A Brief History of Planetary Science

Astronomy 311Professor Lee

CarknerLecture 2

Ancient Astronomy

They noticed that some things moved with respect to the stars: Five “wandering stars” (planets)

Also transient things like comets and meteors

Caracol

Ancient Observing

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 ZenithTo Zenith

SyeneAlexandria

To Sun7

7

Eratosthenes’s Experiment• Measure length of shadow,

find angle between Sun and zenith:

• • Subtract angles measured

at both cities: • If D is distance between

the two cities (756km), the circumference of the Earth is:

•

zenith

L=length of shadow

Sun

H =heightof 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 Phases of Venus

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

Uranus (William Herschel, 1781) Pluto (Clyde Tombaugh, 1930)

In the the 1990’s large infrared

telescopes confirm a large, well populated, zone of small icy bodies beyond Neptune Called the Kuiper Belt

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

Next Time

Meet in planetarium Read Chapter 1.1-1.5 Do homework

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 (~1500-1700AD) 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