Transcript Lecture 2
Lecture 2 Astro 1001 6/1/07 Group Work • When people say “day”, “month”, or “year” which kind of day, month, and year do they mean? • You have a sundial that indicates that the solar time is 3pm. The longitude of Minneapolis is about -90 degrees. What time is it (solar time) in Greenwich, England (hint: 360/24 = 15) Stars and Constellations • About 2000 stars are visible to the naked eye at night – Probably ~20 or so in the city • The constellations that astronomers used were determined by the IAU – Based on European myths – Asterisms are parts of constellations The Celestial Sphere • No depth perception in space, so it appears that all of the stars (and planets) lie on a sphere centered around Earth – Celestial Poles – Celestial Equator – Ecliptic The Local Sky • This is the sky that you personally observe – Horizon – Zenith – Meridian • We use these features to define various coordinate systems Celestial Coordinates • Several different ways – Altitude and direction (azimuth) – Right Ascension and Declination • Declination is how high something is in the sky – Can be positive or negative • RA is how far something is from Meridian – Measured in units of time RA and Dec Angular Sizes • Angular size is the angle that appears to separate two objects – Depends on the distance to the objects • We use degrees, arcminutes, and arcseconds – Can write angles as 35°27’35” More Backyard Astronomy • Constellations you see vary with latitude • Rise and set times of constellations vary with longitude • Circumpolar stars are those that never rise or set but simply make circles • Constellations along the ecliptic are called constellations of the zodiac Seasons • Seasons are related to the tilt of the Earth • Light is more concentrated when Earth is tilted towards the Sun • Important parts of the year – Solstices – Equinoxes Precession • The Earth is not a perfect sphere • Things that are not perfect spheres wobble as they spin • Earth wobbles on 26,000 year time scales • Tropic of Cancer is where the Sun is directly overhead on the summer solstice Precession cont Solar Eclipses • Moon is directly in between Earth and Sun – Moon must be a new moon • Umbra is where the Sun is completely blocked – Usually about 300 km in diameter • Penumbra is where the sun is partially blocked – Usually around 7000 km in diameter Lunar Eclipses • Must occur when the moon is full • Lasts longer, more common than solar eclipses • Moon often becomes red Planets • Planets wander through the zodiacal constellations (unlike stars) • Planets exhibit retrograde motion – Not easily explained with a celestial sphere Group Work • Page 50 of your text suggests an experiment you can do with a friend that demonstrates retrograde motion. Perform this demonstration. Why Nobody Figured This Out • Greeks (correctly) thought that a heliocentric model should cause stellar parallax Heliocentricity cont • Greeks (correctly) determined that either: – Earth orbits the Sun but the stars are so far away that you can’t detect stellar parallax – There is no parallax because Earth is stationary • Stars are really far away – 1 parsec = a parallax arcsecond Ancient Astronomy • Many cultures developed astronomy • One important thing was to identify equinoxes/solstices • Some cultures paid particular attention to the moon – Metonic cycle allows calendars to be somewhat synchronized The Greeks • We pay particular attention to the Greeks because they were the first (that we know of) to rely on natural models • Greeks usually relied on geocentric models Famous Greek Philosophers • Thales – Successfully predicted solar eclipse – Created first scientific model • Anaximander – Invented celestial sphere – Knew that the Earth had to be curved Philosophers cont. • Pythagoras – Argued that Earth is a sphere – Largely mystical/aesthetic basis • Plato/Eudoxus – Heavenly objects move in perfect spheres – Nested spheres account for different motions Philosophers cont. • Aristotle – Earth was at the center of everything • Ptolemy – Synthesized previous ideas into a single model – Could account for retrograde motion very well Copernicus • Proposed heliocentric model • Primarily due to aesthetic reasons – His model was less accurate than Ptolemy’s Tycho Brahe • Made very high quality naked eye observations – Telescope not invented until after his death • Observed the heavens changing • He hired Kepler • Never very successful in creating a model Johannes Kepler • At first was still stuck on circles • Could handle east-west predictions with circles – Couldn’t handle north-west variations • Eventually realized the orbits could be ellipses – Lucky! Ellipses Kepler’s Three Laws • Planets orbit the Sun in ellipses – Sun is at one focus – Perihelion is the closest point, aphelion is the most distant point • Planets sweep out equal areas in equal times – Planets move a great distance at perihelion, less quickly at aphelion The nd 2 Law Kepler’s Laws cont • Distant planets orbit the Sun at slower average speed, obeying a precise mathematical relationship – p2 ά a3 – p2 = k * a3 • Kepler was incorrect about why planets obeyed his laws Galileo • Increased the important of experiments – Rolling balls demonstrated that Aristotle was wrong • Observed things that countered astronomical beliefs at the time – Moon is imperfect – Moons orbiting Jupiter Scientific Method • Relies on hypothesis and predictions • Hypothesis and theories are NOT the same in science • Observations often refute hypothesis Hallmarks of Science • Science only addressed questions that can be disproven (in principle) • Modern science relies only on natural explanations • Science progresses through the creation and testing of models – Simple models are preferred over complex ones Verifiable Observations • Scientific data must be repeatable • Eyewitness accounts notoriously unreliable • Objectivity is important – Individual scientists try to be objective – Science as a whole is more objective than individuals Astrology • Astrology seeks to explain human events by the position of the Sun, planets, Earth • Astrology and astronomy used to be closely related • Astrological thinking is really reliant on Earth centered reasoning The Basis of Astrology • Constellations are special – No, they aren’t • Positions of planets in constellations are important – Entirely based on appearances • Astrologers insist that all planets are important – Why didn’t ancient astrologers predict other planets – Why is Pluto important if “Xena” isn’t? Is Astrology Scientific? • Nowadays astrologers often don’t make testable predictions • Testable predictions turn out to be wrong as often as chance dictates – Hundreds of scientific tests have never shown horoscopes to be more accurate than chance would allow • End result: astrology is useless for predicting past, present, or future events Group Work • UFOlogy is the “study” of UFOs. Almost all people who practice UFOlogy believe that UFOs are explained in part by alien spacecraft visiting Earth. Is the basis of such a field scientific? Why or why not? For some information, see page 80 of your textbook.