THE NIGHT SKY, continued

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

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PLANETS VS. STARS

All ancient civilizations realized the PLANETS WANDERED w.r.t. the FIXED STARS.

Sometimes they (Venus, Jupiter, Mars, Saturn, Mercury) moved

  • faster than the stars across the sky
  • slower than the stars across the sky
  • even sometimes moved backwards w.r.t. to the stars
    a RETROGRADE LOOP (for Mars, Jupiter and Saturn)

    Many believed that the location of the planets and Sun within various constellations
    (on the ecliptic -- these are the ZODIAC constellations) at the time of one's birth determined one's fate.
    Some foolish people still believe in the PSEUDO-SCIENCE of Astrology even today,
    even though it has been shown to be incorrect by dozens of experiments.
    Why? People remember the rare, random, correct predictions and tend to forget
    the far more common incorrect ones -- this is a psychological question,
    not an astronomical one.

    HOW CAN YOU TELL A PLANET FROM A STAR?
    (without watching for its wandering w.r.t. the stars, which is
    noticeable only over several nights.)

    STARS (usually) TWINKLE and PLANETS (usually) DON'T

    Stars are SO far away, that even though they are 100's of times the PHYSICAL SIZES
    of planets they are roughly only 1/1000th of their ANGULAR SIZES.

  • So stars look like points, and planets are little disks.
  • The turbulence in the earth's atmosphere causes light from celestial objects to jump around ("seeing").
  • When we look at a faint object we focus its light on the FOVEA
    at the back of the eye (most sensitive part of the RETINA).
  • The jumping of a point image (star) takes it off the fovea and it appears to
    temporarily disappear -- it twinkles (the light is still in the eye, but hits a less sensitive part).
  • While some of the light from a planetary disk leaves the fovea, enough remains
    on it so that we don't interpret its image as going away for a while.

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    The time it takes the earth to spin once on its axis
    is the SIDEREAL DAY -- stars appear in the exact same location.
    This is 23 hours, 56 minutes and 4 seconds long.

    The SOLAR DAY is the (average) time between local noons
    (when a shadow of a vertical stick is shortest).
    This is 24 hours, 0 minutes and 0 seconds long.

    Why the difference?

  • in one sidereal day the orientation of stars is the same
  • BUT the earth has moved 0.986 degrees around the sun
    in its orbit. Therefore, for the sun to be overhead again
    the earth must spin another 3 minutes and 56 seconds.

    This implies stars RISE about 4 MINUTES EARLIER EACH NIGHT
    as measured by our clocks and watches, which are
    set to solar time in a PARTICULAR TIME ZONE

  • BUT ARE NOT SET TO LOCAL TIME, which depends on one's exact LONGITUDE.
    I.e. local noon-- when the sun is highest in the sky occurs
    sooner in Boston than in Atlanta according to
    a clock set to the Eastern Standard time
    (and closest to 12 EST in Philadelphia, which is right near the center of the time zone)

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    We then went through an ``in class lab'' exercise on Eclipses.

    We will review eclipses and cover a few more details in the next lecture.

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