What we see in the sky.

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What we see in the sky.
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How do we describe it?(constellations)
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Constellations

• There are 88 constellations.
• These can be subdivided into 8 constellation
  families. (Ursa Major, Zodiacal,Perseus,
  Hercules, Orion, Heavenly Waters, Bayer Group,
  and LaCaille)
• See Menzel, "A Field Guide to the Stars and
  Planetes"
• Or http//www.seds.org/Maps/Const/constS.html
• Or web search on constellation families

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Alt/Azimuth Method of Locating Positions in the
Sky
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Celestial Sphere

• Imaginary sphere of infinite radius with the
  earth at its center. It is used for describing
  the positions and motions of stars and other
  objects. For these purposes, any astronomical
  object can be thought of as being located at the
  point where the line of sight from the earth
  through the object intersects the surface of the
  celestial sphere. In astronomical coordinate
  systems , the coordinate axes are great circles
  on the celestial sphere. In most systems of this
  type, the reference points are fixed on the
  sphere, so the two coordinates needed to locate a
  body are relatively constant.

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Celestial Equatorial System
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Celestial Equatorial System

• a Right ascension (RA) is the angle from the
  vernal equinox.
• d Declination (DEC) is the angle up from the
  celestial equator.
• Vernal equinox

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Other systems

• Ecliptic coordinate system
• Very useful for objects in our solar system
• Uses the ecliptic as the primary circle and the
  north and south ecliptic poles.
• Galactic coordinate system
• Useful for phenomena within our galaxy
• Uses the galactic plane to define the primary
  circle.

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Cycles of the Sun
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Terrestrial Time Systems

• You are familiar with terms like second, day,
  week, month, and year.
• Astronomers have historically defined these in
  terms of the Earths rotation, the Moons orbital
  motion, and Earths revolution about the Sun.

• Today the second is a fundamental unit of time
  defined as 9,192,631,770 periods of the light
  emitted in the transition between two energy
  levels in a Cs-133 atom.
• Hydrogen maser clocks keep time to a precision of
  1 sec on 30 million years.

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Time Systems

• Day - historically defined as the interval
  between two successive upper transits of a
  celestial reference point.

• Upper transits occur when celestial reference
  point crosses the celestial meridian.
• Celestial meridian is an imaginary line drawn
  north point of the horizon, the zenith, and the
  southpoint of the horizon.

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Time Systems - Sidereal Time

• Sidereal time is divided into 24 sidereal hours.
• One sidereal hour corresponds to 15 degrees of
  the Earths rotation with respect to the stars.

• Vernal equinox is the zero point for sidereal
  time.
• Local sidereal time is the hour angle of the
  vernal equinox with respect to the celestial
  meridian.
• Because the vernal equinox precesses westward,
  the actual period of the Earths rotation is
  0.008 sec longer than the sidereal day.

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Solar Time

• Apparent solar time is the hour angle of the real
  Sun plus 12 hours.
• Length of the apparent solar day is not constant
  due to the eccentricity of the Earths orbit.
• To avoid inconveniences we use a mean solar time.
• Standardization achieved with 24 time zones.

• Hour angle is the angle with respect to the
  celestial meridian.
• Mean solar time uses the hour angle of a
  fictitious mean Sun which moves at the average
  angular rate of the true Sun.
• Zero degrees longitude runs through the
  observatory in Greenwich England.
• Standard time at Greenwich is know as Greenwich
  Mean Time (GMT)

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Variations in Solar Day
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Sidereal vs. Solar Day
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SUN and the Ecliptic
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Equinoxes and Solstices

• Equinoxes occur when the Sun crosses the
  celestial equator. This is where the ecliptic and
  celestial equators intersect.
• Here the axis of rotation of the Earth is
  inclined neither toward nor away from the Sun.
• There are two equinoxes per year, Vernal and
  Autumnal.

• Solstices occur when the Sun is at its furthest
  from the celestial equator.
• Here the axis of rotation of the Earth is
  inclined toward (or away) from the Sun
• There are two solstices per year, Summer and
  Winter

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Seasons

• During the year, the Sun does not shine equally
  on both hemispheres of the Earth except during
  the equinoxes. That is to say the angle of the
  light from the Sun makes with the surface of the
  Earth is different at different locations
  (latitudes).

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Motions of the Moon
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Moon - Earth System

• The Moon rotates about its own axis even as it
  revolves around the Earth.
• The period of the Moons rotation matches the
  period of its revolution.
• This is due to tidal locking.
• The period for this rotation/revolution is
  approximately 28 days. (One moonth)
• Sidereal period 27.32days, Synodic period
  29.53

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Phases of the Moon

• Phases of the Moon are caused by our shifting
  perspective on the sunlit suface of the Moon.
• Just as with the Earth, half the surface of the
  Moon is sunlit at all times. (Although the part
  of the surface illuminated by the Sun changes as
  the Moon rotates.)

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