What is Astronomy? The branch of science that deals with celestial objects, space, and the physical universe as a whole. Considered the 1 st science Earliest written records – Babylonians (~1600 B.C.) Also evidence from early Chinese, Central American, and North European cultures such as Stonehenge
Early Astronomy – Ancient Greece The Greeks took measurements of distant objects such as the Sun and the moon. Famous Greek philosopher, Aristotle, concluded Earth is round because it always cast a curved shadow on the moon. Another Greek astronomer, Hipparchus, determined the location of almost 850 stars.
The Geocentric Model The Greeks believed in a geocentric universe, in which the known planets and the Sun revolved around the Earth. The path of an object as it goes around another object in space is called an orbit. Beyond the planets was an invisible sphere on which the stars traveled daily around Earth celestial sphere The Greeks attempted to explain the movements of all celestial bodies in space by using this geocentric model.
Ptolemy Presented a geocentric model of the universe called the Ptolemaic System, with fixed stars in the background. Although Ptolemy’s theory was wrong in that the planets do not orbit Earth, it was able to account for the planets’ apparent motions, which he called epicycles We concluded the planets’ individual rotation periods from this
Heliocentric Model Planets revolve around the Sun not the Earth Aristarchus (270 B.C.) first developed the theory Though much evidence was provided to support a heliocentric universe, the Earth- centered (geocentric) view dominated Western thought for 2000 years.
Nicolaus Copernicus (1473 – 1543) After Ptolemy, very few advances were made in astronomy Copernicus reinvented the heliocentric theory, which challenged the Church By placing the Sun at the center of the Solar System, he forced a change in worldview His model still fails modern knowledge when dealing with orbits
Tycho Brahe (1546 – 1601) Astronomy’s 1 st true observer Measured positions of planets and stars to best accuracy at the time Showed Sun is much farther from Earth than the Moon His measurements were used to support the geocentric theory So, even though his observations were very good at the time, his result was still wrong
Johannes Kepler (1571 – 1630) Brahe’s assistant who carried on his work Did not agree with Brahe’s view of a geocentric universe Formulated 3 Laws of Planetary Motion System of determining how all of the planets move around the Sun Still used today Caused a major shift in scientific thought
Kepler’s 3 Laws of Planetary Motion 1 st Law – Law of Ellipses The orbit of a planet around the Sun is an ellipse (oval-shaped) with the Sun at one focus
Kepler’s Laws of Planetary Motion 2 nd Law – Law of Equal Areas A line joining a planet and the Sun sweeps out equal areas in equal intervals of time A planet travels faster when it is closer to the Sun and slower when it is farther from the Sun
Kepler’s Laws of Planetary Motion 3 rd Law – The Law of Harmonies The square of a planet’s orbital period (the time it takes to orbit the Sun) is proportional to the cube of its semi-major axis (distance to the Sun) Orbital period expressed in Earth years Semi-major axis expressed in astronomical units (AU) Which is the distance between the Earth and Sun 1 AU = 150 million km or 93 million miles
Galileo Galilei (1564 – 1642) Italian scientist during the Renaissance Contributions: Descriptions of the behavior of moving objects Major improvements to the telescope He constructed his own telescope and used it to study the sky, making many important discoveries supporting Copernicus’s view of the universe.