Astronomy 161 - Introduction To Solar System Astronomy

How old is the Earth? This lecture reviews the idea of cyclic and linear time, since how you view time determines whether the question of the age of the Earth is even meaningful. We then review various ways people have estimated the age of the Earth, starting with historical ages that equate human history with the history of the Earth proper, and then see how various physical estimates, which do not make an appeal to human history, were made. This brings us to the technique of radioactive age dating of the oldest rocks, leading to our current best estimate of 4.5+/-0.1 Billion years for the age of our planet. Recorded 2006 Oct 30 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Telescopes, equipped with advanced electronic cameras and spectrographs, are the primary tools of the astronomer. This lecture reviews the types of telescopes and observatory sites, and discusses radio and space telescopes, and reviews briefy the observing facilities at Ohio State. Recorded 2006 Oct 27 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

Why does each chemical element have its own unique spectral-line signature? How do emission- and absorption-line spectra work? This lecture is the second part of a two-part exploration of the interaction between matter and light, today discussing how the unique spectral-line signatures of atoms are a reflection of their internal electron energy-level structure. We will discuss energy level diagrams for atoms, excitation, de-excitation, and ionization, and do a short demonstration with gas-discharge tubes and slide-mounted diffraction gratings. For podcast listeners, the last portion of the class is the demo, which we do not, unfortunately, have the resources to videotape. Recorded 2006 Oct 26 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

How do matter and light interact? This lecture is the first of a two-part lecture on the physical basis of spectroscopy. Today we will discuss the Kelvin Absolute Temperature scale, which provides a measure of the internal energy content of matter, and Kirchoff's empirical Laws of Spectroscopy, along with the Stefan-Boltzmann Law and the Wein Law to describe the continuous emission from a blackbody. We will end by briefly describing the suggestive properties of emission- and absorption-line spectra, whose explanation in the details of atomic structure will be the topic of the next lecture. Recorded 2006 Oct 25 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 Oct 19 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 strongly correlate with Lunar Phase. We also look at 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 then explore the combined effects of tidal braking of the Earth, which slows the Earth's rotation and increases the length of the day by about 23 milliseconds per century, and causes the steady Recession of the Moon, which moves 3.8cm away from Earth every year. Tidal effects are extremely important to un

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 to measure the masses of astronomical objects. Recorded 2006 Oct 17 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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 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 2006 Oct 16 in 100 Stillman Hall on the Columbus campus of The Ohio State University.

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.