Astronomy 161 - Introduction to Solar System Astronomy

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 their consequences. We are now ready, next week, to examine the role of Gravity and finally explain the orbits of the planets. Recorded 2006 Oct 13 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 Copernican Heliocentric System, however, were to bring him into conflict with the Roman Catholic Church. Recorded 2006 Oct 12 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 geometric description of planetary motions that set the stage for their eventual physical explanation by Isaac Newton a generation later. Recorded 2006 Oct 11 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 the Copernican system, and sets the stage for the scientific revolution of the following generations. Recorded 2006 Oct 10 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 systems of Hipparchus and Ptolemy. We will also briefly discuss what is known of Aristarchus' mostly-lost heliocentric system, which was to so strongly influence the work of Copernicus. The ultimate expression of an epicyclic Geocentric system was that described by Claudius Ptolemy in the middle of the 2nd Century AD, and was to prevail virtually unchallenged for nearly 14 centuries. Recorded 2006 Oct 9 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 discuss retrograde motion, the apparent westward reversal of motion seen at opposition in the superior planets and inferior conjunction in inferior planets. The quest to describe the very complex motions of the planets marks the birth of science, and will be the central theme of next week's lectures. Recorded 2006 Oct 5 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 celestial bodies, and the division of the day into hours, minutes, and seconds. We will also discuss the difference between the Solar and Sidereal days, and the introduction of timezones used in modern civil timekeeping. Recorded 2006 Oct 3 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 different declinations, determines to total amount of solar heating per day, and drives the general weather. It has nothing, however, to do with how far away we are from the Sun at different times of the year. Finally, the direction of the Earth's rotation axis slowly drifts westward, taking 26,000 years to go around the sky. This "Precession of the Equinoxes" represents a tiny change that is still measureable by pre-telescopic observations, and means that at different epochs in human history there is a different north pole star, or none at all! Recorded 2006 Sep 28 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 Obliquity of the Ecliptic, and four special locations along the Ecliptic: the Solstices and Equinoxes. This will set the stage for much of our discussions in rest of this section. Recorded 2006 Sep 27 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 sky. An important wrinkle is that what part of the sky we see at any given time depends on both where we are on the Earth, and what date/time it is. This gives us the start of the coordinate system we need to begin our exploration of motions in the sky in the next lectures. Recorded 2006 Sep 26 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 of The Ohio State University.

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.