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

Ancient Astronomy  Ancient people had a much better view of the sky than we do today  They noticed that some things moved with respect to the stars:     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:  Calendars   Religious or Mystical Reasons    Our place in the universe has deep spiritual significance

Early Middle Eastern Civilization Using astronomy to chart the seasons was a key job of the Priest-Kings of the fertile crescent.

The Days of the Week English Sunday Monday Tuesday Wednesday Thursday Friday Saturday Planets Sun Moon Mars Mercury Jupiter Venus Saturn Old English Sun Moon Tiw Woden Thor Frigg Seterne Latin Solis Lunae Martis Mercurii Jovis Veneris Saturni

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

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

Eratosthenes’s Experiment Measure length of shadow, find angle between Sun and zenith: arctan (L/H) =   Subtract angles measured at both cities:  =  1 -  2 If D is distance between the two cities (756km), the circumference of the Earth is: C=(360/  ) X D zenith L=length of shadow Sun H = height of stick Angle = 

Geocentric Solar System  Earth is at center of Solar System    Developed between ~200BC (Hipparchus) and ~200AD (Ptolemy)

Retrograde Motion and Epicycles

Heliocentric Solar System  Sun at center of Solar System  First proposed by Aristarchus (~300BC)   More comprehensive model developed by Copernicus (~1500 AD) 

Uraniborg

Tycho, Kepler and the Motions of the Planets  Tycho Brahe carefully observed the planets for 20 years at Uraniborg ( AD)   Planets move in elliptical orbits and follow mathematical laws 

Galileo’s Observations

Galileo and the Telescope  Made many important observations starting in 1610, including:      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  Why do the planets move?  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:  Uranus (  Neptune (  Pluto (  No other large planets

Discovering Pluto

Beyond Pluto

The Kuiper Belt and Oort Cloud  What is beyond Pluto?   Jan Oort theorized an additional sphere of comets beyond the Kuiper Belt (Oort Cloud) 

Mare Imbrium - Apollo 15

Viking Lands on Mars

The Grand Tour - Voyager II

Space Missions  A series of space missions since the early 1960’s have allowed close up views of the planets   This data has provided enormous insight into the history and nature of the solar system

Exoplanets

Extrasolar Planets  In the 1990’s first detection of planets outside of the solar system   Our solar system is not unique, billions of planets in the galaxy

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 planets plus cometary region  Space missions have allowed the detailed study of each planet  Planets exist around other stars