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Celestial Observations
        The Celestial Sphere

All celestial objects, (sun, stars, moon,
  planets), from Earth appear to be on
  the surface of an imaginary sphere
  surrounding Earth.
This imaginary sphere is called the
  Celestial Sphere.
    Celestial Observation Terms

Horizon = where celestial sphere meets Earth.
Zenith = point on celestial sphere directly
  above observers position.
Altitude = objects distance, in degrees, above
Azimuth = objects distance, in degrees,
  measured clockwise from due north position
  (due north is 00 azimuth).
                Star Paths

All celestial objects appear to move from east
  to west across the sky in an arc. Why?
Circumpolar stars move in
 counterclockwise circles between the
 northern horizon and Polaris and never set.
The apparent daily motion of celestial objects
  is due to Earth’s rotation.
Rotation is the turning of an object on its axis.

Celestial objects are not actually moving from
  east to west across the sky, rather the earth
  is rotating under them.

Revolution is the movement of one
  celestial object around another.
The path along which an object travels
  during a revolution is called an orbit.
Earth’s Revolution
       What are Constellations?

Constellations are stars that appear to be
  grouped in patterns that form the outlines
  of “things” (people, animals, objects).
The night sky is divided into 88 constellations.
Circumpolar constellations are those
  constellations that are always visible as they
  never set below the horizon.
                Star Shifts
Constellations shift, rise and set, (similar to
  the rise and set of the Sun) during a night
  due to the rotation of the Earth.
Constellations in the sky slowly change from
  one night to the next as a result of the
  revolution of Earth around the sun.
Constellations rise approximately 4 minutes
  later each night.
        Planetary Motions
• On a daily basis planets and stars
  appear to move from east to west.
• Over long periods of time the planets
  appear to move eastward relative to the
  stars behind them.
         Celestial Models
Geocentric         Heliocentric
 Theory: this       Theory: this
 theory, put        theory, put
 forward by the     forward by the
 ancient            Polish scientist
 Egyptian           Copernicus,
 astronomer         states that the
 Ptolemy, states    Sun is the
 that the Earth is  center of the
 the center of      universe.
 the universe.
Ptolemy and Geocentric Theory
Copernicus and Heliocentric Theory
              Orbital Forces
Johann Kepler developed the “Laws of
  Planetary Motion” which states that planets
  orbit in ellipses.
Unlike a circle the center of an ellipse consists
  of two fixed points called foci.
                Kepler’s First Law:
The orbits of the planets around the Sun are
  ellipses, with the Sun at one of the foci.
         Kepler’s Laws
Kepler’s second law states that:
An imaginary line joining a planet to the
  Sun will sweep over equal areas in equal
  periods of time.
Perihelion = when the planet is closest to
 the Sun (moving the fastest).
Aphelion = when the planet is farthest
 from the Sun (moving the slowest).
 Evidence of Earth’s Rotation
• Foucault Pendulum: the greater the
  latitude the greater the hourly change
  in direction.
• Coriolis Effect: is the tendency of
  matter moving across the Earth’s
  surface to be deflected from a straight-
  line path.
• Other evidence includes the day night
  cycle, star paths and satellite photos.
Apparent Motions of the Sun
• Due to the rotation of the Earth, the
  Sun appears to move in an arc across
  the sky from east to west.
• The tilt of the Earth’s axis, 23.50,
  causes the location of the direct rays
  of the Sun to change in a cyclic
        Change of Seasons

• Three factors cause the seasonal
   – Earth’s revolution around the sun
   – Tilt of the Earth’s axis
   – Parallelism of the axis
       Time and Earth Motions
• Sidereal day is the time period for Earth to
  make one complete rotation on it’s axis (23
  hrs, 56 min, 4 sec.).
• Apparent solar day is about 4 min longer
  (based on solar noon to solar noon) to
  account for the additional rotation needed as
  a result of Earth’s revolution around the
• Since our orbital speed varies, therefore the
  apparent solar day varies, the mean solar
  day (24 hrs) is used for daily timekeeping.
             Moon Motions

• Moon orbits earth once every 27 1/3 days.
• The barycenter is the gravitational center
  the Earth and Moon revolve around
  (1700km beneath surface of Earth).
• Sidereal month (27 1/3 days) is time for
  the Moon to revolve around the Earth.
• Synodic month (29 ½ days) is the time for
  the Moon to complete a cycle of phases.
          Phases of the Moon
• The illuminated portion of the moon that
  faces Earth varies in a cyclic pattern called
• New moon = when the entire illuminated
  portion is facing away from Earth.
• Waxing period = when right hand portion
  seems to grow more lighted.
• Full moon = when the entire illuminated
  portion is facing toward Earth.
• Waning period = when the left hand portion
  shrinks in illumination.
• Solar eclipse occurs when the Moon passes
  between the Earth and the Sun.
  – Umbra is the part of the Moon’s shadow in
    total darkness
  – Penumbra is the part of the Moon’s shadow in
    partial darkness.
  – Annular eclipse occurs when the Moon is at
    apogee and the outer edge of the Sun is visible.
• Lunar eclipse occurs when the moon passes
  through the Earth’s shadow.
Solar and Lunar Eclipse