Astronomy 100


Lectures Table of Contents Astro 100

Section 1 Review

Following are what I consider the most important topics covered in this section.

Naked Eye Astronomy and Tools of Astronomy

  • The Scale of the Universe

    • Relative scales of planets, plant orbits, the Sun and other stars, galaxies, etc.

    • Powers of 10 and orders of magnitude

    • Scientific notation (e.g., 1 A.U. = 1.5 x 108 km)

    • Names of basic units and conversions (e.g., meter, nm, ly, K, degree, arcminute, arcseconds, etc.)

    • The speed of light

  • Apparent Motions of the Sun and Stars; Calendars and Seasons

    • Daily motion of the Sun and stars

      • Sun and stars move from east to west during day (Earth's rotation)

    • The celestial sphere -- a model of the apparent Universe

      • north and south celestial poles

      • celestial equator

      • zenith 

      • horizon 

      • altitude, latitude

      • precession (about 26,000 years per cycle)

    • Annual motions of the Sun and stars

      • Sun slips from west to east among stars about 1o per day or 30o per month (Earth's orbit around Sun); as a result, a constellation rises about two hours earlier after a month.

      • ecliptic (zodiac constellations) 

      • Sun appears to move along ecliptic -- north of celestial equator in our summer, south of celestial equator in winter.

  • Seasons on Earth

    • Tilt of Earth's axis (23.5o) causes Sun's motion north-south-north throughout year. (So if the Earth were tipped less than the angle, seasons on Earth would be less severe.)

    • Seasons are result of (1) angle of sunlight and (2) duration of daylight due to tilt

    • Seasons have nothing to do with distance from Sun

  • Phases of the Moon; Tides; Eclipses

    • Phases of Moon caused by our changing view of Moon, illuminated by Sunlight, as the Moon orbits Earth once/month.

    • Tides caused by difference in Moon's pull on front of Earth vs. on back of Earth --> stretches out the oceans into high and low tides.

      When do you expect a low  tide during the Full Moon?

    • Solar eclipse: Moon blocks Sun (seen from Earth) -- Moon casts shadow on Earth.

    • Lunar eclipse: Earth blocks Sun (seen from Moon) -- Earth casts shadow on Moon (When do we expect to a partial lunar eclipse?)

  • Planetary Motions

    • Planets move mostly in ecliptic

    • Inferior planets (Venus, Mercury) stay close to Sun

    • Superior planets (esp. Mars) sometimes move in retrograde loops


  • Historical Astronomy/Gravity and Orbits

    • Greeks founded basis of Western science

      • Aristotle -- geocentric Universe

      • Ptolemy -- added epicycles to explain retrograde loops, etc.

    • Copernicus -- heliocentric Universe

    • Tycho Brahe -- made accurate observations (e.g., a supernova and planet positions)

    • Kepler -- used Tycho's data, supported Copernican view, plus:

      • All planets move in ellipses, the Sun at one focus

      • planets move faster closer to Sun --> sweep out equal areas in equal times

      • P2yr = A3AU (The equation applies to objects in the solar system, including comets.)

    • Galileo -- used telescope to see moons around Jupiter, sunspots, stars in the Milky Way, phases of Venus, craters on Moon, "ears" or companion stars on Saturn; supported Keplerian view

    • Newton -- studied all of the above,  he found the three laws:

      • inertia (constant and straight motion unless a force is inserted)

      • F=ma (a --- acceleration that is the rate of change in speed, direction, or both)

      • reaction (equal and opposite forces)

      Plus Universal Law of Gravity: F=Gm1m2/r2

      Newton realized how gravity works to cause Moon to "fall" towards Earth without getting closer.

  • Light: The Electromagnetic Spectrum 

    Electromagnetic radiation

    • is a form of energy

    • includes radio, microwave, infrared, optical, ultraviolet, X-ray, and gamma ray radiation

    • can be described as waves with length , frequency , and speed c

    • can also be described by photons with energy E=h

    • has colors determined by and

    • can be dispersed into its component colors by a prism or a grating

    The intensity of the radiation

    • obeys inverse square law: I 1/r2

    • has a relation with the apparent magnitude m of an object as I 10-2/5m

  • Temperature and Black Body Radiation

    • Temperature  is a measure of the average speed of the particles in a gas 

      • units: Kelvin; 

      • absolute zero

    • thick gases warmer than absolute zero glow = black body radiation, which is continuous emission from thermal motion of electrons

    • Wien's law tells wavelength of peak of black body spectrum (hotter --> bluer; colder --> redder): peak = 2.9/T, with in mm and T in degrees Kelvin

    • Stefan-Boltzman law (hotter --> brighter, colder --> fainter): the integrated radiation energy (heat) E T4

  • Atoms and Spectral Lines

    • atoms consist of nuclei (protons and neutrons) and electrons

    • electrons orbit nuclei in discrete (not random) energy levels

    • if electron absorbs photon with right energy, can jump to higher energy level --> absorption lines in spectrum

    • electron may later emit same color photon and jump back down to original level, or emit different color photons matching different transitions --> emission lines in spectrum

    • Transition between energy levels of an atom may also caused by a collision with another atom or electron (e.g., collisional excitation or ionization)

    • each element (H, He, C, N, O....) has different energy levels --> unique fingerprint of spectral lines

  • The Doppler Effect

    • The amount of shift in wavelength due to the relative motion of a light source to an observer is given by:

      / = Vr/c
      where is the change in induced by the motion, i.e., o -
    • Blue shift and red shift


Lectures Table of Contents Astro 100

Houjun Mo Astronomy 100