ASTRO 1102 1st Edition Lecture 2Outline of Last Lecture I. Goals for ASTRO 1102/1104 II. Why Take It?III.MethodsIV. Course PhilosophyV. Rules of the CourseVI. Schedule and Extra InformationOutline of Current Lecture I. The Main PointII. General Introduction to the SkyIII. Coordinates: Getting Your BearingsIV. Earth’s Spin and Orbital MotionV. Constellations and Astrologya. Astrology ≠ Astronomy!VI. Classical Astronomya. What Did The Ancients Know?b. What Did They Think They Knew?VII. Modern AstronomyCurrent LectureThe Main PointThe movements of the Sun, Moon, planets, and stars that we observe every day and night are apparent, not actual motions, because we are moving as well!General Introduction to the SkyWatching and knowing the sky is the key to understanding our place in the Cosmos. For most of human history, knowing the sky was critical for survival.The stars you see in the night sky vary depending on where you are on the Earth.- At the North Pole, the zenith corresponds exactly with the north Celestial Pole, so that the stars appear to move in a circle.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- At the equator, the stars seem to move around the horizon.Coordinates: Getting Your BearingsSome key terms:- Horizon: 360 degrees around, where the Earth’s surface intersects with the apparent position of the sky.- Zenith: The point directly overhead, which has an altitude of 90° (straight up).- Celestial Sphere: The imaginary sphere on which objects in the sky appear to reside when observed from Earth; it is a useful construct for understanding the geometry of Earth’s horizon, zenith, rotation axis, etc.- Celestial Pole: Where the projected rotation axis of Earth intersects with the celestial sphere.- Celestial Equator: The extension of Earth’s equator onto the celestial sphere.- Rotation Axis: The axis upon which the Earth spins.- Latitude: The angular north-south distance between Earth’s equator and a location on Earth’s surface. People living in different places on the Earth see different motions of the night sky.Earth’s Spin and Orbital Motion• Earth spins once per day on its axis• Earth orbits around the Sun once per year• Ecliptic: Earth’s orbital plane/path; also the apparent path of the sun in the sky.• Earth’s axis is tilted relative to its orbit or ecliptic.o The tilt angle (obliquity) is about 23.5 degrees.o The tilt causes the seasons.o The tilt angle changes the angle of sunlight striking the Earth's surface.o At a fixed location on the Earth, the angle of the sunlight varies with time.o Other planets have different tilts, and thus different types of seasons.Constellations and AstrologyConstellations are patterns of stars in the sky used by ancient peoples for navigation and folklore. They provide a "roadmap" to the sky, but no real scientific information (the stars happen to appear adjacent in the sky, but most are far apart in space).During one year, the Sun appears to travel through a group of constellations along the ecliptic: this zone is called the zodiac.The planets also move through the same zone since they are (essentially) in the same orbital plane as the Earth.Astrology ≠ Astronomy!• Astrology is an ancient system of beliefs that claimed that the positions of the Sun, Moon, and planets governed the actions and events in our lives.• Astrology’s ability to predict actions or personality traits has been repeatedly tested and disproved.• Yet astrology and horoscopes continue to pervade society and the media - why?• Therapeutic power of guidance and advice..?• Connection to a simpler, less technological time..?• Astrology is entertainment, not science.Classical AstronomyMany ancient civilizations had sophisticated concepts of astronomy and cosmology (calendars, navigation, crop & river cycles, comets, supernovae, Venus, etc.) Much of modern astronomy derived from the Greeks and Romans (~2000-2500 years ago).What Did The Ancients Know?• The Earth is round.o Pythagoras (560-480 B.C.): “perfect spheres"o Aristotle Stars rise higher or lower as one travels north or south The Earth’s shadow on the moon during lunar eclipses is round Masts of ships appear before hulls they approach the shore• Earth’s Diameter ~12,700kmo Eratosthenes (~200 B.C.)o Discovered with use of sticks, shadows, and pacing the distance from Alexandria to Syene on the 22nd of June, observing the changing position of the sun from one place to another.o Derived diameter = 12.500 km (1.5% error!)• Precision of the Equinoxeso Hipparchus’ data (~150 B.C.)o Earth’s axis precesses like a top.• Relative size of the Earth and Moon (~4:1)o Aristarchus (~310-230 B.C.)o Derived from the relative size of the shadow of Earth on the Moon during eclipses• Distance to the Moon (60x radius of the Earth)o From geometry: angle subtended by the moon and its size.• Distance to the Suno Aristarchus (~310-230 B.C.)o If you look at the Sun, Earth, and Moon system during a quarter moon, a right angle is formed.o Aristarchus used this, along with simple trigonometry, to conclude that the Sun ismuch further away from Earth than the Moon is, even though he didn’t get the measurement right.• Relative Size of the Sun of the Moono Aristarchus deduces that since the sun is much farther away than the moon and still large in the sky, it must be extremely large and thus the central body in the solar system.o This model was overlooked for many years because of an “argument from authority.”What Did They Think They Knew?• The Earth is the center of the Universeo simplest explanation for apparent motionso nostellar parallax observedo a comforting result• Sun, Moon, and Planets orbit Eartho Ptolemy (~140 A.D.)o Model comprised of many complex circular pathso The model could still make predictions.o Ptolemaic (geocentric) cosmology would be the dominant paradigm for 1,000+ years.Modern Astronomy• Nicholas Copernicus (1473-1543)o Proposed heliocentric cosmology. The Earth and planets circle the Sun. Only the Moon orbits Earth.o Very much at odds with the prevailing wisdom of the time, especially religion.o A simple model; it fit the data.• GalileoGalilei (1564-1642)o First to create a true astronomical telescope and use ito Discovered sunspots, the moons of Jupiter (1609), explored the features of