Understanding the night sky

• • We measure the sky using angles.


  Pinky= 1 degree

  3 finger (index, middle and ring finger)= 5 degree

  closed fist= 10 degree

  Index and pinky= 15 degree

  thumb and pinky= 25 degree
• Full circle = 360 degree = 2 pi radians
• 
  

  Angles in degrees= 360 degrees

  Angles in Arcminutes= 21600' minutes

  Angles in Arcseconds= 1296000" seconds
• Whose Idea was angular measurement = 360 degrees?

  The ancient Sumerians around 3000BC.
• An objects's angular size appears smaller if it is farther away.
• A proper motion is how much an object moves (in angle on the sky compared to the stars) per unit time
• Angular speed is an object moving with a velocity will have a proper motion because its physical position changes with time.
• Angular resolution is the angle at which 2 points of light can be distinguished
• Anatomy of the milky way

  A) Globular clusters

  B) Disc

  C) Stellar halo

  D) sun

  E) bulge

  
• Stars at different distances all appear to lie on the celestial sphere
• • North celestial pole is directly above the Earth's North pole.
  • South celestial pole is directly above the Earth's South pole.
  • Celestial equator is a projection of Earth's equator onto sky.
• Ecliptic is the sun's apparent path through the celestial sphere.
• Zenith: the point directly overhead.

  Horizon: all points 90 degrees away from zenith.

  Meridian: line passing through the zenith and connecting N and S points on the horizon.

  A) Zenith

  B) Horizon

  
• Latitude : position north and south of the equator.

  Longitude : position east and west of prime meridian.
• The sky varies with latitude but not longitude.
• • Stars near the north celestial pole are circumpolar and never set.
  • We cannot see stars near the south celestial pole.
  • All other stars (sun, moon, planets) rise in east and set in west.
• As the Earth orbits the sun, the sun appears to move eastward along the ecliptic.
• At midnight, the stars on our meridian are opposite the sun in the sky.
• Why do the constellations we see depend on latitude and time of year?

  • Depends on latitude because your position on earth determines which constellations remain below the horizon.
  • Depends on time because Earth's orbit changes the apparent location of the sun among the stars.
• Seasons depend on how Earth’s axis affects the directness of sunlight.

  1. Summer solstice - June
  2. Winter solstice - December
  3. Spring equinox - March
  4. fall equinox - September
• Sun's path across the sky:

  • Summer Solstice: highest path; rise and set at most extreme north of due east.
  • Equinoxes: sun rises due east and sets due west.
  • Winter solstice: lowest path; rise and set at most extreme south of due east.
• Seasonal changes are more extreme at high latitudes
• Earth's axis points in the same direction (to Polaris) all year round, so its orientation relative to the sun changes as Earth orbits the Sun.
• Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when sunlight is less direct.
• Axis tilt is the key to the seasons.
• How does the orientation of Earth's axis change with time?

  Earth has a 26000 year precession cycle that slowly and subtly changes the orientation of Earth's axis.
• Sidereal day: Earth rotates once on its axis in 23 hours, 56 minutes, 4.07 seconds.
• Solar day: the sun makes one circuit around the sky in 24 hours.
• A solar day is longer than a sidereal day by about 1/360 because Earth moves about 1 degree in orbit each day.
• The phases of the moon are a 29.5 -day cycle.
• Phases of the moon:

  A) first quarter

  B) gibbous

  C) full

  D) gibbous

  E) last quarter

  F) crescent

  G) crescent

  H) Waxing

  I) waning

  
• Waxing: Moon visible in afternoon/evening; gets fuller and rises later each day
• Waning: moon visible in late night/ morning; gets less full and sets later each day.
• Sidereal month: moon orbits Earth in 27.3 days. Earth and Moon travel 30 degrees around the sun during that time.
• Synodic month: a cycle of lunar phases; takes 29.5 days ( which is 1/12 longer than a sidereal)
• We only see one side of the moon because of synchronous rotation- the moon rotates exactly once with each orbit.
• Eclipse: earth and moon cast shadows. The moon is in front of the sun.
• Solar eclipses can occur only at new moon; Moon's shadow on Earth
• solar eclipse can be partial, total, or annular
• Partial solar eclipse: The Moon partially obscures the Sun.