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.