THE NIGHT SKY

We began by finishing our discussion of types of science:

What about: anthropology, history, political science, psychology, sociology?

  • These social or "soft" sciences rely to one extent or another on
    scientific methods, but also invaraibly carry a great deal of preconceptions that allow for
    many disparate interpretations to be drawn from the same data.
  • While in the natural or "hard" sciences, the range of interpretations is
    usually much less.

    Aside from this OBSERVATIONAL ---EXPERIMENTAL dichotomy, since the
    advent of calculus many sciences have distinguished these
    approaches from THEORETICAL science, driven by applied mathematics.

    ASTRONOMY IS AN OBSERVATIONAL SCIENCE.

    ASTROPHYSICS IS AN EXPERIMENTAL/THEORETICAL/OBSERVATIONAL SCIENCE.

    Today we typically use these terms interchangeably since so much of
    what we learn combines observations with theory and some experimental work.

    Modern science has a third, (nearly) equal aspect: COMPUTATIONAL.

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    We then discussed SCIENTIFIC NOTATION--see APPENDIX 1
    and for the great majority of what was covered in class on this topic.

    When we multiply numbers using SCIENTIFIC NOTATION we multiply
    the numbers in front of the powers of ten (the mantissas)
    but we ADD the powers of ten.
    (2 x 10^4) x (3 x 10^6) = (2x3) x 10^(4+6) = 6 x 10^10
    or 20,000 x 3,000,000 = 60,000,000,000
    (IN THESE WEB NOTES, "^" means "to the power of",
    so 10^4 = 10,000, 10^-4 = 1/10^4 = 1/10,000 = 0.0001)

  • For division we divide the mantissas but SUBTRACT the powers of ten.
    3,800/0.019 = 3.8 x 10^3/1.9 x 10^-2 =
    = (3.8/1.9) x 10^(3-(-2)) = 2.0 x 10^(3+2) = 2.0 x 10^5 = 200,000

    We also discussed the idea of SIGNIFICANT FIGURES:
    do not quote answers to more precision than the least
    precise measurement or number that enters them.

  • Example: (3.0 x 10^1) x (1.53 x 10^2) = 4.6 x 10^3
    (not 4.59) since the first number has only two significant figures.

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    We then discussed the relative sizes of lots of things in the universe.
    The key points are:
    the universe is incredibly large, and even
    the solar system is huge compared to puny people.
    On the other hand, we're much bigger than the wavelength of visible light,
    which is in turn much bigger than atoms,
    while atoms are much bigger than their nuclei!

    See APPENDIX 2 for a useful summary, and
    APPENDIX 3 for some important numbers.

    Right now, you should memorize:

  • Radius of the Sun = 6.96 x 10^10 cm = 7 x 10^5 km
  • Radius of the Earth = 6.38 x 10^8 cm = 6.4 x 10^3 km
  • 1 Astronomical Unit = 1.496 x 10^13 cm = 1.5 x 10^8 km
    (the mean distance between the Earth and the Sun)
  • Number of seconds in a year = 3.15 x 10^7

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    THE NIGHT SKY

    DAILY variations are due to Earth's ROTATION on its axis (from West to East).
    This leads to apparent motions of everything else in the universe.

  • Sun rises (towards E) and sets (towards W)
  • Moon rises (towards E) and sets (towards W)
  • Equatorial zone stars also rise and set.
  • Circumpolar Stars travel in counterclockwise circles around the North Celestial Pole.
    The star Polaris, at the end of the Little Dipper, is very close to the NCP.
  • (In the S. Hemisphere, they would go in clockwise circles around
    the South Celestial Pole).
  • The relative positions of the stars are (very nearly) fixed with respect to each other.
  • The Sun, Moon and PLANETS have positions that vary w.r.t. the fixed stars.

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    The North Celestial Pole is the projection of the Earth's north pole onto the sky.

  • Polaris (the North Star) is very close to this point.
  • it is 34 degrees above the horizon in Atlanta, and really
    DEFINES our LATITUDE

    The South Celestial Pole is the projection of the Earth's south pole onto the sky.

    The CELESTIAL EQUATOR is the projection of the Earth's equator onto the sky.

    The Sun moves on a path through the sky (w.r.t. the stars)
    called the ECLIPTIC

  • It is tilted by 23.5 degrees from the Celestial Equator.
  • The VERNAL EQUINOX occurs when the Sun, moving on
    the Ecliptic, crosses the Celestial Equator moving from S to N.
  • this is roughly March 21st
  • The Moon and the planets also wander w.r.t the stars,
    but always stay close to the Ecliptic.

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    SEASONAL/ANNUAL
    variations are due to Earth's REVOLUTION / ORBIT, around the Sun:

  • Sun rises due E and sets due W on the VERNAL and AUTUMNAL EQUINOXES.
  • Sun rises most N of E and sets most N of W on the SUMMER SOLSTICE.
  • Sun rises most S of E and sets most S of W on the WINTER SOLSTICE.

    At a given latitude, the same circumpolar stars are seen
    year-round, BUT different groups of equatorial zone stars rise
    and set at different times of year.

  • At the north pole, ALL stars are circumpolar
    and are visible basically for all six months of "winter"
    while ths sun is up all six months of "summer".
  • At the equator, ALL stars are EQUATORIAL, and rise
    and set each night, while Polaris is on the N. horizon.

    The Sun (and the Moon and planets) appear to move through a
    group of CONSTELLATIONS we call the ZODIAC.

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    FINISH READING CHAPTER 1