AstronomyAstronomyThe Science that Studies The Science that Studies

The UniverseThe UniverseAncient GreeksAncient Greeks

ToTo

Isaac NewtonIsaac Newton

Geocentric Model of the UniverseAristotle 384 BC -1600’s AD

• The Moon, the Sun and the Planets (Mercury, Venus, Mars, Jupiter and Saturn) go around Earth within a hollow celestial sphere of Stars

Heliocentric Model of the Universe 312 BC to Present

• Sun centered universe with the Earth and the other planets orbiting the Sun

Ptolemaic System (AD141)Ptolemaic System (AD141)

• Planets move in circles about the Earth• Planets also moved in tiny circles in their

orbit, which were called epicycles• The epicycles explained an apparent

backwards motion of the planets against the stars called “retrograde”

The Birth of Modern Astronomy

• Nicolaus CopernicusNicolaus Copernicus (1500 AD) concluded (1500 AD) concluded Earth is a planet and Earth is a planet and the Sun is at the center the Sun is at the center of its motionof its motion

Tycho Brahe1600 AD

• Built an observatory in Denmark

• Constructed the most precise astronomical measuring devices of his time that measured the angles of the stars and planets

Johannes Kepler1600 AD

• An assistant to Tycho Brahe• Used Tycho’s observations

to develop his three laws of planetary motion:

1. Each planet travels in an ellipse about the Sun

2. Every month a line drawn from the Sun to a planet would “sweep out” the same area

3. There is a single mathematical relationship for all the planets traveling around the Sun, describing the length of time to get around (orbital period) relative to how far they were from the Sun (the further out the longer to get around)

Galileo Galilei1600 AD

• His most important contributions were his His most important contributions were his descriptions of the behavior of moving objects.descriptions of the behavior of moving objects.

• With his telescope he discovered : With his telescope he discovered : four moons orbiting Jupiterfour moons orbiting Jupiter planets are not just points of light like starsplanets are not just points of light like stars Venus has “phases” like our MoonVenus has “phases” like our Moon the Moon’s surface was not smooththe Moon’s surface was not smooth the sun had sunspotsthe sun had sunspots

Sir Isaac Newton1700 AD

He developed the concept of He developed the concept of universal universal gravitationgravitation: every body in the universe : every body in the universe attracts every other body with a force that attracts every other body with a force that depends on the mass of each and the depends on the mass of each and the distance of separationdistance of separation

• Two times theTwo times the mass mass = Twice the force = Twice the force

• Two times the Two times the distancedistance = 1/4 = 1/4thth the force the force

• Three times the Three times the distancedistance = 1/9 = 1/9thth the force the force

He explained the He explained the reasonsreasons for Kepler’s laws for Kepler’s laws

Summary & Reviewuse these question to guide you in writing your summary

1. Which model of the structure of the universe did Copernicus use?

2. List three of Galileo’s contributions.

3. Which law did Isaac Newton give to us?