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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?
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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
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
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!
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
Kepler’s Laws cont • Distant planets orbit the Sun at slower average speed, obeying a precise mathematical relationship • p2ά 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.
