Journey Through Early Astronomy: Stars, Planets, and Pioneers

Astronomy-The Original Science Imagine that it is 5,000 years ago. Clocks and modern calendars have not been invented. How would you tell time or know what day it is? One way to tell the time is to study the movement of stars, planets and the moon. Studying the ancient skies was so important that ancient people built observatories. Over time, the study of the night sky became the science of Astronomy. Today Astronomy is known as the study of the universe.

Early Greek Astronomers • Eratosthenes calculated circumference of the earth within 5% accuracy • Aristarchus proposed a heliocentric cosmos and easily explained the motion of the Sun through the heavens • His writings were lost at Alexandria along with Hypatia

Who’s Who of Early Astronomy • The careful work of early astronomers helped people understand their place in the universe. Almost everything early astronomers knew about the universe came from what they could discover with their eyes and minds. • Not surprisingly, most early astronomers thought that the universe consisted of the sun, the moon and the planets. They thought that the stars were at the edge of the universe.

Aristotle • Believed the planets and stars were on a set of 55 crystalline spears that rotated around by a prime mover. • This was geocentric or Earth centered theory.

Aristotle • 384-322 B.C. • Member of Plato’s Academy • Philosopher and Scientist • One of the 1st Physical Models of the Cosmos • Geocentric

Ptolemy: An Earth-Centered Universe • A Greek Astronomer-around 100 AD • Ptolemaic Theory-he wrote a book that combined all of the ancient knowledge of astronomy that he could find. He then expanded on it with careful mathematical calculations. • Ptolemy thought that the Earth was at the center of the universe and that the other planets and the sun revolved around the Earth. • Although his theory was incorrect, it predicted planetary motion better than any other theory at the time. • His theory was the most popular for the next 1,500 years. (Geocentric Theory)

Ptolemy • Geocentric • Attempted to explain retrograde motion • Used Hipparchus idea of epicycles and deferents • Added the equant • Very complicated • Inconsistent rules

Ptolemy’s Solution Planet Epicycle Earth Equant Deferent

Nicholas Copernicus: A Sun-Centered Universe • A Polish astronomer (1543) • Revolutionized astronomy with his new theory • Heliocentric theory-the sun is at the center of the universe, and all of the planets, including the Earth, orbit the sun. • The theory correctly explained the movement of the planets around the sun but it did not replace Ptolemy’s theory immediately. • When Copernicus’s theory was accepted, major changes in science and society were taking place.

Copernican Revolution • 1473-1543 A.D. • Revived early heliocentric models • Retrograde motion simply explained • Circular Orbits • No public support

Tycho Brahe: A Wealth of Data • Danish astronomer, late 1500’s • Used several tools to make the most detailed astronomical observations that had been recorded to date. • Brahe favored a modified version of Ptolemy’s theory; the sun and the moon revolved around the earth and that other planets revolve around the sun. • While his theory was not correct, Brahe recorded very precise observations of the planets and stars that helped future astronomers.

Hans Lippershey • 1608 • Invented the first refracting telescope • Was intended as a device of war to spy on other ships at sea.

Johannes Kepler: Laws of Planetary Motion • Was Brahe’s assistant-continued the work after Brahe’s death • 1609-after much analysis of the Brahe’s data, Kepler concluded that all of the planets revolve around the sun in elliptical orbits and that the sun in not the exact center of the orbits. • Stated his ideas in three laws of planetary motion: • 1-the sun is the center of universe and the planets revolve around it in elliptical orbits. • 2-the planets move faster when their orbits bring them closer to the sun. • 3-a mathematical formula used to determine the distance of a planet from the sun. • These laws are still used today.

Kepler’s 1st Law • Elliptical Orbits not Circular!

Ellipse Geometry Major Axis Foci Minor Axis Eccentricity=How oval is it?

Kepler’s 2nd Law • Equal Areas in Equal Times • Describes Single Planet is orbit around the Sun • Perihelion-closest approach to Sun • Aphelion-farthest distance from Sun

Kepler’s 3rd Law • The more distant a planet’s orbit is from the Sun, the slower it moves • Compares planets to one another • Period- time for one complete orbit around sun • The Square of the orbital period is equal to the Cube of the average distance from the sun. P2yrs = A3 AU

Kepler’s Triumph • Published first two laws in 1609 • Calculations only for Mars • Decade later he checks all known planets • They work! • Also introduces 3rd law at this time

Summary • Models are constantly being evaluated • Theory is never “true” just not “false” • Example geocentric cosmos • Kepler’s 3 laws accurately and simply describe the motion of the planets • Laid the foundation for work of future scientists

Galileo: Turning a Telescope to the Sky • In 1609, Galileo Galilei became one of the first people to use a telescope to observe objects in space. • He discovered craters and mountains on the Earth’s moon, four of Jupiter's moons, sunspots on the sun, and the phases of Venus. • These discoveries showed that the planets are not “wandering stars” but are physical bodies like the Earth and it gave him proof that the planets did indeed revolve around the sun, as Copernicus had stated.

Isaac Newton: The Laws of Gravity • In 1668 Invented the reflecting telescope • In 1687, Isaac Newton showed that all objects in the universe attract each other through gravitational force. • The force of gravity depends on the mass of the objects and the distance between them. • Newton’s law of gravity explained why all of the planets orbit the most massive object in the solar system---the sun. • Newton once said that “I could see so far because I stood on the shoulders of giants.” He gave credit the observations and ideas of all the scientists who came before him.

William Herschel • 1781 • Approximated the size and shape of the MilkyWay Galaxy

Modern Astronomy • The invention of the telescope and the description of gravity were two milestones in the development of modern astronomy. • In the 200 years following Newton’s discoveries, scientists made many discoveries about our solar system. But they did not learn that our galaxy has cosmic neighbors until the 1920’s.

Edwin Hubble: Beyond the Edge of the Milky Way • In 1924, Edwin Hubble proved that other galaxies existed beyond the edge of the Milky Way. • His data confirmed the beliefs of some astronomers that the universe is much larger than our galaxy. • Today, larger and better telescopes on the Earth and in space, new models of the universe, and spacecraft help astronomers study space. • Computers help process data and control the movement of telescopes. • These tools have helped answer many questions about the universe, yet new technology has presented questions that were unthinkable even 10 years ago.

Grote Ruber • 1937 • Invents first radio telescope

Ptolemy: Geocentric Earth-Centered Universe

Copernicus: Heliocentric Sun-Centered Universe

Kepler: Heliocentric with Elliptical orbits

Theories of the Universe

Galileo: Telescope

Journey Through Early Astronomy: Stars, Planets, and Pioneers