Astronomy 161 - Introduction To Solar System Astronomy - Autumn 2007

How old is the Earth? In this lecture I review the ideas of cyclic and linear time, and how this determines whether or not the question of the age of the Earth is meaningful. I then review various ways people have tried to estimate the age of the Earth, starting with historical ages that equate human history with the physical history of Earth. We then look at physical estimates of the Earth's age that do not make an appeal to human history, but instead seek physical processes that play out over time to make the estimates. This brings us to a discussion of radiometric age dating techniques that use the radioactive decay of isotopes trapped in minerals to identify the oldest Earth rocks and meteorites, and hence establish a radiometric date for the formation of the Earth some 4.55+/-0.05 Billion Years ago. Recorded 2007 Oct 29 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Telescopes outfitted with modern electronic cameras and spectrographs are astronomers' primary tools for exploring the Universe. In this lecture I review the primary types of telescopes and the best observatory sites to locate them, with a brief mention of radio and space telescopes. At the end, I give a brief review of the Ohio State's observing facilities. Recorded 2007 Oct 26 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Why does each element have its own unique spectral signature? how doe emission lines and absorption lines arise? This lecture is the second part of a two-part exploration of matter and light, looking at how the unique spectral-line signatures of atoms are a reflection of their internal electron energy-level structures. Topics include energy level diagrams for atoms, excitation, de-excitation, and ionization. There will be a short demonstration with gas-discharge tubes and slide-mounted diffraction gratings. For podcast listeners, the last portion of the class is the demo, for which we do not unfortunately have the resources to videotape. Recorded 2007 Oct 25 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

How do matter and light interact? This lecture is the first of two that will explore the interaction between light and ordinary matter, and how we measure that with spectroscopy. This lecture introduces the idea of internal energy as quantified by the temperature on the Absolute Kelvin scale, and Kirchoff's empirical Laws of Spectroscopy. We will deal primarily with blackbody spectra emitted by hot solids or hot dense gasses or liquids, the Stefan-Boltzmann and Wien Laws, and introduce emission and absorption line spectra. The next lecture will explain how line spectra arise from atoms and molecules. Recorded 2007 Oct 24 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What is ordinary matter made of? This lecture reviews the basic properties of matter from subatomic to atomic scales, introducing atomic structures, atomic number and chemical elements, isotopes, radioactivity, and half-life, ending with a brief overview of the four fundamental forces of nature: gravitation, electromagnetism, and the weak and strong nuclear forces. Recorded 2007 Oct 23 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What is light? Most astronomical objects are too far away to measure directly. Light is the messenger of the Universe, carrying with it information about objects as near as the Moon and as far away as the most distant objects in the visible Universe. In this lecture we will review the basic properties of light, the electromagnetic spectrum, the inverse square law of brightness, and the Dopper Effect. Recorded 2007 Oct 22 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

How do objects orbit if more than 2 massive bodies are involved? Newton's versions of Keplers 3 Laws of Planetary Motion are only strictly valid for 2 massive bodies. The Solar System, however, clearly has more than 2 massive objects within it. How do we handle this many-body problem? This lecture discusses some of the multi-body gravitational effects seen in our Solar System (and by extension elsewhere). We will describe Lagrange Points for the restricted 3-body problem and consequences like the Trojan Asteroids of Jupiter, long-range gravitational perturbations and their aid in discovering the planet Neptune, close encounters that can dramatically alter the orbits of comets and give us ways to slingshot spacecraft into the outer and inner Solar System without huge expenditures of fuel, and orbital resonances that can amplify small long-range perturbations and either stabilize or destabilize orbits. All of these effects play a role in the Dynamical Evolution of our Solar System that we will see througho

Why are there two high tides a day? This lecture examines tides caused by the 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 correlate with Lunar Phase. We will also discuss body tides raised on the Moon by the Earth, and how that has led to Tidal Locking of the Moon's rotation, which is why the Moon always keeps the same face towards the Earth. We end with a discussion of the combined effects of tidal braking of the Earth, which slows the Earth's rotation by about 23 milliseconds per day century, and causes the steady Recession of the Moon by 3.8cm away from Earth every year. Tidal effects are extremely important to understanding the dynamical evolution of the Solar System, as we'll see time and again in the second half of the class. Recorded 2007 Oc

Why do Kepler's Laws work? In this lecture I will describe Newton's generalization of Kepler's Laws of Planetary Motion so that they will apply to any two massive bodies orbiting around their common center of mass. I will introduce families of open and closed orbits, the circular and escape speeds, center-of-mass, conservation of angular momentum, and Newton's generalized version of Kepler's 3rd Law. The latter is a powerful tool for using orbital motions as our only way to measure the masses of astronomical objects. Recorded 2007 Oct 16 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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.