See Figure 2-19.             f l REFERENCE ELLIPSOID POINT ON EARTH'S SURFACE Figure 2-19  Geodetic Coordinates c.  Geodetic coordinates are computed and adjusted as part of a geodetic network.  All the points in the network are "common" to all the other points in that network.  They are also "common" to points extending and adjusted from that network.  Geodetic networks   can be adjusted together to complete a national network such as the National Geodetic Reference System (NGRS) in the United States. 2-20  Astronomic Coordinates a.  Astronomic coordinates are those values which define the position of a point on the surface of the earth or the geoid and reference the local direction of gravity.  Note:  The term Astronomic Coordinates can also be used to refer to the location of a celestial body; however, that system is not discussed in this chapter.  Astronomic positions are often used to establish and define horizontal datums. An ellipsoid is oriented in such a way that a line through a point which is perpendicular to the geoid (vertical) is also perpendicular to the ellipsoid (normal) and the geoid separation is generally zero at that point.  At that point the geodetic and astronomic coordinates are the same.   b.  The astronomic latitude is the angle formed by the intersection of the plane of the celestial equator and the plumb line (perpendicular to the geoid).  It is equal to the angle formed by the plane of the observers horizon and the rotational axis of the earth. Astronomic latitude results directly from observations of celestial bodies, uncorrected for the deflection of the vertical.  The term applies only to the position of points on the earth.  Astronomic longitude is the time that elapses from the moment the celestial body is over the Greenwich Meridian until it crosses the observers meridian..  It results directly from observations of celestial bodies, uncorrected for the deflection of the vertical.  See Figure 2-20. DRAFT 2-10 ASTRONOMICAL LONGITUDES OF SELECTED PRIME MERIDIANS WITH REFERENCE TO THE GREENWICH MERIDIAN MERIDIAN OFFSET FROM GREENWICH Amsterdam, Netherlands Reformed Church, West Tower 4° 53' 01"  E Athens, Greece Observatory, Geodetic Pillar 23° 42' 59"  E Batavia (Djakarta), Indonesia Old Tidal Guage 106° 48' 28"  E Bern, Switzerland Old Observatory 7° 26' 22"  E Brussels, Belgium Observatory 4° 22' 06"  E Copenhagen, Denmark New Observatory 12° 34' 40"  E Ferro, Canary Islands (By definition 20° west of Paris) 17° 39' 46"  W Helsinki, Finland Observatory 24° 57' 17"  E Istanbul, Turkey Hagia Sophia 28° 58' 50"  E Lisbon Portugal Castelo San Jorge, Observatory 9° 07' 55"  W Madrid, Spain Observatory 3° 41' 15"  W Oslo, Norway Observatory 10° 43' 23"  E Paris, France Observatory 2° 20' 14"  E Pulkovo, Russia (USSR) Observatory 30° 19' 39"  E Rome, Italy Monte Mario 12° 27' 08"  E Stockholm, Sweden Observatory 18° 03' 30"  E Tirane, Albania First-order Trig Point 19° 46' 45"  E Figure 2-21    Astronomic Longitudes of Prime