2.1 History of Astronomy

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

Ptolemaic System

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.

The Birth of Modern Astronomy

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.

A hypothetical planet orbits the sun a distance of 3 AU. What is its orbital period? A. 2.8 Earth years B. 3.0 Earth years C. 5.2 Earth years D. 9.0 Earth years Answer: C