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 ( B.C.) Aristotle ( 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 Greeks Eratosthenes ( B.C.) First to calculate tilt of Earth's axis First to calculate tilt of Earth's axis Calculated circumference of the Earth 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, ,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 ( 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 ( ) – 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 3 rd 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 ( ) – Used instruments to measure the locations of “heavenly bodies” – His observations were far more precise than any made before

Modern Astronomy Johannes Kepler ( ) 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 ( ) Created the first telescope in 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 ( ) 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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