Astronomy 161 - Introduction to Solar System Astronomy - Autumn 2007

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, 2007 Sept 19, and run through Friday, 2007 Nov 30. New lectures will appear shortly before 6pm US Eastern Time each day there is a regular class. Recorded 2007 Sep 19 in 4037 McPherson Lab on the Columbus campus of The Ohio State University.

What is Astronomy? What is Science? What is the course all about? Brief introductory remarks after going over course mechanics on the first day of Astronomy 161 for Autumn Quarter 2007. Recorded 2007 Sep 19 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What are our units of measure in astronomy? To begin our exploration of astronomy, we first 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. This lecture is a quick review of scientific notation and the metric system. For measuring the vast distances in astronomy, we need to introduce two special units: the Astronomical Unit for interplanetary distances, and the Light Year for interstellar distances. We end with a discussion of mass and weight, and the distinction drawn in physical measurements that differs (a little) from everyday usage. Recorded 2007 Sep 20 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What are the constellations? 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 by various cultures. Recorded 2007 Sep 21 in 1000 McPherson Lab 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 ideas of a Flat-Earth to Aristotle's compelling demonstrations in the 3rd century BC that the Earth was a sphere. We then discuss two famous classical measurements of the circumference of the Earth by Eratosthenes of Cyrene in the 3rd century BC and Claudius Ptolemy in the 2nd century AD. Recorded 2007 Sep 24 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Where are we? Where is someplace else? And how do I get there from here? 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. This lecture includes a review of angular units and 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 elements of the coordinate system we will need to begin our exploration of motions in the sky in the next lectures. Recorded 2007 Sep 25 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Why do celestial objects appear to rise and set every day? How does this depend on where you are on the Earth, or the time of year? In today's lecture we we set the heavens into motion and review the two most basic celestial motions. Apparent Daily Motion reflects the daily rotation of the Earth about its axis. Apparent Annual Motion reflects the Earth's annual orbit around the Sun. We introduce the Ecliptic, the Sun's apparent annual path across the Celestial Sphere, and note four special locations along the Ecliptic: the Solstices and Equinoxes. This sets the stage for many of the topics of the rest of this section. Recorded 2007 Sep 26 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Why do we have different seasons? This lecture explores the consequences of the tilt of the Earth's rotation axis relative to its orbital plane combined with the apparent annual motions of the Sun around the Ecliptic. The most important factor for 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 over the ground. This, combined with the different length of the day throughout the year, determines to total solar heating per day and so drives the general weather. It has nothing 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 2007 Sep 27 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What are the Phases of the Moon? 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 2007 Sep 28 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Among the most amazing sights in the sky, eclipses of the Sun and Moon have long fascinated us. This lecture describes the eclipses of the Sun and Moon, their types, and how often they occur. Recorded 2007 Oct 1 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What time is it? Telling time is the oldest practical application of astronomy. Today's lecture is the first of a 2-part lecture on the astronomical origins of our methods of keeping time and making calendars. This lecture reviews the divisions of the year into the solstices, equinoxes, and cross-quarter days, the division of the year into months by moon phase cycles, months into weeks, and the division of the day into hours by marking the location of the Sun in the sky Recorded 2007 Oct 2 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

How do we make calendars? This lecture explores the astronomical origins of our calendars. We start by discussing lunar and solar calendars and their hybrids in history and tradition (for example, the Islamic Lunar Calendar and the Hebrew Luni-Solar Calendar), and then describe the Julian and Gregorian Calendar reforms that attempt to align the calendar with the seasons of the year with greater degrees of precision. Recorded 2007 Oct 3 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

How do the planets move across the sky? This lecture discusses the motions of the 5 naked-eye planets (Mercury, Venus, Mars, Jupiter, and Saturn) as seen from the Earth. We introduce the major configurations of the planets, and then discuss their apparent retrograde motions. The apparent motions of the planets are far more complex than those of the Sun, Moon, and stars, and present a great challenge to understand. The centuries long effort to understand these motions was to give birth to modern science. Recorded 2007 Oct 4 in 1000 McPherson Lab 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 2007 Oct 8 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

In 1543, Nicolaus Copernicus revived Aristarchus' Heliocentric System in an attempt to rid Ptolemy's geocentric system of the un-Aristotelian idea of the Equant. He desired to create a model of the planets that would please the mind as well as preserving appearances. Rather than reinstate the ideal of the Aristotelian World View, he was to set the stage for its overthrow after nearly 2000 years of supremacy, and within two centuries give birth to the modern world. This lecture describes the astronomical world from the end of the classical age until the birth of Copernicus, and then describes his revolutionary idea of putting the Sun, and not the Earth, at the center of the Universe. Recorded 2007 Oct 9 in 1000 McPherson Lab on the Columbus campus of The Ohio State University. NOTE: Due to a recorder malfunction, only the first 15 minutes of this lecture was recorded.

Because my voice recorder malfunctioned 15 minutes into my Lecture on Copernicus on 2007 October 9, I've added this recording of my Copernicus lecture from Autumn Quarter 2006. It is the same basic material, but since I generally improvise on a basic outline, there will be some differences. Personally, I liked this year's lecture better, but this will at least cover most of the same material. Oh well.

In the generation following Copernicus, the question of planetary motions was picked up by two remarkable astronomers: Tycho Brahe and Johannes Kepler. Tycho was a Danish nobleman and brilliant astronomer and instrument builder whose high precision naked-eye measurements of the stars and planets were to be the summit of pre-telescopic astronomy. Kepler was the talented German mathematician who was hired by Tycho and succeeded him after his death who was to use Tycho's data to derive his three laws of planetary motion. These laws swept away the vast complex machinery of epicycles, and provide a geometric description of planetary motions that was to set the stage for their eventual physical explanation by Isaac Newton a generation later. Recorded 2007 Oct 10 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Tycho reached the limits of what 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 describes his many discoveries with the telescope. These observations electrified Europe in the early 17th century, and set the stage for the final 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 2007 Oct 11 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Copernicus, Kepler, Tycho, and Galileo together gave us a new way of looking at the motions in the heavens, but they could not explain why the planets move they way the do. It was to be the work of Isaac Newton who was to sweep away the last vestiges of the Aristotelian view of the world and replace it with with a new, vastly more powerful 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. Recorded 2007 Oct 12 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What is Gravity? Starting with the properties of falling bodies first formulated by Galileo, Newton applied his three laws of motion to the problem of Universal Gravitation. Newtonian Gravity is a mutually attractive force that acts at a distance 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 is held in its orbit by the same gravity that works on the surface of the Earth. In effect, the Moon is perpetually "falling" around the Earth. Recorded 2007 Oct 15 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.