ge101

• Geodetic astronomy


  The art and science for determining, by astronomical observations, the positions of points on the Earth and the azimuths of the geodetic lines connecting such points

  
• Astronomy


  The scientific study of the universe beyond the Earth, especially the observation, calculation, and theoretical interpretation of the positions, dimensions, distribution, motion, composition, and evolution of celestial bodies and phenomena
• Celestial sphere


  • An imaginary surface whose center is the earth and where all celestial bodies are contained
  • The reference framework for celestial reference system

  
• Celestial sphere


  1. Rotates around its axis on an east-to-west direction

  2. The points of intersection of the axis of rotation of the earth and the sphere are known to be the celestial poles

  3. Perpendicular to this axis is a great circle called the celestial equator

  
• Celestial coordinate systems


  • Horizon System (HS)
  • Hour Angle System (HAS)
  • Right Ascension System (RAS)
  • Ecliptic System (ES)

  
• Horizon System


  • Primary reference: the horizon
  • Secondary reference: vertical circles
  • Parameters: altitude, azimuth

  
• Hour Angle System


  • Based on the location of the observer
  • Parameters: local hour angle, sidereal hour angle, Greenwich hour angle

  
• Right Ascension System


  • References: equator, hour circles
  • Parameters: declination, right ascension

  
• Ecliptic System


  • References: ecliptic, equinoctial colure
  • Parameters: ecliptic latitude, ecliptic longitude

  
• Celestial coordinate system parameters


  • Altitude
  • Azimuth
  • Declination
  • Hour Angle
  • Right Ascension
  • Ecliptic Latitude
  • Ecliptic Longitude

  
• The position of any celestial body may be defined by angular measurement from two planes at right angle to each other and passing through the center of the sphere

  
• The daily motion of the stars and their positions within the celestial sphere at any instant are intimately related to time

  
• The measurement of time is directly dependent upon the apparent motion of the stars
• Winter/Summer Solstice


  Point on the ecliptic defined when the sun's declination is greatest
• Equinoctial Colure


  A great circle in the celestial sphere passing through the celestial poles and the equinoxes

  
• Solsticial Colure


  A great circle in the celestial sphere passing through the celestial poles and the solstices
• Ecliptic Poles


  Points of intersection of the line perpendicular to the ecliptic and the celestial sphere

  
• Ecliptic Parallel


  Intersection of any plane parallel to the ecliptic and the celestial sphere

  
• Ecliptic Meridian


  Great circle in the celestial sphere that passes through the ecliptic poles

  
• Obliquity of the Ecliptic


  The angle between the plane of the ecliptic and the plane of the equator

  
• The measurement of time is directly dependent upon the apparent motion of the heavenly bodies caused by the earth's rotation
• Transit or Culmination


  The instant when a heavenly body is on the observer's meridian

  
• Upper Transit


  When a heavenly body is positioned on the side of the meridian containing the zenith

  
• Lower Transit


  When a heavenly body is on the other side of the meridian
• Time systems


  • Apparent Solar Time
  • Sidereal Time
  • Mean Solar Time
  • Standard Time
• Apparent Sun or True Sun


  The sun which we see
• Apparent Time or True Solar Time


  Determined with respect to the true sun, conveniently measured by a sun dial
• Apparent Solar Day


  The time required for one apparent revolution of the true sun about the earth, begins when the true sun starts to appear on the lower branch of the observer's meridian

  
• The length of an apparent solar day is not constant due to the non-constant velocity of travel of the apparent sun and the elliptical path taken by the apparent sun
• Sidereal Day


  Begins when the vernal equinox is on the observer's meridian and above the horizon

  
• Sidereal Time


  Equal to the hour angle of the vernal equinox referred to the meridian at any point of the earth surface

  
• Mean Sun


  A fictitious body that is assumed to move along the celestial equator at the average or mean rate of motion of the real sun during the year

  
• Civil Time or Mean Solar Time


  The standard time generally used, measured by watches and clocks, the hour angle of the mean sun
• Local Civil Time


  The time based on the central meridian of an observer
• Civil Day or Mean Solar Day


  The time required for one revolution of the mean sun, begins at 0 hours or midnight, mean noon is when the mean sun is on the observer's meridian, divided into 24 hours (AM and PM)
• Greenwich Civil Time or Universal Time or Greenwich Mean Time


  The standard time for Greenwich, used as reference for figuring out longitudes anywhere on the earth's surface
• Equation of Time


  The quantity needed to convert mean time into apparent time or vice versa, can be positive or negative
• Equation of Time = Apparent Solar Time - Mean Solar Time
• The difference in solar time between two points on the earth's surface is equal to the difference in their longitudes
• When travelling from west to east, time advances one hour for each 15 degrees of longitude, and when travelling east to west, time is set back one hour for each 15 degrees of longitude