Bodies.

Mercury: Closest to the Sun, no moons, extreme temperature variations due to lack of atmosphere.
Venus: Earth's twin, hottest planet; has no moons; longer day than year.
Earth: Supports life; has one moon.
Mars: Known as the red planet, thin atmosphere, has evidence suggesting past life; has two moons.

Jupiter: Largest planet (10x Earth's size), 92 moons and small rings; shortest rotation period of 9.9 hours.
Saturn: Prominent ring system and 83 known moons.
Uranus: Tilted rotational axis (98 degrees); 27 known moons.
Neptune: Known for its storms (large blue spots); 14 moons.

Located in an asteroid belt between Mars and Jupiter; irregularly shaped; collisions with Earth can occur as meteors.

Composed of rock and ice, originating from the Oort Cloud or Kuiper Belt.
- Components:
- Nucleus: Solid core; up to 10 km wide.
- Coma: Best seen as the vapor cloud around the nucleus.
- Tail: Always points away from the Sun due to solar wind.