History of Astronomy

Our Universe

• Earth is one of nine planets that orbit the sun• The sun is one star in 100 billion stars that

make up our galaxy- The Milky Way• Our galaxy is one of billions of galaxies in the

universe• Views of our galaxy and Universe have

changed tremendously throughout time

Early Astronomy

• Astronomy- science that studies the universe• Deals with properties of objects in space and the laws

through which the universe operates

Early Astronomy- Ancient Greeks

• Aristotle (384-322 B.C.)– Concluded that Earth was round from

observations of the curved shadow on the moon when Earth passes between sun and moon

• Eratosthenes (276-194 B.C.)– Calculated circumference of the Earth• Used angles of noonday sun in two locations on the

planet

Early Astronomy- Ancient Greeks

• Hipparchus (2nd century B.C.)– Determined the location of 850 stars and

categorized them according to brightness– He measured the length of a year and developed a

method for predicting lunar eclipses

Models of the Universe

• Greeks believed in the Geocentric model of the universe• Earth is the center and planets, sun, and moon orbit

around Earth

• Aristarcus (312-230 B.C.)– First to propose a heliocentric model- Earth and

other planets orbit the sun• Still- the Geocentric model dominated for

nearly 2,000 years

Ptolemaic System

• Claudius Ptolemy published the Ptolemaic system in 141 A.D.

• This system accounted for the movements of the planets, but was still a Geocentric model which was determined to be inaccurate

• Ptolemy discovered retrograde motion

Retrograde Motion

• Planets appear to stop, reverse direction, and resume normal motion

Modern Astronomy

• Nicolaus Copernicus (1473-1543)– Concluded that Earth was a planet and proposed a

model of the solar system with the sun at the center (Heliocentric)

– Tycho Brahe (1546-1601) • Used instruments to measure the locations of “heavenly

bodies”• His observations were far more precise than any made

before• Johannes Kepler started his astronomy career as an

assistant to Brahe

Modern Astronomy

• Johannes Kepler (1571-1630)• 3 laws of planetary motion– Discovered that planetary orbit around the sun is

not in a circle but an ellipse (oval shape)– Also discovered that speed of planets changes as

revolve around the sun• Faster as they get closer to the sun (perihelion) slower

as it gets further from the sun (aphelion)

Kepler’s 3 Laws of Planetary Motion

• Law 1– The path of each planet around the sun is an

ellipse, with the sun at one focus, and other focus symmetrically opposite

Kepler’s 3 Laws of Planetary Motion

• Law 2– Each planet revolves around the sun so that it

sweeps over equal areas in equal time intervals- therefore planets travel faster as they near the sun

Kepler’s 3 Laws of Planetary Motion

• Solar distances of the planets can be calculated when their periods of revolution are known

• Distances are expressed in astronomical units (AU)

Galileo Galilei (1564-1642)

• Created the first telescope in 1609- previous to this invention all astronomical discoveries were made without aid of a telescope– Telescope magnified objects 3X

Galileo’s Discoveries• Four moons orbiting Jupiter– Disproved that Earth was center of universe

• Planets are circular disks and Earthlike– Disproved that planets were points of light

• Venus has phases like the moon– Shows that Venus orbits its light source (the sun)

• Moons surface is not smooth• Sun has sunspots, or dark regions– Helped to calculate the rotational period of the sun

Sir Isaac Newton (1642-1727)

1. Was the first to formulate and test the law of universal gravitation– This helps to explain why planets move in ellipses

and not in a straight line• Gravity is directly proportional to mass of an object and

inversely proportional to distance• Greater mass = Greater gravitational attraction• Greater distance = less gravitational attraction

– Gravity Pulls objects towards the sun = elliptical orbits