A Brief History of Planetary Science Astronomy 311 Professor Lee Carkner Lecture 2

Ancient Astronomy   They noticed that some things moved with respect to the stars:   The Moon   Also transient things like comets and meteors

Caracol

Ancient Observing  People all over the world built devices to help observe the sky  Two basic purposes:   When does planting season begin?   Our place in the universe has deep spiritual significance

Ancient Greek Astronomy  Greeks used reason and mathematics to study the sky  Greek discoveries:   Relative distance to Sun and Moon   Earth’s diameter

Finding the Size of the Earth Sunlight To Zenith Syene Alexandria To Sun 77 77

Eratosthenes’s Experiment arctan (L/H) =   Subtract angles measured at both cities:  =  1 -  2 C=(360/  ) X D zenith L=length of shadow Sun H = height of stick Angle = 

Geocentric Solar System   Most obvious explanation of fact that everything seems to go around the Earth   Developed between ~200BC (Hipparchus) and ~200AD (Ptolemy)

Retrograde Motion and Epicycles

Heliocentric Solar System   First proposed by Aristarchus (~300BC)   More comprehensive model developed by Copernicus (~1500 AD)  Better explains retrograde motion, relative brightness and positions of planets

Uraniborg on the Island of Ven

Tycho, Kepler and the Motions of the Planets   His successor Johannes Kepler used Tycho’s data to determine three laws of planetary motion (early 1600’s)   You can predict how they will move

Galileo’s Observations

Galileo and the Telescope  Made many important observations starting in 1610, including:   Mountains on the Moon    Galileo’s writings were condemned by the church, but presented hard evidence  Careful observation and theorizing by Copernicus, Brahe, Kepler and Galileo disproved the seeming obvious and incontrovertible geocentric model.

Newton and Gravity   Isaac Newton used Kepler’s Laws to discover gravity (~1700):   The universe is governed by universal rules

Discovering the Other Planets  Careful observation led to the discovery of:   Neptune (Adams and Le Verrier, 1846)   No other large planets  In the the 1990’s large infrared telescopes confirm a large, well populated, zone of small icy bodies beyond Neptune 

Space Missions   We now know compositions, atmospheric properties, temperatures, and morphologies for nearly every planet  This data has provided enormous insight into the history and nature of the solar system

Exoplanets

Extrasolar Planets   A planet’s gravity produces a slight periodic motion of the central star  Our solar system is not unique, billions of planets in the galaxy

Next Time Meet in planetarium Read Chapter Do homework Observing Tonight 9-10 pm –If clear (check webpage) –Bring at least two copies of telescope template

Summary  Pre-civilized  Sun, moon and planets move  Can be used to determine seasons  Greek (~300BC -300AD)  It is possible to measure their properties  Copernican Revolution (~ AD)  Copernicus -- Planets (including the Earth) orbit the Sun  Kepler -- Planets have elliptical orbits and their motions are governed by laws  Galileo -- planets have features like the Earth (mountains, satellites)

 Newton and Physics (~1700 AD)  Planets follow laws of physics  Gravity accounts for orbital motion  Modern (20th Century)  Solar System consists of 9 (now 8) planets plus cometary region  Space missions have allowed the detailed study of each planet  Planets exist around other stars