Seeing the Sky

1
Seeing the Sky

• Naked-Eye Astronomy

2
Naked-Eye Astronomy

• Stars
• Planets (move relative to stars)
• Sun (disk about 1/2 degree)
• Moon (disk about 1/2 degree)
• Other (comets, meteors, UFOs, etc.)

3
Angles

• Can only measure angles on the sky (circle spans
  360 degrees 1 degree 60 minutes 1 minute 60
  seconds)
• Angular distance is the angle on the sky between
  two celestial objects
• Use your fist (about 10 degrees at arms length)

4
Sky Directions

• Face South (sun at noon), then
• North is to your back
• West is to your right
• East is to your left(Northern hemisphere!)

5
Sky Motions

• Only see angular speeds (angle covered per unit
  time-day, month, year)
• Motions are relative with respect to some
  reference (stars, horizon)
• Same object can have different motions at the
  same time relative to different references

6
Stars

• Patterns fixed constellations (88 official)
  no visible change over human lifetimes
• Daily Rise in east, set in west (relative to
  horizon)
• Seasonal Different constellations visible at
  different seasons

7
Moon

• Daily Rises in east, sets in west (relative to
  horizon)
• Monthly Cycle of phases angle relative to the
  sun (opposition-full)
• Monthly Moves eastward relative to zodiacal
  stars
• Eclipses can occur at new (solar) or full (lunar)

8
Sun

• Daily Rise in east, set in west (relative to
  the horizon)
• Due south at noon (northern hemisphere!)
  greatest angle (for the day) above horizon at
  noon
• Seasonally noon height varies (highest, summer,
  lowest winter) rising, setting points also vary

9
Sun

• Moves eastward relative to stars
• Moves through constellations of the zodiac, about
  360 degrees in one year (about 1 degree per day)
• Path in sky relative to the stars defines the
  ecliptic

10
Planets

• Daily Rise in east, set in west (relative to
  horizon)
• Long-term Move eastward relative to stars
  (zodiac)
• In regular cycles, move westward relative to the
  stars (retrograde)

11
Planets-Long term

• Mercury, Venus stay near sun(morning, evening
  stars)
• Mars, Jupiter, Saturn anywhere in zodiac relative
  to sun
• Average angular speeds of eastward motion through
  zodiac Mercury, Venus, Mars, Jupiter, Saturn

12
Planets-Retrograde

• Mercury, Venus retrograde only near sun (when
  moving from evening to morning star)
• Mars, Jupiter, Saturn retrograde only when
  opposite the sun in the sky (opposition, 180
  degrees apart)