What is Matter? This lecture reviews the nature of matter from subatomic to atomic scales, and introduces the ideas of atomic structure, atomic number (number of protons), the elements, isotopes, radioactivity, and half-life. We conclude with a brief overview of the four fundamental forces of nature: gravitation, electromatgnetic, and the strong and weak nuclear forces. Recorded 2006 Oct 24 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What is Light? This lecture reviews the basic properties of light, introducing the inverse square law of brightness and the Doppler Effect. Recorded 2006 Oct 23 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

How do we prove physically that the Earth rotates on its axis and revolves around the Sun? Newtonian physics was so compelling that it was mostly accepted before there were ironclad physical demonstrations of the Earth's daily rotation about its axis and annual revolution (orbit) around the Sun. This lecture reviews three of these demonstrations: the Coriolis Effect, the Foucault Pendulum, and Stellar Parallaxes. This ties up the last loose-end of the Copernican Revolution. Recorded 2006...

Why are there two high tides a day? This lecture examines another of the consequences of gravity, the twice-daily tides raised on the Earth by the Moon. Tides are a consequence of differences in the gravity force of the Moon from one side to the other of the Earth (stronger on the side nearest the Moon, weaker on the side farthest from the Moon). The Sun raises tides on the Earth as well, about half as strong as Moon tides, giving rise to the effect of Spring and Neap tides that...

Why do Kepler's Laws work? This lecture discusses how Newton applied his Three Laws of Motion and the Law of Universal Gravitation to the problem of orbits. Newton generalized Kepler's laws to apply to any two massive bodies orbiting around their common center of mass. We discuss these new, generalized laws of orbital motion, introducing the families of open and closed orbits, circular and escape velocity, center-of-mass, conservation of angular momentum, and how orbital mechanics is used...

What is Gravity? This lecture reviews the law of falling bodies first described by Galileo, and then Newton's explanation in terms of his Law of Universal Gravitation. Gravity is a mutually attractive force that acts between any two massive bodies. Its strength is proportional to the product of the two masses, and inversely proportional to the square of the distance between their centers. We then compare the fall of an apple on the Earth to the orbit of the Moon, and show that the Moon...

The work of Copernicus, Kepler, and Galileo all contributed to a new way of looking at the motions in the heavens, but did not explain why they move that way. Enter Isaac Newton, who within a few years swept away the last vestiges of the Aristotelian view of the world and replaced with a new, powerfully predictive synthesis, in which all motions, in the heavens and on the Earth, obeyed three simple, mathematical laws of motion. This lecture introduces Newton's Three Laws of Motion and...

Tycho did as much as could be done with the naked eye, a new technology was required to extend our vision, the telescope. This lecture introduces Galileo Galilei, the contemporary of Kepler who was in many ways the first modern astronomer, and his discoveries with the telescope. These observations were to electify Europe in the early 17th century, and begin the final intellectual dismantling of the Aristotelian view of the world. Galileo's claims that they constituted proof of the...

In the generation following Copernicus, the question of planetary motions was picked up by two remarkable astronomers: Tycho Brahe, the brilliant Danish astronomer whose precise measurements of the planets represented the highest expression of pre-telescope astronomy, and Johannes Kepler, the brilliant and tormented German mathematician who used Tycho's data to derive his three laws of planetary motion. These laws were to sweep away the vast complex machinery of epicycles, and provide a...

In 1543, Nicolaus Copernicus re-introduced the Heliocentric idea of Aristarchus of Samos in an attempt to purge Ptolemy's geocentric system of the un-Aristotelian idea of the Equant. His goal was to derive a model that, in his words, pleased the mind as well as preserved appearances. What he started, without intending, was a profound revolution in thought that was to overturn both Ptolemy and Aristotle within two centuries, and help give birth the the modern world. This lecture looks at...

What are the origins of the Geocentric and Heliocentric models put foward to explain planetary motion? This lecture begins a new unit that will chart the rise of our modern view of the solar system by reviewing the highly influential work by Greek and Roman philosophers who elaborated the first geocentric and heliocentric models of the Solar System. We discuss the various geocentric systems from the simple crystaline spheres of Anaximander, Eudoxus, and Aristotle through the Epicyclic...

