History of Astronomy

Our Universe• Earth is one of nine planets (if you include

Pluto) 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

Indicating Time• A.D. stands for Anno Domini, which is Latin for

"year of our Lord," and it means the number of years since the time of Jesus Christ.

• Some people use C.E. and B.C.E. instead. These stand for Common Era, and Before Common Era. People use these in order to avoid Christian references.

• How many years ago was 500 B.C.?

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 (eclipse), and he noted that travelers going south see southern constellations rise higher above the horizon.

Early Astronomy- Ancient GreeksEratosthenes (276-194 B.C.)• First to calculate tilt of Earth's axis• Calculated circumference of the Earth– Used angles of noonday sun in two

locations on the planet– Finding the difference in the angles

and knowing the distance between the two locations, the circumference can be calculated.

– He calculated circumference to be ~ 39,690 - 46,620 km. • The circumference of the Earth

around the poles is now measured to be ~40,008 km.

Early Astronomy- Ancient Greeks

• Hipparchus (190 – 120 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• From Earth,

other Planets appear to stop, reverse direction, then resume normal motion• Video

Modern Astronomy• Nicolaus Copernicus (1473-1543)– Combined information from other

astronomers to make a better argument for the sun being at the center of

the universe (Heliocentric theory)– Described the order of the planets as you

move away from the sun – Earth 3rd planet from sun

– Said that the stars were fixed in space (did not move) and were very far away compared to the distance to the sun.

Modern Astronomy• Tycho Brahe (1546-1601) –Used instruments to

measure the locations of “heavenly bodies”–His observations were

far more precise than any made before

Modern Astronomy• Johannes Kepler (1571-1630)• started his astronomy career as an assistant to

Brahe• 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. For the planets there is usually nothing currently at the other focus.

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 when they are closer to the sun.

Kepler’s 3 Laws of Planetary Motion

• Law 3– The closer an object is to the body it is orbiting, the

faster is has to be traveling so that it does not get pulled in by the bodies gravitational pull (he didn’t use the term gravity).

– Therefore, farther planets from the sun don’t have to travel as fast as closer planets in order to stay in orbit.

– Thus, we find that Mercury, the innermost planet, takes 88 days to orbit the Sun but Pluto takes 248 years to do the same.

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• 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)• 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

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