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Astro 1010 - Exam 1 Study Guide

Chapter 1: A Modern View of the Universe

  • Our place in the universe:
    • Earth is one planet.
      • Formal definition of a planet is required knowledge.
      • Dwarf planets like Ceres or Pluto fail to meet the definition.
    • The Solar System:
      • Independent of any…
      • Star systems may have more than one star.
      • Star systems are usually organized into pairs of binary stars.
      • Planets are tiny compared to the size of the solar system.
    • Star cluster:
      • Two types: open and globular (more detail in Astronomy 1020).
      • Stars not in clusters (like the sun) used to be in clusters when younger.
    • The Milky Way galaxy:
      • Galaxy: A collection of star clusters and independent stars orbiting a common center (super-massive black hole).
      • The center of the galaxy is in the direction of the Sagittarius constellation.
    • Local Group galaxy cluster:
      • Major objects: Andromeda galaxy, Milky Way galaxy, Triangulum galaxy.
      • Several dozen other galaxies (smaller than the big 3).
    • Laniakea Supercluster:
      • Only one relatively small part of…
    • The entire universe:
      • The totality of all energy and matter that exists.
  • Units and Conversions
    • How to convert units, what units are useful in astronomy.
      • The astronomical unit (AU).
      • The light-year (ly).
      • The parsec (pc).
  • Look-back time:
    • The further away astronomers look, the further back in time we are seeing due to light-travel delay.

Chapter 2: Discovering the Universe for Yourself

  • The celestial sphere
    • Stars appear to be painted on the celestial sphere, but are at varying distances from us.
    • Celestial poles and celestial equator: Earth’s poles and equator projected outward onto the apparent sphere of the sky.
    • Ecliptic
      • Path of the sun across the celestial sphere over a year.
      • Represents the plane of Earth’s orbit around the sun.
      • Other planets are also always found along the ecliptic.
  • Constellations:
    • Imaginary patterns of stars along the celestial sphere.
  • The local sky
    • Horizon: Where the sky appears to meet the ground from your point of view.
    • Azimuth: Direction along the horizon (North/South/East/West) where a star is located.
    • Altitude: How high above the horizon (in angular units) a star is located.
  • Angular Size vs. Physical size equation
    • Focus on how to use it
  • Stars rise and set
    • Due to Earth’s rotation.
  • Circumpolar stars
    • Some stars can be circumpolar; that is, they never set from your perspective on Earth’s surface.
  • Seasons
    • The earth has seasons due to its axial tilt.
    • June solstice: Longest day of the year in the northern hemisphere (summer), shortest day of the year in the southern hemisphere (winter).
    • December solstice: Reverse of above.
    • Equinoxes: 12 hours of day and night everywhere on earth.
  • The phases of the moon
    • Phases caused by the moon’s changing position relative to the Earth-sun line as it orbits the Earth.
      • The sun always illuminates one side of the moon.
      • But we can only see the side of the moon that is facing Earth.
    • Waxing vs. waning
    • Order: New, Waxing Crescent, First Quarter, Waxing Gibbous, Full, Waning Gibbous, Third Quarter, Waning Crescent, back to New
    • Location of the moon in its orbit during each of the above phases.
    • What the moon looks like when seen from Earth in each of the above phases.
  • Synchronous rotation
    • The moon spins on its axis in exactly the same amount of time it takes to orbit the Earth.
  • Eclipses
    • Locations of Earth / sun / moon during a lunar or solar eclipse.
    • Why don’t we see an eclipse every month?
    • Phase of the moon during a lunar or solar eclipse.

