Warm-up Explain the difference between precession and nutation. (check your notes from last week) TEST FRIDAY

The Universe

Organization (from large to small) Universe - made of galaxies Galaxies - made of many stars Some stars have planet systems Earth orbits one particular star (our Sun)

Galaxies Galaxy- a collection of stars, dust, and gas bound together by gravity Barred Elliptical Spiral Irregular

Big Bang theory The theory that the universe began as a point and has been expanding ever since. Not an explosion into space but an expansion of space Matter going along for the ride

Big Bang theory Competition between the outward expansion of the universe and inward force of gravity as the matter in the universe slows the expansion

Nebula- A large cloud of gas and dust in interstellar space; a region in space where stars are born or where stars explode at the end of their lives

Solar System Formation Collapse of the solar nebula caused heating at the center Planetesimals (small planets) formed in the surrounding space The central mass of the nebula became the Sun Planets formed from the surrounding materials

Solar System Formation There are an estimated 100 billion stars in the Milky Way alone!

Terrestrial Planet Formation Coalescence of small particles High temperatures developed, melting interiors

Rotation- the spin of a body on its axis Revolution- the motion of a body that travels around another body in space; one complete cycle of the orbit Orbit- the path that a body follows as it travels around another body in space

Kepler’s First Law Johannes Kepler, using Brahe’s accurate observations of planetary positions, demonstrated that planets orbit the Sun in an ellipse

Kepler’s Second Law A line between the Sun and a planet sweeps equal areas in equal times

Kepler’s Third Law The square of the orbital period (P) of a planet equals the cube of the semi-major axis of the orbital ellipse (a) P2 = a3

Kepler’s Third Law As planets get farther from the sun, the length of their orbit increases The amount of time to complete one orbit around the sun (period) also increases

Astronomical Unit AU Earth’s average distance from the Sun 1.5 x 108 km Used as base for measuring the distance between all planets Used in the next diagram – third law

Why Do Planets Orbit? In 1684, Isaac Newton published his mathematical and physical explanation of the motions of celestial bodies. Law of Universal Gravitation – any two bodies attract each other with a force that depends on their masses and the distance between the two bodies.