Ancient Astronomers Egyptians Egyptians Babylonians Babylonians Native Americans Native Americans Chinese Chinese Celts Celts Many Others Many Others

Early Observers Stonehenge Big Horn Medicine Wheel Loanhead of Daviot Pyramids of Giza

Greek Astronomers Tried to explain the motion of celestial objects Tried to explain the motion of celestial objects Built Cosmological Models to explain observations Built Cosmological Models to explain observations Stars, Moon, Sun easily explained Stars, Moon, Sun easily explained Planets (wanderers) not so easily explained Planets (wanderers) not so easily explained

Scientific Method Use models to explain observations Use models to explain observations Test models Test models Theories are never “true” just “not false” Theories are never “true” just “not false”

Chain of Scientific Method ObservationExplanation Prediction

Early Greek Astronomers Eratosthenes calculated circumference of the earth within 5% accuracy Eratosthenes calculated circumference of the earth within 5% accuracy Aristarchus proposed a heliocentric cosmos and easily explained the motion of the Sun through the heavens Aristarchus proposed a heliocentric cosmos and easily explained the motion of the Sun through the heavens His writings were lost at Alexandria along with Hypatia His writings were lost at Alexandria along with Hypatia

Aristotle B.C B.C. Member of Plato’s Academy Member of Plato’s Academy Philosopher and Scientist Philosopher and Scientist One of the 1 st Physical Models of the Cosmos One of the 1 st Physical Models of the Cosmos Geocentric Geocentric

Ptolemy Attempted to explain retrograde motion Attempted to explain retrograde motion Used Hipparchus idea of epicycles and deferents Used Hipparchus idea of epicycles and deferents Added the equant Added the equant Very complicated Very complicated Inconsistent rules Inconsistent rules

Ptolemy’s Solution EarthEquant Deferent Epicycle Planet

Triumph and Tragedy Geocentric Model persisted for many, many, many, many years Geocentric Model persisted for many, many, many, many years Lack of observable Stellar Parallax Lack of observable Stellar Parallax Painter’s Thumb-Parallax Painter’s Thumb-Parallax Stars are too far away! Stars are too far away!

Copernican Revolution A.D A.D. Revived early heliocentric models Revived early heliocentric models Retrograde motion simply explained Retrograde motion simply explained Circular Orbits Circular Orbits No public support No public support

Copernican Retrograde

Copernicus says… The celestial spheres do not have just one common center The celestial spheres do not have just one common center The Center of Earth is not the center of the universe but instead only the center of gravity and of the lunar orbit The Center of Earth is not the center of the universe but instead only the center of gravity and of the lunar orbit All spheres revolve around the Sun All spheres revolve around the Sun

He continues…. The firmament is far away The firmament is far away Apparent motions of the firmament and Sun are actually due to Earth’s motions Apparent motions of the firmament and Sun are actually due to Earth’s motions The apparent retrograde and forward motion of the planets is actually due to the Motion of the Earth The apparent retrograde and forward motion of the planets is actually due to the Motion of the Earth

Finally Somebody Gets It! Johannes Kepler Johannes Kepler Worked for Tycho Brahe, inherited his data Worked for Tycho Brahe, inherited his data Heliocentric model (29 years!) Heliocentric model (29 years!) Found orbit shapes by observation from different points in Earth’s orbit Found orbit shapes by observation from different points in Earth’s orbit

" By the study of the orbit of Mars, we must either arrive at the secrets of astronomy or forever remain in ignorance of them." Johannes Kepler " By the study of the orbit of Mars, we must either arrive at the secrets of astronomy or forever remain in ignorance of them." Johannes Kepler

Kepler’s 1 st Law Elliptical Orbits not Circular! Elliptical Orbits not Circular!

Ellipse Geometry Minor Axis Major Axis Foci Eccentricity=How oval is it?

Kepler’s 2 nd Law Equal Areas in Equal Times Equal Areas in Equal Times Describes Single Planet is orbit around the Sun Describes Single Planet is orbit around the Sun Perihelion-closest approach to Sun Perihelion-closest approach to Sun Aphelion-farthest distance from Sun Aphelion-farthest distance from Sun

Kepler’s 3 rd Law The more distant a planet’s orbit is from the Sun, the slower it moves The more distant a planet’s orbit is from the Sun, the slower it moves Compares planets to one another Compares planets to one another Period- time for one complete orbit around sun Period- time for one complete orbit around sun Let’s see all three in action Let’s see all three in action Let’s see all three in action Let’s see all three in action

Kepler’s Triumph Published first two laws in 1609 Published first two laws in 1609 Calculations only for Mars Calculations only for Mars Decade later he checks all known planets Decade later he checks all known planets They work! They work! Also introduces 3 rd law at this time Also introduces 3 rd law at this time

Summary Models are constantly being evaluated Models are constantly being evaluated Theory is never “true” just not “false” Theory is never “true” just not “false” Example geocentric cosmos Example geocentric cosmos Kepler’s 3 laws accurately and simply describe the motion of the planets Kepler’s 3 laws accurately and simply describe the motion of the planets Laid the foundation for work of future scientists Laid the foundation for work of future scientists