Ancient Astronomy Neolithic Astronomy Stonehenge ~ 2800 – 1700 BCE

The Americas Machu Picchu – the Intihuatana or “hitching post of the sun” Several of the major cultures had complex structures that reveal astronomical alignments Marked solstices, equinoxes, and positions of the Moon.

The Caracol at Chitzen Itza, Yucatan ~ 800 AD Venus was the ‘Great or Ancient Star”. Observations from slits in circular structure used to record times of its rising and setting. Measurements of Venus and moon used to establish a calendar and for timekeeping.

N. America – Medicine Wheels in western plains Stone alignments in a spoked wheel pattern some with large cairns that marked sight lines All are on high places with a clear view of the horizon. Medicine Wheel in Big Horn Mtns, Wyoming Date from ~

Geocentric or Ptolemaic model Problem – apparent backward motions of planets -- retrograde

The Heliocentric Model Copernicus Kept circular orbits and epicycles, explained retrograde motion

Galileo 1564 – 1642

Galileo’s famous observations with the telescope Surface of the Moon – mountains, craters Phases of Venus – Saturn had “ears” 4 moons of Jupiter Milky Way made of 1000’s of stars Sun was blemished

Galileo’s 3 decisive discoveries for Heliocentric model : 1. Surface features on the Moon 2. Jupiter’s moons and motions 3. “Full” phases of Venus None of these made sense in geocentric model, but they were all perfectly OK in Copernicus’ universe. The phases of Venus are especially decisive.

Tycho Brahe 1546 – 1601

Brahe couldn’t measure stellar parallax and therefore couldn’t confirm Copernican model -- proposed his own

Thomas Digges – English mathematician, astronomer 1546 – 1595 Contemporary with Brahe and Galileo, but his model goes a step further "This orb of stars fixed infinitely up extends itself in altitude spherically, and therefore immovable the palace of felicity garnished with perpetual shining glorious lights innumerable, far excelling over [the] sun both in quantity and quality the very court of celestial angels, devoid of grief and replenished with perfect endless joy, the habitacle for the elect."

Johannes Kepler The Astronomical Unit ( 1 AU) distance Earth to the Sun 93 x 10 6 mi, 150 x 10 6 km

Kepler’s 3 laws of planetary motion 1. The orbits of the planets are ellipses with the Sun at one focus 2. The line joining the planet and the Sun sweeps out equal areas in equal times. 3. Period of revolution proportional to distance from Sun If a = average distance from the Sun and P = orbital period, then P 2 = (constant) x a 3 If P in years, a in astronomical units then P 2 = a 3

Kepler’s third law -- the numbers a (AU) P (yr) a 3 P 2 MERCURY VENUS EARTH MARS JUPITER SATURN

Isaac Newton Astronomer, mathematician, physicist – father of physics, mechanics Laws of Motion and Law of Gravity

The 3 Laws of Motion and the Law of Gravity Some basic concepts: Speed – how fast an object is moving mi/hr, km/s Velocity -- vector – speed + direction Acceleration – rate of change in velocity Inertia – property of an object resists change in state of rest or motion 1 st Law -- An object remains at rest or in motion (in a straight line) unless acted upon by an outside force

2 nd Law -- The acceleration of an object is directly proportional to the force acting on it. F = ma Defines: Force Mass (m) - total amount of material in an object weight depends on the force of gravity on an object with mass m 3 rd Law -- (Law of Reaction) Whenever a force is exerted on an object there is an equal and opposite reaction

Law of Gravity -- the force of attraction between two objects is directly proportional to their masses and inversely proportional to the square of the distance between them ( attractive force between M and m ) F Grav = G x M x m / (distance) 2. Laws of Motion + Gravity explained Kepler’s Laws Newton’s form of Kepler’s 3 rd law: P 2 = 4  2 /  G x a 3 / (M 1 + M 2 ) Why the difference ? Orbital Motion is due to inertia plus gravity – the force of acceleration towards the center -- centripetal acceleration