Chapter 3: The Science of Astronomy

  • Practical benefits:
    • Keeping track of time: sundials, almanacs, calendars
    • Navigation (see chapter S1)
  • Ancient Middle Eastern / Islamic scholars:
    • Recorded the names we still use for most of the stars visible to the naked eye.
  • Ancient Greek scholars
    • Determined the approximate radius of the Earth
    • Created a mathematical description of a geocentric model of the solar system.
      • Earth in the center of the universe, sun and planets orbit it. (incorrect!)
      • Apparent retrograde motion was a problem for the geocentric model.
        • Epicycles / deferents were used in an attempt to explain this motion.
        • In reality, this motion is caused by the Earth “pulling ahead” of another planet as they orbit the sun.
      • Lack of observable stellar parallax used as proof of the geocentric model and a stationary Earth. In truth, stars were just really, really far away!
  • Renaissance scholars
    • Nicholas Copernicus: created the first mathematically rigorous description of a heliocentric model of the solar system.
      • Sun in the center of the solar system, all the planets orbit it. (correct!)
      • His model was crippled by the assumption of perfectly circular planetary orbits, making Copernicus’s model no more accurate at predicting planetary motion than the geocentric model of the ancient Greeks.
    • Tycho Brahe: precise naked-eye observations of the positions of the stars, planets, and moon. Provided crucial data used by…
    • Johannes Kepler
      • 3 Laws of planetary motion:
        • Planetary orbits are ellipses (not circles) with the sun at one focus
        • Planets sweep out equal areas in equal amounts of time
        • P^2 = a^3
    • Galileo Galilei
      • First to observe the night sky through a telescope. His observations more-or-less killed the geocentric model of the universe.
        • Shadows of mountains and valleys on the moon: heavenly objects are not perfect spheres.
        • Moons orbiting Jupiter: clearly objects in the sky can orbit something other than Earth!
        • Phases of Venus: should not be possible to see the phases in the geocentric model, yet expected in the heliocentric model.
      • Got in trouble with the Inquisition for all of the above. Their theology was based on a geocentric world-view.
  • Science vs. Pseudoscience
    • The scientific method.
    • Law: A simple statement (usually presentable as an if-then statement) about some aspect of nature.
    • Theory: A well-tested set of models and laws that describe some aspect of nature. Typically form the basis of entire branches of science.
    • Hallmarks of good science
      • Relies solely on natural causes
      • Progresses through the testing of models
        • Occam’s razor: the simplest model with the fewest assumptions is most often (but not always!) the correct one.
      • Models make testable predictions. If predictions don’t agree with observations, the model must be updated / edited or abandoned entirely.
    • Hallmarks of pseudoscience
      • Continues to make use of disproven hypotheses
      • Ignores evidence that disagrees with a hypothesis
      • Hypotheses that cannot be disproven through experiment or observation
      • Deliberate misuse of terminology and / or statistics.
      • Lack of peer review
      • Predictions are vague or exaggerated
      • Claims opponents to the hypothesis are part of a conspiracy
      • Lack of any progress (hypothesis never generates new predictions)
      • Preference for attacking dissenters rather than providing evidence.
    • Astronomy is a science. Astrology is a pseudoscience. Don’t confuse them!

Chapter S1: Celestial Timekeeping and Navigation

  • Sidereal day vs. solar day
  • Sidereal month vs. synodic month
  • Conjunction vs. opposition
  • Zenith: directly overhead
  • Meridian: arc from due south on the horizon, to zenith, to due north
  • Coordinates on the celestial sphere
    • Right Ascension (RA): east/west position of a star. Measured in hours, minutes, and seconds.
    • Declination (Dec): north/south position of a star. Measured in degrees, arcminutes, and arcseconds.
      • Celestial equator: 0° declination
      • North celestial pole: 90° declination
      • South celestial pole: -90° declination
    • Zero RA = sun’s location on the celestial sphere during the March (spring) equinox, which is one of two places the ecliptic crosses the celestial equator.
  • The sun at key locations on Earth:
    • Arctic / Antarctic Circles
      • Arctic circle: the sun is circumpolar during the June solstice. The further north you go, the more time the sun spends as a circumpolar star.
      • Antarctic circle: the sun is circumpolar during the December solstice. The further south you go, the more time the sun spends as a circumpolar star.
      • The sun is never circumpolar south of the Arctic circle / north of the Antarctic circle.
    • Tropics
      • Tropic of Cancer: sun passes through zenith on the June solstice.
      • Tropic of Capricorn: sun passes through zenith on the December solstice.
      • Equator: sun passes through zenith on both of the equinoxes.
      • The sun never passes through zenith if you are north of the Tropic of Cancer or south of the Tropic of Capricorn.
  • Finding your location on Earth
    • Northern hemisphere only: your latitude = the altitude of Polaris in the night sky.
    • Each hour of difference between time zones: 360° in one full rotation / 24 hours for Earth to rotate once = 15° difference in longitude for every 1 hour difference in time zones.