How do the planets move across the sky? This lecture will review planetary motions, specifically the apparent motions of the five classical planets (Mercury, Venus, Mars, Jupiter, and Saturn) as seen from the Earth. We will discuss the classical division of the naked-eye planets into inferior (Mercury and Venus) and superior (Mars, Jupiter, and Saturn) planets, and describe their main configurations in the sky: conjunction, opposition, maximum elongation, and quadrature. We will then...

Why are there leap years? This lecture explores the astronomical origins of the calendar. We will discuss lunar and solar calendars and their hybrids in history and tradition (for example, the Islamic Lunar Calendar and the Jewish Luni-Solar Calendar), and the Julian and Gregorian Calendar reforms. Recorded 2006 Oct 4 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

What time is it? This lecture is the first part of a two-part exploration of the astronomical origins of our time-keeping and calendar conventions. Today we will discuss the division of the year into seasons by the motions of the Sun, and the oft-forgotten origins of our holidays in in the solar Quarter and Cross-Quarter days, the division of the year into 12 months based approximately on the cycle of lunar phases, the traditional division of the month into weeks reflecting the seven moving...

Eclipses of the Sun and Moon are among the most glorious spectacles in the sky. This lectures looks at the causes and types of eclipses, and how often they occur. Recorded 2006 Oct 2 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

How does the Moon appear to move through the night sky? This lecture introduces the Moon, and describes the monthly cycle of phases. Topics include synchronous rotation, apogee and perigee, the cycle of phases, and the sidereal and synodic month. Recorded 2006 Sep 29 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Why do we have different seasons? This lecture looks at the consequences of the tilt of the Earth's rotation axis relative to its orbital plane (the Obliquity of the Ecliptic) combined with the apparent annual motions of the Sun around the Ecliptic. The important factor determining whether it is hot or cold at a given location at different times in the year is "insolation": how much sunlight is spread out on the ground. This, combined with the different length of the day when the Sun as at...

Why do celestial objects appear to rise in the East and set in the West? How does this depend on where you are on the Earth, or the time of year? Today we set the heavens into motion, and look at the two most basic types of celestial motions. Apparent daily motions are a reflection of the daily rotation of the Earth about its axis. The apparent annual motions are a reflection of the Earth's orbit around the Sun. To describe the Sun's apparent annual motion, we introduce the Ecliptic, the...

Where are we? Where is someplace else? How do I get from here to there? These are questions we need to answer both on the Earth and in the Sky to assign a location to a place or celestial object on the surface of a sphere. We start by introducing angular units, and use them to describe the terrestrial system of latitude and longitude on the spherical Earth. We then define the Celestial Sphere, with its Celestial Equator and Poles, and begin to define an analogous coordinate system on the...

What is the shape and size of the Earth? This lecture traces historical ideas about the shape of the Earth, from ancient flat-Earth models to the compelling demonstrations by Aristotle in the 3rd century BC that the Earth was a sphere. We then discuss ways people measured the size of the Earth, describing the results of Eratosthenes of Cyrene in the 2nd century BC and Claudius Ptolemy in the 2nd century AD, and their impact. Recorded 2006 Sep 25 in 100 Stillman Hall on the Columbus campus...

What are the constellations, and how have they be named and used by many different cultures throughout human history? We will review the most basic feature of the night sky, the 6000 visible stars sprinkled about the sky, and introduce the idea of constellations, reviewing their history and uses. We'll end with a brief discussion of where stars get their names. Recorded 2006 Sep 22 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Before we can begin our exploration of astronomy, we need to develop a common language for notating large numbers, and introduce the basic units of length, mass, and time that we will use throughout the quarter. We will first re-introduce the basic metric system, explaining how these units have a physical basis. For measuring lengths in astronomy, we need to introduce two special units: the Astronomical Unit, which is used to discuss interplanetary distances, and the Light Year, used...

Welcome to the Astronomy 161 Lecture Podcasts. This is a brief message from me explaining the podcasts, and welcoming new and old listeners. University. Lectures will begin on Wednesday, 2006 Sept 20, and run through Friday, 2006 December 1. New lectures will appear shortly before noon US Eastern time each day there is a regular class. Recorded 2006 Sept 18 on the Columbus campus of The Ohio State